Bisexuality: A Biological1 Perspective

Milton Diamond 2

First published in: Erwin J. Haeberle and Rolf Gindorf, eds.: Bisexualities - The Ideology and Practice of Sexual Contact with both Men and Women, New York, Continuum, 1998, pp. 53-80

Biological Diversity
Mammals: Ungulates, Primates, and Rodents
Kinsey Scale
Survey Data
Polygraph Studies
Social Background and Sexual Orientation
Upbringing and Sexual Rearing
Cross-Cultural Studies
Cross-Sex Rearing
Genetic Studies
Concluding Comments

There is little doubt that much of the present impetus to study the human phenomena loosely called bisexuality stems from the AIDS epidemic and interest in the transmission vectors of HIV disease. While this attention is of value for itself and relatively new from the perspective of psychology, sociology, and the humanities, the broader study of bisexuality is a relatively classical subject in biology, This is particularly true when our discussion encompasses botanical and invertebrate species. Even were we to limit our discussion to mammalian forms, this holds true. And the broad study of sexual orientation and partner preferences, how an individual chooses another for mating, is certainly not a subject of interest limited to the human species.

The biological definition of bisexuality is relatively dear. Biological bisexuality refers to the presence in the same individual of male and female structures related to the production, delivery, or storage of gametes. This may be normal and typical for each individual of that species, a common or rare variation, or a condition of sequential or simultaneous hermaphroditism. When behaviors are referred to, the definition becomes somewhat more vague and complicated.

Behavioral definitions revolve around activities associated with insertion or use of a sperm delivery organ being classified as male and those behaviors associated with receiving sperm or egg fertilization being categorized as female. As the behaviors become more complicated and courtship is extended, the definitions are broadened. They enlarge to encompass those behaviors which somehow might be construed to facilitate these basic behaviors in a sex-linked manner that is typical for the species. When humans are the species in focus, the biological definition remains the same, but at core, bisexuality refers to activities or inherent interests of an individual for sexual/genital involvement with both male and female partners. And for humans, the behavioral definitions take on social and political significance which often prejudice interpretation.


Biological Diversity

LEAVING ASIDE THE huge diversity among botanicals, the different bisexual types found among animal species are legion. Some examples should suffice. Among the better known invertebrate species are Crepidula fornicata and Anonymus virilis. Crepidula is a clam-like mollusk which is often found, as its name might suggest, with individuals piled one atop each other. The first individual to secure a perch acts as a female for the individual that alights on it. This second individual attaches to the first and produces sperm which inseminates eggs produced by the one below. When individual number two is found by a third, number three attaches and becomes the male to the converted individual below which now produces ova. This succession of male to female changes continues in turn as each new individual joins the pile. Each new top member produces sperm to fertilize eggs produces by the previous top male which now produces eggs. The gonads to provide these gametes are mature and appropriately activated by the situation encountered by the individual Crepidula (Brusca and Brusca 1990).

Anonymus virilis is a flatworm member of the phylum Platyhelminthes with another colorful popular name: "twenty-four penises." This aquatic species creeps on different substrata and seemingly inserts its twenty-four penises (stylets) into anything soft. It continues until it eventually does so to a conspecific. In so doing, while vaginae are available, it may essentially puncture the other's skin and deposit sperm in a process called hypodermic impregnation. The recipient produces eggs for appropriate fertilization as a female. In another instance, however, the impregnated member may insert its own penises into a different individual and deposit sperm. Basically, each individual can function as a male and female (Hyman 1951). While self-fertilization cannot occur, simultaneous fertilization can. This is true for many species of this phylum. Some species of leeches also copulate in this manner (Najarian 1976).

With consideration of vertebrates there are still many examples of fascinating bisexuality. Perhaps fish best demonstrate this. Among fish there are true examples of bisexual characteristics (Demski 1987; Thresher 1984). Two groups exhibiting different evolutionary strategies will again serve as examples showing the great diversity in ways bisexuality is manifest.

The seas basses, fish of the family Serranidea, like the flatworms, are simultaneous hermaphrodites. Within seconds, members of this group can mate acting as a male and releasing sperm in one instance and then acting as a female releasing ova the next. Hypoplectrus, a species of fish called hamlets, are well known in this regard (Thresher 1984).

The reef-dwelling wrasse Thalasoma duperry (family Labridae) in contrast, is a species which demonstrates sequential hermaphroditism. Some few individuals start off life as males, but most are born as females. Then, depending upon those conspecifics with which it interacts, the female either remains a female or develops into a male (Ross, Losey, and Diamond 1983).

Typically, these fish mate promiscuously in quite sex-specific ways.3 Their male or female behavior is definitely linked to the spawning process. With some fish species, sex change occurs in the largest individual present in the absence of an opposite-sexed individual (see Fishelson 1975, and Shapiro 1979), but in Thalasoma the female-to-male sex change requires visual dues from smaller conspecifics. A mature female alone will not become a male. When two females are placed together, however, the larger of the two will become a male in the absence of another larger fish (which is usually a male). This change is not dependent on the sex or color of the smaller fish, only on behavioral cues from a smaller conspecific. A smaller companion fish of a similar but not identical species won't serve as a proper stimulus to change.

The sex change of the larger of the two, however, can be inhibited by the presence of an even larger conspecific of either sex. Sex change not only involves behavioral shifts but replacement of developing oocytes with the development of active spermatogenesis (Ross, Losey, and Diamond 1983). The stimuli involved in these changes are visual, not pheromonal.

On the reef, a female probably changes sex when the relative numbers of larger and smaller conspecifics change within her home range. Considering the reef life of these fish, where many species cohabit and population densities may be relatively large, this flexible mechanism of insuring both fertile males and a large reproductively active female population is a good strategy. The crucial matter to be emphasized here is the dual requirement for both a stimulus by one type of partner and the absence of another type. This duality might be considered the basic principal of behavioral bisexuality. Such conditions will allow certain individuals, which already possess the potential, to dramatically switch their sex and behavior.

Among many birds, structural bisexuality has been reported as a rarity. Behaviorally, some species of gulls (western gulls, ring-billed gulls, and California gulls) have been reported to occasionally demonstrate female-female courting, mounting, and sharing of a nest site (Conover, Miller, and Hunt 1979; Hunt and Hunt 1977; Ryder and Somppi 1979). In these instances, however, there was a notable shortage of males, and this behavior is not considered routine. Some handreared domestic geese have also been reported to demonstrate homosexual-like activities (Lorenz 1967). Nevertheless, the phenomenon of homosexual or bisexual activities among birds is rare.


Mammals: Ungulates, Primates, and Rodents

THE PHENOMENON OF hermaphroditism or structural bisexuality becomes rarer as we probe higher in the evolutionary scale. Some examples, however, do exist. The common domestic cow provides one of the best illustrations, known to all cattle breeders. In the condition known as Freemartin (Lillie 1917), a female Bovine fetus, while in utero with a male twin, becomes "masculinized" by the gestational androgens present. These are sufficient to modify both her internal and external genitals. These females are almost always sterile, and in a manner similar to that induced in rats (discussed below), the female demonstrates male like behaviors. Twinning in cattle is a rare situation.

Even among normal cows, female-female mounting may occur when one is in estrus. This is a sexual mount to be sure and does not appear to be a displacement activity in the absence of a male. If a bull were present, however, he would mount the female and female-female mounting would be less likely to occur. A bull mounting a bull, however, is rarely seen.

In the sheep (Ovine), a closely related ruminant species, female-female mounting and the Freemartin condition are both rare. In this species, twinning is quite common. Interestingly, in the only known example of its kind in mammals, males reared together, without any experience with females, will occasionally develop a relationship in which they mount each other and then prefer mounting each other (without intromission) to mounting a female (Schein 1991).

In the field, primates seem heterosexual in almost all instances. Rarely does any activity occur which might be considered preferentially homosexual. While male-male or female-female mounting is not uncommon, it is almost always a demonstration of dominance or status in the primate hierarchy or appears to be a substitute behavior. With this in mind, some instances of homosexual-looking behavior have been reported. Hardy, cited in Weinrich (1982), claims that among languors (Presbytis entellus) male-male mounting occurs in situations of sexual excitement where the males do not have females available for mounting. Yamagiwa (1987) also cites male-male activity among wild gorillas as a result of a relative lack of females. Some male-male and female-female sexual-appearing activity has been reported for the Bonobo (Pan paniscus) (de Waal 1990). The Bonobo is a chimpanzee like primate with very human like activities including occasional ventral-ventral mounting and copulation. Their activities also include occasions of same-sex behaviors which can be considered mutual masturbation, kissing, genital play, and genital apposition. De Waal (1990, p. 387) explains these behaviors as serving not erotic, but important tension-regulating functions.

In all of these nonhuman mammalian situations it appears that homosexual or bisexual activities occur when, as mentioned, for Thalasoma above, there is present the dual requirement of both a stimulus by one type of partner and the absence of another type. Such conditions allow certain individuals, which already possess the potential, to switch from their preferred sex and behavior. This means both that the environmental situation must be conducive to the behavior and that only certain individuals and certain species will respond.

In contrast to spontaneous homosexual or bisexual behavior, there is a large literature on the induction of male-to-female and female-to-male behaviors among mammals. Starting with the classical paper by Phoenix, Goy, Gerall, and Young (Phoenix et al. 1959) working with the guinea pig, many have reported on paranatal influences on mammalian sex behavior development. While different from our normal understanding of bisexuality, some of these findings are instructive for the present discussion.

With good justification, much has been made of the fact that androgens given to neonatal female rats can result in these individuals, when adults, displaying malelike mounting behavior when placed with a receptive female. Similarly, male rats castrated paranatally, and thus having their endogenous androgens removed, will demonstrate femalelike lordoses when placed with a normal adult male (for reviews of this large area of research, see Whalen 1968; Feder 1981; and McEwen 1983).

Several features of these experiments warrant emphasis: (1) the individual test female is placed with a receptive female, which she mounts in a malelike fashion. She is not tested with a male. (2) The individual test male is not placed with a receptive female, to see if he will mount her or if he will lordose. He is instead placed with an intact adult male toward which he lordoses.4

Also worthy of note is that even androgens, when given neonatally to males, can adversely affect their male sexual behaviors. For instance Diamond, Llacuna, and Wong (1973) have shown that testosterone propionate can be as deleterious as estrogen in decreasing ejaculation, intromission, and mounting. But noteworthy for our present discussion, the treatment also resulted in a significant increase in oral-genital licking. This is a typical male-sex behavior and extremely rare for females.

The significance of all these findings is great in showing how behaviors can be reorganized by hormone manipulation during critical stages of development. Male and female rodents can, with relative case, be experimentally induced to behaviors that are not typical for members of their sex. And this may indeed have theoretical relevance to how human behavior might be modified by endogenous idiophatic events.

However, for most direct relevance to human sexual orientation, the studies referred to, with comments (1) and (2) above, may be argued not to have demonstrated homosexual or bisexual behaviors since the test animals were not given a choice to see which type of partner, male or female, they might have preferred or if they would mate with both. They were shown only to have been given the ability, when placed in a research situation, to demonstrate behaviors typical of the opposite sex. Studies where the experimental animals were given a choice would be of value.

While it is now known that androgens convert to estrogens within the rat brain (for review, see Baum 1979) and this can produce the deleterious effects to mounting and ejaculation referred to above, it does little to explain the simultaneous increase in male oral-genital behavior. Also, there is little evidence to demonstrate that while these animals can be induced to show opposite-sex behaviors, they prefer same-sex sexual partners.

The point to be made is that definitions of opposite sex behavior and assertions of homosexuality and bisexuality have to be very clear and specific. If too broad, they obscure more than they reveal.

Comparable work to that in rodents has been done with other species including primates (e.g., Phoenix, Goy, and Resko 1968). Suffice it to say that males and females can be experimentally induced, with hormone manipulation, to show behaviors typical of opposite-sex individuals. The magnitude of the influence is dependent upon the time, duration, and size of the endocrine manipulation. With primates, much more of a biological push is needed to alter inherent tendencies.

One well-known set of primate studies is worth emphasis here, even though it does not involve hormonal intervention. The studies of Harlow and coworkers (Harlow 1961) are often cited to show how social input is crucial for sexual development. That issue cannot be denied, but another aspect of these studies bears emphasis. A normal rhesus monkey raised in isolation will not react appropriately enough to mate when placed with a willing partner. However, it is clear that such an isolated male will still try to mount or insert. While his mounting attempts will be aberrant, his behaviors will be male, not female. Similarly, a previously isolated female will show deviant female patterns, not male ones. She will try to "present" and "receive." The basic biological drives are intact in both sexes, but social learning is needed so males can learn where and how to "insert" and females where and how to "receive." The inherent mate and female patterns are there to be augmented and refined by learning and practice. So-called preferential bisexual behavior has not been reported for primates.



KINSEY ET AL. (KINSEY, Pomeroy, and Martin 1948; Kinsey et al. 1953) clearly state they thought the term bisexual best reserved for nonhuman species with biological characteristics of both male and female. Although quite rare, true hermaphroditism or intersexuality, with. both ovarian and testicular tissue present, does exist in humans and its presence has long been well known (for review, see Armstrong 1964 and Overzier 1963). Much more common are conditions of pseudohermaphroditism in which, judging from the appearance of the external genitalia, the impression rather than the actuality of hermaphroditism exists. The behavioral disposition of individuals with these hermaphroditic or pseudohermaphroditic conditions is usually in keeping with their genetic-endocrine heritage, although they may be reared erroneously (Diamond 1965, 1968, 1976/77, 1982).


Kinsey Scale

WITH THE USE OF their now classical "scale" for sexual orientation, Kinsey and colleagues (1948, 1953) evaluated individuals in terms of both behavior and psychic orientation. Determinant were the sex of the partners with whom the respondent had sexual experiences and the sex of the partner about whom the respondent was fantasizing. This seven-point Kinsey scale ranges from exclusive heterosexual behavior (0) to exclusive homosexual behavior (6). An individual with an equal number of male and female partners would be rated 3 on the scale (K=3). Someone with only incidental activity with someone of the same sex would be rated I (K=1). Someone with only incidental activity with someone of the opposite sex would be rated a 5 (K=5).

The Kinsey scale can be used to evaluate an individual's actual behavior or preferred behavior. In the latter mode it is an index of the sex of the preferred partner toward whom one might be attracted. This is seen as the prime measure of sexual orientation or sexual preference. Thus, heterosexuality, homosexuality, and ambi- or bisexuality are seen as terms of psychological sexual orientation or sexual preference.

Very few people in any society studied sustain anything like an equal balance of both same-sex and other-sex partners for a long period of time. Behaviorally, those warranting a 3 on the scale are rare. On the other hand, it is not rare for an individual to be married having sex with his or her spouse while fantasizing about a partner of his or her own sex. To smooth out this disparity, an individual's behavioral and psychological scores may be averaged to get a single composite index. This can result in an overall rating of K=3 but it's meaning might be misunderstood.

Behaviors rated 1 through 5 were termed ambisexual by Kinsey et al. Most individuals were found to be exclusively or predominantly heterosexually or homosexually active over years and decades; bisexual behavior over a lifetime was a minority practice. It is worth recalling that Kinsey et al. (1948, 1953), in addition to the 0 to 6 scale, also used an x in their ratings. This was for individuals, not at all rare, who were neither sexually aroused nor attracted to either males or females. These individuals were not erotically interested in sexual contact of any kind. They constituted some 2% of their male and about 5-18% of their female respondents.

Considering sexuality in humans, it should be clear that the terms heterosexual, homosexual and bisexual refer better to behaviors than to people. Used in this context, the terms indicate the sex of the erotic/love/affectional partners a person prefers; whether these partners are of the same sex or opposite sex or either. Other than this, the labels may say very little about the person involved. A banker who prefers samesex or opposite-sex partners, for example, might have more things in common with other bankers than with a plumber having similar erotic preferences. For just such reasoning, Kinsey et al. (1948, 1953) disparaged use of the terms homosexual, heterosexual or bisexual as labels for their respondents.

While Kinsey and his colleagues did not label people by their "score" and use their ratings as nouns to refer to individuals, others were and are not so fastidious. The use of the terms as labels is actually common. The labels "homosexual" and "heterosexual" seem convenient to discuss individuals or populations of persons involved in same-sex or opposite-sex activity. Those who are labeled, however, are not always appreciative. First, the labels are often stigmatizing. Second, individuals who engage in homosexual behavior often find themselves caught in a crossfire. Politically active gays, and those who would discriminate against them, want such persons to identify as homosexual whether or not the individual identifies as such. More ambiguous has been the status and labeling of those with ambisexual behavior. Individuals who exhibit both same-sex and opposite-sex erotic activities or fantasies often see themselves as heterosexual or homosexual, not both or bi. On the other hand, many lay persons consider anyone with any homosexual experience, regardless of how infrequent, a "homosexual."

Some persons who feel strongly on this issue see individuals who exhibit ambisexual preference as "fence sitters" who should really belong to either the heterosexual or homosexual camp rather than as male or female bisexuals with their own arousal patterns. Others, cited by Rubin (1983), just as vociferously contend that those who engage in ambisexual activities would benefit from their own group identity, and call for being open as a bisexual, just as one might be open about being homosexual. This seems the present political zeitgeist.


Survey Data

This question of definition or label or self-identity, as used by either investigators or the people investigated, is of more than polemic interest. It affects the nature of the data collected and its validity.

As the term bisexual came more into general use and the nature of bisexuality more under scrutiny, various investigators established their own definitions. While Kinsey et al. (1948, 1953) considered ambisexuals as those demonstrating K=1 through K=5, Weinberg and Williams (1974, 1975), Bell and Weinberg (1978), and Green (1987) considered bisexuals as those whose behaviors and fantasies averaged K=2 through K=4. Haeberle (1978) categorized heterosexuals as K=0-2, homosexuals as K-4-6, and ambisexuals as K-2-5. All these investigators were coping with the reality that the term often obscures as much (if not more) than it reveals. It is extremely rare that an individual would both live and fantasize an erotic life with someone who was not sexually arousing, but an occasional or even prolonged "fling" might occur. Necessity might also force such behavior. Blumstein and Schwartz (1972) have written: "Bisexuality ... gives a misleading sense of fixedness to sex-object choice ... the preferable term, [is] Ambisexuality, connoting some ability for a person to eroticize both genders under some circumstances."

In this manner, in contrast with true erotic predilection, any individual may exhibit hereto-, homo-, or bisexual behaviors for many reasons which have nothing to do with sexual arousal or interest (see Diamond 1984, 11-12). Blumstein and Schwartz (1977) discuss individuals' sex-object choice changing in many ways and many times over a life cycle, with the individuals often unaware of their ability to change, and early sexual experiences not being the final or even the strongest determinant of adult sexual expression.

With these ambiguities, among others, the size of the "true" ambisexual population - or heterosexual population, for that matter - is difficult to establish. The data on males is limited and the data on females even more so. Aside from the study by Kinsey et al. (1953) there is no known large-scale survey of females which documents bisexuality. And neither the male nor the female Kinsey studies were conducted among random samples, so they may be of little interest in this regard. To use them to identify groups, as has been done so often in the past, might misrepresent not only the size of this population, but that of any group extracted from the data set.

Kinsey et al. (1948) reported, in one of their most controversial findings: "it appears that nearly half (46%) of the [male] population engages in both heterosexual and homosexual activities, or reacts to persons of both sexes, in the course of their adult lives [p. 656]." By extension, with heterosexuals (K=O) representing 50% of the population and exclusive homosexuals (K=6) representing 4% of the population, ambisexuality should be considered almost as common as heterosexuality. Extracting from the original Kinsey data, even if we combine the K=1 behaviors with exclusive heterosexual behavior and K=5 with exclusive homosexual behavior, the size of the remaining ambisexual population remains significant. Indeed, these numbers have been challenged.

It has been said that, in contrast with the female volume, which was reissued, one reason the Kinsey et al. (1948) male volume was never reissued in paperback or later editions is due to concern with the statistical value of their sample and its lack of randomness. This was a major criticism at the time of publication (Terman 1948) and remains so today. Gebhard (1972), one of the co-authors of the 1948 and 1953 studies, discusses this and other shortcomings of the original study. He points out that the original study had a high percentage of prison inmates, particularly among the noncollege-educated population, which tends to inflate the proportion of those with ambisexual experience in the total study population.

Figures given by Kinsey et al. (1953: table 142) for female ambisexual behavior, K=2-5, were something between 4 and 11% for those twenty to thirty-five years of age. Some 1 to 3% demonstrated exclusively homosexual, K=6, activities.

But even if the numbers are more or less a true reflection of the population having engaged in both same-sex and opposite-sex activity, we know nothing of the motivation for the behavior. What percentage represents true bisexual arousal compared with sexual experimentation or curiosity? Considering that many male respondents were prison inmates or had prison experiences, what percentage represents a forced sexual activity or purely displacement activity?

Suffice it to say, the actual representation of homosexuals or ambisexuals in any population is under dispute both scientifically and politically. Despite the AIDS epidemic as a prod to such research, attempts at a national U.S. study to obtain such data have been stymied by political and sexophobic considerations (Booth 1989). Attempts to similarly frustrate such research in Britain were successful for a period (Wellings et al. 1990) but were overcome.

Nevertheless, some information is available. A study of several thousand self-identified homosexual males in the United States, the Netherlands, and Denmark by Weinberg and Williams (1974) indicated that about 1 in 5 Americans were sexually active as K=2, 3, or 4 and about 1 in 10 Dutch or Danish gay men were similarly active. Their findings for Americans are at variance with three more recent studies: Bell and Weinberg (1978), McWhirter and Mattison (1984), and Diamond and Higa (in preparation). These newer studies are of U.S. males and their findings are relatively consistent (see Table 1).

For the newer studies of males who identify as engaging in homosexual behavior, about three out of four or more claim to be exclusively homosexual and fewer than one in ten claim they ever had more than incidental sex with a female. Unfortunately, all four of these studies also are nonrandom.

Difference among the findings of the original Kinsey et al. (1948) study, the early Weinberg and Williams (1974) study, and the latter research offered in Table 1 may reflect behavior changes over time. The evolving so-called "Age of Aquarius" and movement toward decreased sexual restrictions might have fostered among homosexual oriented males or females a lessened feeling of obligation to engage in heterosexual activities, and for heterosexuals may have provided an increased availability of partners to more readily satisfy their erotic desires.

A recent probability sample household survey in Dallas (Dixon et al. 1991) by the Centers for Disease Control found only 7.3% of their male population (N=701) reporting having had any male-male sexual contact from 1978 to 1989 and a just-analyzed 1989 random sample of Hawaii residents found less than 3% of males (N=1,024) and 1.2% of females (N=987) as having engaged in same-sex or bisexual activity (Diamond, Ohye, and Wells in preparation).

Studies cited by Schover and Jensen (1988, 47) for a randomly selected population of Danish women found only 1 of 625 women from twenty-two to seventy years of age admitting to having had a homosexual experience.

A recent study by the Dutch research group NISSO (National Institute for Social Sexological Research) and Utrecht University has the first known national random sample data of male and female sexual behavior (Sandfort and Van Zessen 1991). It is worth considering in some detail. In a 1989 study using face-to-face interviews (N=421 males and 580 females), only about 13 in 100 males admitted to ever in their life having a homosexual experience and only about 3.3% would consider themselves homosexual (K=5-6) and an additional 4.5% of their male sample would self-label themselves as bisexual (K=2-5). Only 1.2% of the males had bisexual activities in the preceding twelve months.

Among females, only about 10 in 100 reported ever having had homosexual activity and only about 3% of the female population would consider themselves bisexual (K=2-5). Less than 1% of the female population had had bisexual activities in the preceding year. Fewer than 0.5% of the interviewed women considered themselves homosexual.

Most importantly in regard to these data, a follow-up study of individuals who initially refused to be interviewed found males not to differ significantly from the main sample on all relevant variables (lifestyle, sexual risk behavior, sexual preference). And while the females that were not in the original study, but found in the follow-up, reported more risk behaviors and were more likely to have had homosexual experiences and a current homosexual self-labeling, the number involved is too small to estimate how much the basic figures might be modified (Sandfort and Van Zessen 1991).

Lastly, findings from a most recent British national random study (Wellings at al. 1990) echo these findings. These researchers report only "9% of men and 4% of women reported some homosexual experience, and 5% of men and 1% of women reported ever having a homosexual partner."

These recent British, U.S., Danish, and Dutch reports appear fairly uniform in results and offer very different findings than would have been predicted by previous research. And the comparatively liberal attitudes toward homosexual or bisexual activity in Denmark or the Netherlands, if not in Britain or the U.S., must be considered strongly. These studies taken together certainly seem to indicate that bisexuality, and indeed homosexuality, may be less common than previously considered. This appears whether the group surveyed are self-identified gays or from random samples.

Looking at the phenomenon from another perspective, it is safe to say that most individuals, most of the time, and for most of their lives, have sex with only males or females, not both. On the other hand, some individuals, at least for a few years, engage in sex with both. Aside from that, with the exception of the Netherlands and the U.K., for the present we have no good data for any country on the actual size of its homosexual and bisexual population nor how diverse are its sexual interests. Without a reliable random study for the United States and other countries, we have to consider that a population of men - the size of which we do not know - do not associate with homosexual groups or consider themselves as such, but do demonstrate ambisexual activity. But adding consideration of the data from Dallas and Hawaii, we might anticipate that the number will be significantly smaller than previously thought.5


Polygraph Studies

SO FAR, WE have discussed assessment of sexual orientation by fairly obvious methods. Most studies directly ask subjects to self-identify on the basis of the type of partner - male, female, or both - with whom they experience sexual activities or to whom they are attracted. Or they ask respondents the number of male and female partners they have had in a specific time period. Since the advent of AIDS the interval of concern is usually from 1978 to the present. Field observers such as anthropologists might obtain such information by direct observations, and clinicians in their offices might, in their own way, learn how individuals select partners.

Another method is available: genital plethysmography. This is a laboratory technique of assessment believed to bypass an individual's possible attempt to either pretend or mask socially controversial homosexual or bisexual arousal or interest. In societies such as ours, personal reports may be self-serving or might even reflect biases of which the individual him or herself is unaware. Polygraph studies supposedly reveal a core orientation.

This technique utilizes the physiological phenomenon of genital vasocongestion which occurs when an individual sees or hears stimuli found sexually arousing. Essentially, these responses are measured by placing a vaginal photoplethysmograph probe in a female's vagina or a penile strain gauge or volume-measuring device around a male's penis and recording the subject's responses to different stimuli. The visual stimuli might be slides, movies, or video depictions of nudes or frank sexual activities. To such stimuli reflexive blood flow responses occur (vasocongestion and/or erection) with greater deflections of the recording device interpreted to indicate greater erotic interest to the stimuli. Pupillary responses (Hess 1968, Hess and Polt 1960) and other physiological measures have also been used to this end. Genital vascular responses however, seem to be better known and more reliable.

While pupillary and genital responses are probably a valid reflection of erotic interest, the responses might also be to novelty, shock, or something else. Nevertheless, practiced test givers can most usually differentiate the responses of erotic interest from other causes. Many studies have documented the heuristic and practical value of such measures.

This technique has been found valuable in differentiating those who find homosexual stimuli appealing from those who find heterosexual stimuli appealing, and vice versa, those who find homosexual or heterosexual stimuli unappealing.

In this area, where the work of many might be cited, e.g., Abel et al. (1975), Conrad and Wincze (1976), Mavisskalian et al. (1975), McConaghy (1967, 1978), Zuckerman (1971), and the work of Freund (1963, 1974, 1982, 1984, 1989) is classic. Basically Freund, in summarizing his years of experience (personal communication), feels that individuals, males or females, are best considered either gynephilic (attracted/aroused by females) or androphilic (attracted/aroused by males) rather than homosexual or heterosexual. His many investigations in this area lead him to conclude that, while individuals may engage in sexual activity with either males or females or both, very few actually show reflexive responses to both. His basic emphasis is in keeping clear the distinction between what arouses individuals and what they do. These two features may or may not be in concert. From the data presented in Table 1, it is interesting to note that among individuals who consider themselves homosexuals, more claim bisexual attractions than engage in bisexual behavior. Why this is so remains an intriguing question.


Social Background and Sexual Orientation

WHAT CAUSES THIS androphilia or gynephilia? One thing we can say for certain: It is not a simple matter of rearing. Three types of studies illustrate this. The first focuses on an individual's upbringing and family constellation. The second looks at cross-cultural research, and the third inspects those cases, rare but not unknown, where individuals arc reared socially opposite to their biology. As with the animal data, due to space restrictions, calling attention to a few salient studies will have to suffice.


Upbringing and Sexual Rearing

TWO TYPES OF studies are important here. The first I will call "watching the children and the second I will call "watching the family." The work of Green (1987) typifies the first type of study. Green followed prepubescent boys with obvious effeminate behavior (N=66) and boys who were masculine (N=56) for fifteen years, to see how they would develop. In U.S. culture being a "sissy' is socially difficult and stigmatized. Formal and informal forces are usually marshaled against the practice. How the boys would react to family, society, and establishment was the subject of the investigation. Of the families involved with the "sissy" boys, most tried on their own to discourage the effeminate behavior, and a minority of the parents even entered their sons into a formal treatment program. Nevertheless, the study found that, when interviewed as adults, out of "two-thirds of the original group of 'feminine' boys reveal that three-fourths of them are homosexually or bisexually oriented. By contrast, only one of the two-thirds of the previously 'masculine' boys is homosexually or bisexually oriented."

The second type of study is typified by the work of Bell, Weinberg, and Hammersmith (1981) in the United States and Siegelman (1981) in Great Britain. These investigators looked for features in the family constellation and background of adult heterosexuals, homosexuals, and bisexuals. Their basic finding was that no common parameter of family or upbringing could be linked causally to sexual orientation. These workers on both sides of the Atlantic basically concluded they could find no correlation between any aspect of an individual's childhood or adolescent experiences and their homosexual or bisexual activities. Most homosexuals are reared as heterosexuals in apparently conventional households.

Bell, Weinberg, and Hammersmith (1981) cautiously conclude: "Exclusive homosexuality seemed to be something that was firmly established by the end of adolescence and relatively impervious to change or modification by outside influences. For the bisexuals, by contrast, a homosexual preference seemed to emerge later and to be more tied to learning and social experiences." (p. 211) and "our findings are not inconsistent with what one would expect to find if, indeed, there were a biological basis for sexual preference" (p. 216; emphasis in original).

A study combining features of both types of research is also instructive. Mandel et al. (1980, 1979) followed the development of boys raised in households where the parental influence was openly lesbian. They concluded: "analysis of the children's data has not revealed any sexual identity conflict or homosexual interest. Relationships with fathers and other males do not differ significantly [from that of boys reared in heterosexually patented families]." In summarizing results on both boys and girls these authors state: there is "no evidence of gender conflict or poor peer relations" for samples of children reared by lesbian mothers (Hotvedt and Mandel 1982). Similar work has been done studying boys who grow up in households patented by openly gay males. These boys, when adult, like those brought up in lesbian households, also were heterosexually oriented without conflict or homosexual interest (Green 1978). Also, neither homosexuals nor heterosexuals perceived their parent's personalities differently (Newcomb 1985).


Cross-Cultural Studies

AGAIN, ONLY A FEW of the studies appropriate here will be mentioned. Three very different types will serve as examples. Whitam and Mathy (1986) studied, in detail, homosexuality in four different cultures: Brazil, Guatemala, the Philippines, and the United States. Across these quite diverse societies these sociologists found many similarities in how homosexual lifestyles were manifest. Examples of such behaviors included preferences in occupational interests, to involvement in entertainment and the arts, to Cross-dressing, and use of their respective languages in effeminate ways. These researchers concluded that the similarities were not culturally instituted but were more likely the result of inherent biological tendencies manifest despite acceptance or rejection by the community. Whitam and Mathy conclude: "The view which emerges from cross-cultural research is that sexual orientation is not highly subject to redefinition by any particular social structural arrangement" (p. 31).

The well known work by Herdt (1981) and Stoller and Herdt (1985) is the second example of this group of studies. These researchers documented a New Guinea culture, the Sambia, where homosexual behavior is taught, encouraged, and institutionalized as a way to transfer masculinity from adults to adolescents; the young boys fellate the adult men to obtain there semen. Moreover, females' bodies are presented as not only unattractive and to be avoided, but poisonously dangerous. Nevertheless, these boys, upon reaching adulthood choose females as regular sexual partners, and are almost always heterosexual. Neither youths nor men report impulses to suck penises and they do not ever engage in anal intercourse.

Schiefenhovel (1990) reports on a similar New Guinea culture, the Kaluli, in which anal intercourse is used to transmit the masculinity-inducing semen between older men and younger boys. He too stresses that heterosexual, not homosexual or bisexual, behavior is the preferred and exclusive outlet for these males when they mature. And this is obtained, despite a severe shortage of adult women due to female infanticide.

A set of studies provide the third type of cross-cultural evidence supporting an inherent sexual orientation independent of upbringing. These are the reports from the Dominican Republic by Imperato-McGinley and colleagues (Imperato-McGinley, Guerrero, and Gautier 1974; Imperato-McGinley and Peterson 1976; Imperato-McGinley et al. 1979). These investigators found a population in which, due to a genetic defect, males were born without penises. They were assumed to be girls and raised accordingly, but at puberty they started to develop penises and respond to their own endogenous male hormones. Despite their rearing, all the boys rebelled against their female status and assumed male identities and life patterns. Occasionally, these studies have been criticized (e.g., Gooren et al. 1990) by calling attention to the fact that, at least for the last twenty years or so, this phenomenon has been understood by the natives in the population and the children have been reared appropriately from birth. Indeed that is true. However, prior to the 1970s this was not so and the sex role switch referred to above occurred before the problem was identified. In those earlier days, the boys were inadvertently and unambiguously reared as girls and then, on their own, switched to live as males.


Cross-Sex Rearing

THERE ARE NOW KNOWN to me three instances, two sets of male twins and one singleton, in which an individual was reared as a female, with surgery and endocrine treatment to alter the biology to facilitate the transformation. The first case involved a set of twins extensively reported upon in the literature (Diamond 1982; Money and Ehrhardt 1972). In this case, as a result of a surgical accident during circumcision, an unambiguously male child had his penis ablated.. Believing that gender identity would be based upon the sex of rearing, the decision was made to rear this individual as a female (Money and Ehrhardt 1972). Originally, it was reported that the induced transition was a success (Money 1973; Money and Tucker 1975).

It is now known, more than fifteen years later, that despite subsequent surgical orchidectomy and treatment with female hormones, this individual never accepted the female status or role as claimed by the early investigators (Diamond 1982). Prior to puberty, the twin, without ever being told of his previous history, rebelled against the imposition of a female status. The twin, at his own request, wanted to live as a male. It is now known that, at eighteen years of age, this individual, who was raised as a girl, sought and had phalloplastic and scrotal reconstruction surgery. He now, at the age of twenty-eight, lives as a male, and seeks females as sexual partners. His adjustment is not without its difficulties, but, to him, seems preferable to imposed life as a female.

The second set of identical twins involves two six-year-old Samoan children who were brought to my attention. It seems that one was causing a great deal of disorder at school. The "female" of this twin set was acting rowdy and picking fights, not only with female classmates but also males. The child's history revealed that, unbeknownst to him to that date, ambiguous genitalia at birth had prompted the surgeons then in attendance to reassign him as a female. With appropriate castration and hormonal follow-up, they convinced the parents to rear the child as a girl.

Despite the rearing as a female, even at this young age, the child rebelled against wearing girls' clothes and following the parents' and teachers' admonitions to "act like a girl." The twins typical play patterns and demeanor were typical for a six-year-old rambunctious boy and such that the brother, in discussion with me, often slipped into using the male pronoun while referring to his twin; e.g., "He, I mean she, swims better than me." Asked to draw a child, the misassigned twin drew an ambiguous figure he identified as male. The child spontaneously expressed the desire to grow up as a boy.

The third case is similar. In the summer of 1990, I was called to review the behavior and condition of a four-year-old child. Here again, the history revealed that due to ambiguous genitalia at birth, the decision had been made to reassign the boy as a girl. The decision was followed with appropriate castration and hormonal therapy and the advice to rear the child as a girl. In consultation with Dr. Richard Green of U.C.L.A., it became apparent that by the age of four this individual was exhibiting marked boyish behavior sufficient to disturb the parents and attending professionals. The child was not easily accepting the female role. The fixedness of behavior patterns along male lines, and aversion to the female role, were strong despite the contrary upbringing.

In these last two instances, the individuals were too young to express erotic interest in a sexual partner. I predict in these cases, as I did in regard to the first twin mentioned above (Diamond 1976/77, 1978, 1979) that despite being reared as girls, they will be gynephilic (Diamond 1982). Despite the writings of some on the power of upbringing, role modeling, and learning, there is no known case anywhere in which an otherwise normal individual has accepted rearing or lift status in an imposed role of the sex opposite to that of his or her natural genetic and endocrine history The removal of penis and testes and imposition of a female rearing has never proven sufficient to overcome the inherent bias of the normal male nervous system.


Genetic Studies

PERHAPS THE STRONGEST evidence that sexual orientation has a biological basis involves genetic studies of human families and twins. In the middle of this century, research was done by F. J. Kallman (1952a, 1952b, 1963). Kallman studied forty monozygotic and forty-five dizygotic male twin pairs in which at least one of the co-twins, at the onset of the study, admitted to homosexual behavior. Among this sample Kallman reported 100% of the monozygotic concordant for homosexuality. The dizygotic twins were seen essentially similar to the general male population. Although others found similar genetic components to sexual orientation, e.g. Schlegel (1962), these works were not accepted by many because they did not jibe with the societal preferences of the times. The mood of the 1950s and 1960s preferred to have human behavior a matter of social construction or free will rather than biological predisposition.

Kallman (1952a, 1952b) also reported finding that if one member of a monozygotic twin pair of brothers rated Kinsey=5 or 6, then the chance that his brother also rated K=5 or K=6 was better than 90%. He reported that if the brothers differed in concordance it was usually within one or two points on the Kinsey scale. The fact that Kallman's numbers seemed to come out so cleanly also encouraged skepticism. Several studies soon followed which reported monozygotic twins not concordant for homosexuality (Green and Stoller 1971; Klintworth 1962; Ranier et al. 1960), and theories which held to a genetic component to homosexuality lost support.

That situation essentially held until the 1980s. The work of Pillard and colleagues (Pillard, Poumadere, and Carretta 1982; Pillard and Weinrich 1986), and of myself with Whitam (1987), Whitam and Dannemiller (1987), and Whitam and Vacha (1988) began to reintroduce evidence supporting a major genetic component to sexual orientation.

Pillard and colleagues studied families and inquired of the sexual orientation of the male and female siblings in some 100 male and 86 female index families (Pillard, Poumadere, and Carretta 1982; Pillard and Weinrich 1986; Weinrich 1987). The index families were chosen for having a known heterosexual or homosexual adult. The brothers and sisters of these index individuals were interviewed or surveyed by mailed questionnaire. Their basic finding is that among brothers, if a family contained one index son who was homosexual, between 20 and 25% of the brothers would also be gay. If the index brother was heterosexual, the chance of other brothers being homosexual was only about 4 to 6%. They did not find any significant familial component to homosexuality among sisters. The data from cases where homosexual females were the index cases is still under review (Weinrich 1987). Eckert and colleagues (Eckert et al. 1986) reported similar findings even though they studied six pairs of monozygotic twins in which at least one member of each pair was homosexually active. They also found that the two male pairs were more or less concordant for homosexuality while the four female pairs were not.

It is interesting to also extract from the data offered by one of these study authors (Weinrich 1987). Weinrich reports that only 10 bisexuals were identified from the total study population of 178 men, 5 to 6% of the population. This is far fewer than would have been predicted from the Kinsey data but is in keeping with the later studies referred to in Table 1 and discussed above.

Some years ago, Frederick Whitam and I began to investigate the concordance of homosexuality and heterosexuality among twins. We reported briefly on our initial findings (Diamond and Whitam 1987a; Diamond, Whitam, and Dannemiller 1987b; Whitam, Diamond, and Vacha 1988) and our study is ongoing. By advertising in gay magazines and newspapers and by word of mouth, we sought twins of whom at least one was homosexual. We indicated we were interested in the twin set whether or not they were identical or fraternal and whether or not the second twin was also gay.

Among our initial respondents of 10 sets of monozygotic twin brothers we found 8 sets concordant for homosexuality. Of the 10 index twins where one rated a K=5 or K=6, 8 of the brothers also rated K=5 or K=6. Among 9 sets of dizygotic brothers, with most conservative analysis, we found 3 sets concordant. Now with 30 pairs of twins in our sample, 18 monozygotes and 12 dizygotes our findings remain similar. There is a 72% concordance for homosexuality among one-egg twins and 33-50% concordance for homosexuality among our two-egg twins. Considering all our twins together (Tables 2 and 3), regardless of zygosity, there is approximately 60% concordance (Diamond, Whitam, and Dannemiller, in preparation). Our data for monozygotic twins are not as strong as that reported by Kallman (1952), but both studies find strong concordance for homosexuality. On the other hand, our findings for dizygotic twins are stronger than his. Concordance among our two-egg twins is in keeping with the 20-25% concordance reported for nontwin brothers by Pillard and colleagues Willard, Poumadere, and Carretta 1982; Pillard and Weinrich 1986). This is appropriate since two-egg twins have gene sets similar to regular brothers.

Of particular interest in looking at bisexuality is our finding, similar to those by Kallman, that when the brothers were concordant for sexual orientation, they were usually so within one rating on the Kinsey scale. For instance, a brother with a rating of K=6 would often be found to have a brother also rating K=6 or K=5. In our findings, when concordance was not found, however, the nonconcordant brother was most often seen with behavior rating K=O. In other words, homosexual brothers either have equally homosexual or heterosexual brothers rather than bisexual ones. Brothers exhibiting K=5 or K=6 behavior (Table 3) rarely have a brother with a rating of K=2 to K=4 (Diamond, Whitam, and Dannemiller, in preparation).

Hirschfeld, early in our century, after reviewing his extensive records, reported: "Homosexuality runs in families in at least 35% of the cases." Homosexuality is congenital: "No outside influence, neither masturbation, nor impotence, nor yet disgust with the opposite sex, or seduction, or Binet's 'fortuitous shock,' can adequately explain the definite orientation of the homosexual impulse toward a certain sexual goal from the first awakening of the sexual impulse or from the first wet dreams."



A FULL DISCUSSION of transsexualism here would be warranted if space permitted.

Unfortunately this is not so. But a few comments are justified. I have personally and extensively interviewed more than 100 transsexuals over some thirty years in sex research. One of the more salient findings is that an individual's feeling about himself or herself, how he or she identifies, not as masculine or feminine, but as male or female, is independent of the sex of the erotic partner preferred. A transsexual believes he or she is of the opposite sex not because of the person's love or erotic object, but independent of it. Since sexual identity holds similarly so for heterosexuals and homosexuals as well, this should not be surprising. It reinforces the thesis that an individual's sexual preference or orientation is a separate level of sexuality and independent of his or her sexual or gender identity. One can be a transsexual who is heterosexually, homosexually, or ambisexually oriented. A more complete discussion of this is available (Diamond 1976/77, 1979, 1984, 1980). It might also be mentioned here that children brought up by transsexual parents do not develop as transsexuals (Green 1978).


Concluding Comments

SOME MATTERS SEEM simple. Among invertebrates, structural and behavioral bisexuality is quite common. And among fish and other lower vertebrates the combination is also not uncommon. For mammals, however, and humans in particular, the structural phenomenon is rare and the behavioral one a matter of definition.

Over the years certain terms have evolved in sexology to try to clarify such definitional problems. One speaks of technical virgins (they have done everything but ... ), of core gender identity (the inner voice which, independent of culture and rearing, says "I am a male" or "I am a female"), and also of primary or true bisexuality.

One can have sex with an erotically unarousing and unattractive partner for a nonsexual reason. We have seen this demonstrated among natives of New Guinea but people of all cultures exhibit sexual behaviors for nonerotic reasons in different circumstances and with different motives. One can of course accept that having sex without erotic interest in the sex of the partner may be another type of bisexuality. But the primary bisexual, or ambisexual, is one who is erotically aroused psychologically by both males and females, not necessarily one who will have sex with both males and females without being aroused by both.

Sometimes the distinction is made between primary and secondary homosexuality. The former are individuals oriented from the onset of their erotic inclinations, and the latter come to their homosexual interest after an initial and extensive heterosexual period or where the sexual behavior is serving nonerotic needs. Comparably, one might speak of primary or secondary bisexuality. In addition to "primary and secondary" terminology, "true" and "pseudo" or "false" are terms which are also used. In this vein it is possible to have true and pseudo or primary and secondary heterosexuality (feigned heterosexual activity by a primary homosexual) but that is rarely considered. Primary heterosexuality is most common, primary homosexuality less so, and primary bisexuality least common.

To be sure, both types of individuals (and more) exist. As Kinsey et al. (1948) have said: "the world is not to be divided into sheep and goats. Not all things are black nor all things white. . . nature rarely deals in discrete categories." The number or percentage of any group in the human population is not clearly known. The more recent evidence shows, however, that primary ambisexuality - sex with both males and females prompted by erotic arousal - is relatively rare. When it exists, while it is much less common than heterosexuality or homosexuality, it is nevertheless, no doubt, the result of the same biological biases which determine either distinct androphilic or gynephilic arousal.

Unlike those individuals in which bias to one arousal pattern predominates, however, the neural centers or programs organizing sexual object choice in the case of bisexuals have been sensitized to both. The more frequent displays of ambisexual behavior are, however, much more a response to social and interpersonal forces than manifest in genital interactions.

The strongest evidence of some biological underpinning to an individual's sexual object choice/partner preference comes from several areas. These were mentioned above: surveys which look for correlations and cause and effect, clinical findings, cross-cultural studies, physiological response research, transsexual cases, family genealogies, and twin studies.6,7 The genetic studies are firmly believed to be the most convincing, and along with the papers already mentioned many others exist (Puterbaugh 1990).8,9

Having a biological underpinning doesn't mean these behaviors cannot be shaped by experience and learning. Studies from many disciplines make it clear that bisexual, homosexual, and heterosexual behaviors can be modified to conform to individual situations, social stereotypes, and mores. On the other hand, many individuals manifest ambisexual and homosexual activities against social dictates to the extent they might even put their social life if not their actual physical life in jeopardy. These behaviors, as with heterosexual ones, are often expressed as compulsive and self-generating and arising from within.

Sexual orientation, as are all other evolutionarily crucial behaviors such as mate selection, is a biologically organized phenomenon with the capacity for social modification and learning. A recent study (Ernulf et al. 1989) has found that a majority of self-identified homosexuals, as do heterosexuals, think their orientation is inherent and constitutional rather than learned or imposed. And they prefer to think of matters this way.

But so saying does not yet mean we have all the answers to the questions of sexual orientation or partner preference. It only means we have a better idea of where we might be more productive in looking for answers. And we must recognize that dealing with humans means we can expect some of the more complicated scenarios that nature can provide.

In ways we do not yet understand, not all sibs of homosexual index individuals show homosexuality even among monozygotic twins. There is obviously more than one set of genes involved. And they apparently interact with genes of other traits and social forces to organize how the final behaviors will be manifest. And why, among monozygotic twin brothers who are not concordant for homosexuality, do only a minority show bisexual behavior whilst the majority are heterosexual (K=O) instead? Intuitively one might expect a higher ratio of bisexuals to heterosexuals. Indeed, it may be that bisexuality is related to homosexuality and heterosexuality but quite different in its developmental pattern. And equally fascinating for us to know is how and why some social situations at some times seem capable of modifying these inherent biases yet seem ineffectual at other times and situations.

As first proposed some twenty-five years ago (Diamond 1965) and many times since, the available evidence indicates manifest sexual orientation is most probably the result of interacting inherent biological forces meshing with environmental and social pressures. The biology sets a predisposition, a bias, with which the individual interacts with his or her surroundings (Diamond 1965, 1976/77, 1978, 1979, 1980, 1982, 1984, 1987a, 1987b; Diamond and Karlen 1980). And particular stages or "critical periods" in development seem more significant than others in organizing these behaviors. Others, even though they once thought otherwise, e.g., Money (1963), are now coming to these same conclusions (Money 1988).

The biased predisposition is believed to lead to a cognitive frame or psychic symbolism which provides an internal standard against which possible behavior choices will be compared. This will hold for considerations of both sexual identity and partner preferences.10 In identity, the only categories conceivable are male and female, but in object/partner choice a spectrum is realizable (Diamond 1976/77, 1979). This spectrum not only can encompass degrees of gynephilia or androphilia, which translate into homosexuality, bisexuality, and heterosexuality, but other attributes as well. Genetics underpins not only fixedness but flexibility as well. Genetic flexibility is no doubt related to the nonconcordance seen in female sibs of index homosexuals.

The internally imposed fixedness may or may not be in keeping with social prejudices. All individuals, going through life, are constantly forced to temper internal desires with external forces. Legal restrictions and social taboos or other motives can move an individual to exhibit behaviors and form sexual and emotional attachments to partners he or she would not otherwise desire, or refrain from relationships that would be preferred. But given a free pick and opportunity, an individual's choice will be manifest (Diamond 1978, 1979). Our biological heritage has given most humans the flexibility to adjust.

It is common in biological studies of behavior and structure to ask how this is adaptive for the species. What are the evolutionary origins or consequences of this feature? This is not unlike a sociologist asking what are the societal origins or consequences for any particular social practice. And if homosexuality or bisexuality has a biological basis, how has it evolved and is it adaptive? Many have, indeed, specifically considered evolutionary aspects of homosexuality from a biological perspective, e.g., Hutchinson (1959), Kirsch and Rodman (1982), Trivers (1971, 1974), and Wilson (1975). Their conclusions are divergent and speculative, and basically run to an advantage by balanced polymorphism where a combination of alleles for homosexuality, and related to homosexuality, provide an increased fitness (Hutchinson 1959) to a theory that clashes between parent and child may make it advantageous for the parents' ability to reproduce and transmit their genes if the minoring does not (Trivers 1974). While interesting to consider, anything said here is obviously speculative.

I would suggest, however, that humans, while having biases in their erotic preferences, are immensely flexible, and that bisexuality and homosexuality represent adaptive mechanisms for satisfying erotic and nonerotic needs and relieving sexual tension in a manner different from but analogous to heterosexuality. And nonheterosexual activities persist since they are no threat to the survival of the species. There are sufficient numbers of humans reproducing with many bisexuals and homosexuals among them.

A last word: For some persons the idea that sexual orientation is biologically biased toward heterosexuality or homosexuality and predisposed is threatening (see Ernulf et al. 1989). Some have expressed consternation that if the developmental biological forces for sexual orientation are known (or even suspected), some governmental, religious, medical, or social agency might use the knowledge to force conformity to a dictated ideal or otherwise modify a potential homosexual outcome, e.g., De Cecco (1987, 1990), Gagnon (1987), Schmidt (1984), Sigusch et al. (1982); see also Dörner (1983). Actually, the argument can more forcefully be made that knowing the social construction of sexual orientation might more easily lead to such a coercion and social manipulation. Even believing that it is social construction that leads to such is probably more dangerous since even despots know it is easier to modify the social arena than the biological. Unfortunately, groups who have the end of homosexuality or ambisexuality as their goal, I fear, need no scientific justification for their malignant ends. They are sufficient in their own ignorance and need no factual information to further their aims. Ignorance and prejudice, more than knowledge and acceptance, will work against humanity. Indeed, the truth is more likely than not to benefit us all.



1. This paper was scheduled as a keynote address as a memorial to Magnus Hirschfeld (1868-1935).

2. The paper, given on July 14, 1990, is dedicated to my daughter Leah Naomi Diamond and her husband, Gerd Harnisch, on the occasion of their wedding in Berlin on July 6, 1990, and to my wife, Grace Hope, deceased in 1989, whose birthday was July 15.

3. The term sex is used when referring to the biological characteristic of male or female. The term gender is only appropriate in a social context for humans. In this vein, a male living as a woman would have male sex and female gender.

4. The terms malelike and femalelike are not arbitrary evaluations. In the rat, as in all nonhuman mammalian species of which we know, the two sexes are quite distinct in their mating patterns. Under normal situations males rarely lordose and females rarely mount.

5. The research by Humphreys (1970) is probably the best-known study documenting that many married men engage in homosexual activities while maintaining heterosexual lifestyles and identities.

6. I have avoided dealing with much of the endocrinological evidence supporting a biological basis of sexual orientation. This is due both to lack of space and also to the controversies and unsettled issues which surround such studies. I think the bulk of evidence does indicate inherent male-female differences in the sensitivity of the CNS to hormonal stimuli, and think that these responses are linked to heterosexual and homosexual indexes. The interested reader, however, can obtain both sides of the argument from the current work of Dörner (1988), Gladue (1990), Gooren et al. (1990), and Meyer-Bahlburg (1984).

7. I must repeat that the evidence for most of these conclusions is derived mainly from studies of males. This is an unfortunate reflection of the present state of research. There is some evidence that the situation in females is different (Pillard and Weinrich 1986; Eckert et al., 1986; Weinrich 1987). More ongoing research on females is needed. Whitam and I are presently replicating our study of male twins with females.

8. The importance of genetics also holds true for an individual's view of himself or herself as male or female, e.g., his or her sexual identity (Diamond 1965, 1977, 1978, 1979, 1982).

9. Several months after the presentation of this paper, a publication appeared (Gooren et al. 1990) with the title "Biological Determinants of Sexual Orientation." Several of the points covered are dealt with in the present paper with differing interpretations. The author did not at all touch on genetic evidence, which is the strongest argument for a biological bias to orientation.

10. I have previously proposed that an individual's sexual profile has five basic levels. Sexual identity, the core sense of being male or female, and sexual object choice/ partnerpreference are two of the five. The other three are reproduction, patterns, and mechanisms (Diamond 1976, 1979, 1980, 1984). These are usually in concert but can assort independently



Abel, G. G., D. H. Barlow, E. B. Blanchard, and M. Mavissakalian. 1975.

Measurement of sexual arousal in male homosexuals: the effects of instructions and stimulus modality. Arch. Sex. Behav. 4: 623-29.

Armstrong, C. N. 1964. Intersexuality in man. In Intersexuality in Vertebrates Including Man, ed. C. N. Armstrong and A. J. Marshall. 349-93. London: Academic Press.

Baum, M. J. 1979. Differentiation of coital behavior in mammals - a comparative analysis. Neurosci. Biobeh. Rev. 3: 265-84.

Bell, Alan P, and Martin S. Weinberg. 1978. Homosexualities - A study of Diversity among Men and Women. New York. Simon and Schuster.

Bell, Alan R, Martin S. Weinberg and Sue K. Hammersmith. 1981. Sexual Preference - Development in Men and Women Bloomington: Alfred C. Kinsey Institute of Sex Research.

Blumstein, Philip, and Pepper Schwartz. 1972.

_. 1977. Bisexuality: Some social psychological issues. J Soc. Issues. 33 (2): 30-45.

Booth, William. 1989. U.S. probe meets resistance. Science 244 (April 28): 419.

Brusca, Richard C., and Gary J. Brusca. 1990. Invertebrates. Sunderland, Mass.: Sinauer Assoc.

Conover, M. R., D. E. Miller, and G. L. Hunt Jr. 1979. Female-female pairs and other unusual reproductive associations in ring-billed and California gulls. Auk 96: 6-9.

Conrad, S. R., and J. P. Wincze. 1976. Orgasmic reconditioning: a controlled study of its effects upon the sexual arousal and behavior of adult male homosexuals. Beh. Ther. 7: 155-66.

de Waal, Frans. 1990. Sociosexual behavior used for tension regulation in all age and sex combinations among Bonobos. In Pedophilia - Biosocial Dimensions, ed. J. R. Feierman. 378-93. New York: Springer-Verlag.

De Cecco, John P. 1987. Homosexuality's brief recovery: pertaining to sex research. J.Sex Res. 23 (1): 106-29.

_. 1990. Homosexuality: nature, nurture, or homophobia? Third International Berlin conference of Sexology. 11 July. Humboldt University, Berlin.

Demski, Leo S. 1987. Diversity in reproductive patterns and behavior in teleost fishes. In Psychobiology of Reproductive Behavior: An Evolutionary Perspective, ed. D. Crew. 1-28. Englewood Cliffs, N. J.: Prentice Hall.

Diamond, Milton. 1965. A critical evaluation of the ontogeny of human sexual behavior. Quart. Rev. Biol. 40: 147-75.

_. 1968. Genetic-endocrine interaction and human psychosexuality. In Perspectives in Reproduction and Sexual Behavior, ed. M. Diamond. 417-43. Bloomington: Indiana University Press.

_ 1976/77. Human sexual development: Biological foundations for social development. In Human Sexuality in Four Perspectives, ed. E A. Beach. 22-61. Baltimore: The Johns Hopkins Press.

_.1978. Sexual identity and sex roles. Humanist (March/April) -. 16-19.

_.1979. Sexual identity and sex roles. In The Frontiers of Sex Research, ed. V. Bullough. 33-56. Buffalo, N.Y.: Prometheus.

_. 1980. The biosocial evolution of human sexuality. Behav. Brain Sci. 3 (2): 184-86.

_. 1982. Sexual identity, monozygotic twins reared in discordant sex roles and a BBC follow-up. Arch. Sex. Behav. 11 (2): 181-85.

_. 1984. Sexwatching: The World of Sexual Behavior. London: Macdonald Co. Ltd.

Diamond, M., and A. Karlen. 1980. Sexual Decisions. Boston: Little, Brown.

Diamond, M., Alfonso Llacuna, and Calvin Wong. 1973. Sex behavior after neonatal progesterone, testosterone, estrogen, or antiandrogens. Horm. Behav. 4 (1-2): 73-88.

Diamond, M., Roy Ohye, and Jesse Wells. In Preparation. AIDS KAP in Hawaii. AIDS Ed. Prev.

Diamond, M., and Frederick L. Whitam. 1987(a]. Sexuality of male twins when one or both are homosexual. Abstract, 8th World Congress for Sexology, June 17-21. Heidelberg.

Diamond, M., F. L. Whitam, and J. E. Dannemiller. 1987(b]. Homosexuality in male twins. Abstract, 13th Annual Meeting of the International Academy of Sex Research, June 21-25, 1987. Tutzing.

_.In Preparation. Homosexuality among male twins.

Dixon, B. W., E. J. Streiff, and A. H. Brunswasser. 1991. Pilot study of a household survey to determine HIV seproprevalence. MMWR 40 (1): 1-5.

Dörner, G. 1983. Letter to the editor. Arch. Sex. Behav. 12 (6): 577-82.

_.1988. Neuroendocrine response to estrogen and brain differentiation in heterosexuals, homosexuals, and transsexuals. Arch. Sex. Behav. 17: 57-75.

Eckert, Elke, Thomas Bouchard, Joseph Bohlen, and Leonard Heston. 1986. Homosexuality in monozygotic twins reared apart. Brit. J. Pychiat. 148: 421-25,

Ernulf, Kurt, Sune Innala, and Frederick Whitam. 1989. Biological explanation, psychological explanation, and tolerance of homosexuals: A cross-national analysis of beliefs and attitudes. Psych. Rep. 65: 1003-10.

Feder, H. H. 1981. Perinatat hormones and their role in the development of sexually dimorphic behaviors. In Neuroendocrinology of Reproduction, ed. N. T. AdIer. 127-58. New York: Plenum Press.

Fieshelson, L. 1975. Ecology and physiology of sex reversal in Anthias squarnipinnis (Peters), (Teleostei: Anthiidae). In Intersexuality in the Animal Kingdom, ed. R. Reinboth. 284-94. New York: Springer-Verlag.

Freund, Kurt. 1963. A laboratory method for diagnosing predominance of homo- and hetero-erotic interest in the male. Beh. Res. Ther. 1 (1): 85-93.

_.1974. Male Homosexuality: an Analysis of the Pattern. In Understanding Homosexuality, ed. J. A. Loraine. New York. Elsevier.

Freund, Kurt, Gerald Heasman I. G. Racansky, and Graham Glancy. 1984. Pedophilia and heterosexuality vs. homosexuality. J. Sex Marit. Ther. 10 (3): 193-200.

Freund, Kurt, Hal Scher, Sam Chan, and Mark Ben-Aron. 1982. Experimental analysis of pedophilia. Behav. Res. Ther. 20: 105-12.

Freund, Kurt, Robin Watsons, and Douglas Rienzo. 1989. Heterosexuality, homosexuality, and erotic age preference. J Sex Res. 26 (1): 107-17.

Gagnon, John H. 1987. Science and the politics of pathology. J. Sex Res. 23 (1): 120-23.

Gebhard, P H. 1972. Incidence of overt homosexuality in the United States and Western Europe. In NIMH Task Force on Homosexuality. Final Report and Background Papers, ed. J. M. Liningood. 22-29. Rockville, MD: National Institute of Mental Health.

Gladue, Brian. 1990. Hormones and neuroendocrine factors in atypical human sexual behavior. In Pedophilia - -Biosocial Dimensions, ed. J. R. Feierman. 274-98. New York: Springer-Verlag.

Gooren, Louis, Eric Fliers, and Keith Courtney. 1990. Biological determinants of sexual orientation. In Annual Review of Sex Research, ed. J. Bancroft. 175-96. Lake Mills, Iowa: The Society for the Scientific Study of Sex.

Green, Richard. 1978. Sexual identity of 37 children raised by homosexuals or transsexual parents. Psychiatry 135 (6): 692-97.

_. 1987. The "Sissy Bay Syndrome" and the Development of Homosexuality. New Haven and London: Yale University Press.

Green, Richard, and Robert J. Stoller. 1971. Two monozygotic (identical) twin pairs discordant for homosexuality. Arch. Sex Behav. 1 (4): 321-27.

Haeberle, Erwin. 1978. The Sex Atlas. New York: The Seabury Press.

Harlow, Harry. 1961. Sexual behavior in the rhesus monkey. Conference: Univ. of Calif., Berkeley,

Herdt, Gilbert H. 1981. Guardians of the Flute: Idioms of Masculinity. New York: McGraw Hill.

Hess, E. H. 1968. Pupillometric assessment. Res. Pychother. 3: 573-83.

Hess, E. H. and J. M. Polt. 1960. Pupil size as related to interest value of visual stimuli. Science 132: 349-50.

Hotvedt, Mary E., and Jane Barclay Mandel. 1982. Children of lesbian mothers, In Homosexuality.- Social, Psychological and Biological Issues, ed. W. Paul, J. D. Weinrich, C. Gonsiorek, and M. E. Hotvedt. 275-85. Beverly Hills: Sage Publications.

Humphreys, R. L. 1970. Tearoom Trade: Impersonal Sex in Public Places. Chicago: Aldine.

Hunt, G. L., Jr., and M. W Hunt. 1977. Female-female pairing in western gulls. (Larus occidentalis) in Southern California. Science 196: 1466-67.

Hutchinson, G. E. 1959. A speculative consideration of certain possible forms of sexual selection in man. Amer. Nat. 93: 81-91.

Hyman, L. H. 1951. The Invertebrates. Platyhelminthes and Rhynchocoela, the Acoelemate Bilateria. New York: McGraw-Hill.

Imperato-McGinley, J., L. Guerrero, and T. Gautier. 1974. Steroid 5a-reductase deficiency in man: an inherited form of pseudohermaphroditism. Science 186: 1213-15.

Imperato-McGinley, J., and R. E. Peterson. 1976. Male pseudohermaphroditism: the complexities of male phenotypic development. Amer. J. Med. 61: 251-72.

Imperato-McGinley, J., R. E. Peterson, T. Gautier, and E. Sturia. 1979. Androgen and evolution of male-gender identity among male pseudohermaphrodites with 5a-reductase deficiency. New Eng. J. Med. 300: 1233-37.

Kallman, F. J. 1952a. Twin and sibship study of overt male homosexuality. Amer. J. Human Genet. 4: 136-46.

_. 1952b. Comparative twin study on the genetic aspects of male homosexuality. J. Nerv. Ment. Dis. 115: 283-98.

_. 1963. Genetic aspects of sex determination and sexual maturation potentials in man. In Determinants of Human Sexual Behavior, ed. G. Winokur. 5-18. Springfield: Charles C. Thomas.

Kinsey, Alfred, Wardell Pomeroy, and Clyde Martin. 1948. Sexual Behavior in the Human Male. Philadelphia and London: W. B. Saunders.

Kinsey, Alfred, Wardell Pomeroy, Clyde Martin, and Paul Gebhard. 1953. Sexual Behavior in the Human Female. Philadelphia and London: W. B. Saunders.

Kirsch, John A. W, and James Eric Rodman. 1982. Selection and sexuality: The Darwinian view of homosexuality. In Homosexuality.- Social Psychological and Biological Issues, ed. J. D. Weinrich, William Paul, John C. Gonsiorek, and Mary E. Hotvedt. 183-96. Beverly Hills: Sage Publications.

Klintworth, G. K. 1962. A pair of male monozygotic twins discordant for homosexuality. J. Nerv. Ment. Dis. 135: 113-16.

Lillie, F. R. 1917. The freemartin a study of the action of sex hormones in the foetal life of cattle. J. Exper. Zeal. 23: 371.

Lorenz, Conrad. 1967. Personal communication.

Mandel, Jane Barclay, and Mary E. Hotvedt. 1980. Lesbians as parents. Haisarts & Praktijk. 4: 31-34.

Mandel, Jane B., Mary E. Hotvedt, and Richard Green. 1979. The lesbian parents comparison of heterosexual and homosexual mothers and their children. Ann. Meet. Amer. Pychol. Assoc.

Mavisskalian, M., E. B. Blanchard, G. G. Abel, and D. H. Barlow. 1975. Responses to complex erotic stimuli in homosexual and heterosexual males. Brit. J. Psychiat. 126:252-57.

McConaghy, N. 1967. Penile volume changes to moving pictures of male and female nudes in heterosexual and homosexual males. Behav. Res. Ther. 5: 43-48.

_. 1978. Heterosexual experience, marital status, and orientation of homosexual males. Arch. Sex. Behav. 7: 575-81.

McEwen, B. S. 1983. Gonadal steroid influences on brain development and sexual differentiation. In Reproductive Physiology IV International Review of Physiology, ed. 1- 0. Greep. 99-145. Baltimore: University Park Press.

McWhirter, David P, and Andrew M. Mattison. 1984. The Male Couple: How Relationships Develop. Englewood Cliffs, N. J.: Prentice-Hall.

Meyer-Bahlburg, Heino F. L. 1984. Psychoendocrine research on sexual orientation - current status and future options. Prog. Brain. Res. 61: 367-90.

_. 1984. Psychoendocrine research and the societal status of homosexuals: a reply to De Cecco. J. Sex. Res. 23 (1): 114-20.

Money, John. 1963. Cytogenic and psychosexual incongruities with a note on space form blindness. Amer. J. Psychiat. 119: 820-27.

_. 1973. Prenatal hormones and postnatal socialization in gender identity differentiation. Neb. Symp. Motivation. 221-95.

_. 1988. Gay, Straight, and In-Between. New York: Oxford University Press.

Money, J., and A. Ehrhardt. 1972. Man and Woman, Boy and Girl. Baltimore: John Hopkins University Press.

Money, J. and P. Tucker. 1975. Sexual Signatures: On Being a Man or Woman. Boston: Little, Brown.

Najarian, H. H. 1976. Sex Lives of Animals Without Backbones. New York: Charles Scribner's Sons.

Newcomb, Michael D. 1985. The role of perceived relative parent personality in the development of heterosexuals, homosexuals, and transvestites. Arch. Sex. Behav, 14 (2): 147-64.

Overzier, C. 1963. Intersexuality, ed. C. Overzier. New York: Academic Press.

Phoenix, Charles H., R. W. Goy, A. A. Getall, and W. C. Young. 1959. Organizing action of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65: 369-82.

Phoenix, C. H., Robert W. Goy, and John A. Resko. 1968. Psychosexual Differentiation as a Function of Androgenic Stimulation. In Perspectives in Reproduction and Sexual Behavior, ed. M. Diamond. Bloomington: Indiana University Press.

Pillard, Richard, Jeanette Poumadere, and Ruth Carretta. 1982. A family study of sexual orientation. Arch. Sex. Behav. 11 (6): 511-20.

Pillard, Richard, and James Weinrich. 1986. Evidence of familial nature of male homosexuality. Arch. Gen. Pychiat. 43: 808-12.

Puterbaugh, Geoff. 1990. Twins and Homosexuality: A Casebook, ed. W. R. Dynes. Garland Gay and Lesbian Series. New York & London: Garland Publishing.

Ranier, J. D., A. Mesnikoff, L. C. Kolb, and A. Carr. 1960. Homosexuality and hererosexuality in identical twins. Psychosom. Med. 22: 250-59.

Ross, Robert, George Losey, and Milton Diamond. 1983. Sex change in a coral-reef fish: Dependence of stimulation and inhibition on relative size. Science 221: 574-75.

Rubin, Sylvia. 1983. How bisexuals face a hostile world. San Francisco Chronicle, 2 Sept. 1983.

Ryder, John P, and Patricia Lynn Somppi. 1979. Female-female pairing in ring-billed gulls. Auk 96: 1-5.

Sandfort, Theo, and G. Van Zessen. 199 1. Sex and AIDS in the Netherlands. In preparation.

Schein, M.W. 1991. Personal communication.


Schiefenhovel, Wulf 1990. Ritualized Adult-Male/Adolescent-Male Sexual Behavior in Melanesia: An Anthropological and Ethological Perspective. In Pedophilia - Biosocial Dimensions, ed. J. R. Feierman. 394-421. New York: Springer-Verlag.

Schlegel, W. S. 1962. Die konstitutionbiologischen Grundlagen der Homosexualität. Z. Menschl. Vererb. Konstitutionslehre 36: 341-64.

Schmidt, G. 1984. Allies and persecutors: Science and medicine in the homosexuality issue. J. Homosex. 10: 127-40.

Schover, Leslie R., and Soren Buus Jensen. 1988. Sexuality and Chronic Illness: A Comprehensive Approach. New York: Guilford Press.

Shapiro, D. Y 1979. Social behavior, group structure, and the control of sex reversal in hermaphroditic fish. Adv. Stud. Behav. 10: 43-102.

Siegelman, Marvin. 1981. Parental Backgrounds of homosexual and heterosexual men: a cross-national replication. Arch. Sex. Behav. 10 (6): 505-13.

Sigusch, V.,.E. Schorsch, M. Dannecker, and G. Schmidt. 1982. Official statement by the German Society for Sex Research (Deutsche Gesellschaft für Sexualforschung e.V.) on the research of Prof. Dr. Günter Dörner on the subject of homosexuality. Arch. Sex. Behav. 11 (5): 445-49.

Stoller, Robert, and Gilbert Herdt. 1985. Theories of origins of male homosexuality. Arch. Gen. Psych. 42 (4): 399-404.

Terman, L. M. 1948. Sexual behavior in the human male: some comments and criticisms. Psychol. Bull. 45: 443-59.

Thresher. 1984. Reproduction in Reef Fishes. Neptune City, N. J.- T.F.H. Publications.

Trivers, R. L. 1971. The evolution of reciprocal altruism. Quart. Rev. Biol. 46 (4): 35-57.

_. 1974. Parent-offspring conflict. Amer. Zool. 14: 249-64.

Weinberg, Martin S., and Colin J. Williams. 1974. Male Homosexuals - Their Problems and Adaptations. New York: Oxford University Press.

_. 1975. Gay baths and the social organization of impersonal sex. Soc. Prob. 23: 124-36.

Weinrich, James D. 1982. Is homosexuality biologically natural? In Homosexuality: Social, Psychological, and Biological Issues, ed. W, Paul, J. D. Weinrich. J. C. Gonsiorek, and M. E. Hotvedt. 197-208. Beverly Hills: Sage Publications.

_.1987. Sexual Landscapes: Why We Are What We Are, Why We Love Whom We Love. New York: Charles Scribners Sons.

Wellings, K., J. Field, A. M. Wadsworth, A. M. Johnson, R. M. Anderson, and S. A. Bradshaw. 1990. Sexual lifestyles under scrutiny. Nature 348 (22 November): 276-78.

Whalen, Richard E. 1968. Differentiation of the neural mechanisms which control gonadotropin secretion and sexual behavior. In Perspectives in Reproduction and Sexual Behavior, ed. M. Diamond. 303-40. Bloomington, Indiana: Indiana University Press.

Whitam, Frederick L., and R. M. Mathy, 1986. Male Homosexuality in Four Societies: Brazil, Guatemala, the Philippines, and the United States. New York: Praeger.

Whitam, Frederick L., M. Diamond, and T. Vacha. 1988. An Analysis of the Sexual Fantasies of Fraternal and Identical Male Homosexual Twins. November 10-13.

Wilson, E. 0. 1975. Sociobiology: The New Synthesis. Cambridge: Harvard University Press.

Yamagiwa, J. 1987. Intra- and intergroup interactions in an all-male group of Virunga mountain gorillas (Gorilla gorilla beringei). Primates 28: 1-30.

Zuckerman, M. 1971. Physiological measures of sexual arousal in the human male. Psych. Bull. 75: 297-329.