In order to understand the complexity of human sexual behavior and its development from infancy to old age, researchers have used a variety of models or paradigms (gr. paradeigma: pattern). For example, they have compared its course to a cycle or interpreted it as a rising and falling curve. Some have also seen it as the unfolding and merging of several partial drives or as the manifestation of a single, comprehensive drive. More recently, one has explained it as the acting out of a script or of a combination of scripts that are being provided by society and internalized by the individual.
Such models are useful as intellectual tools that help us to understand reality; but they are not reality itself. Models are created and used for the purpose of making comparisons that might help to explain what is otherwise incomprehensible. By the same token, however, the models are abandoned when they no longer serve this purpose. Once they are revealed as inadequate, they are replaced with other models that offer a better “fit” and thus a deeper level of understanding. In science, this is known as a paradigm shift. Such shifts always have been, and still are, characteristic of scientific progress.¹
In this course, we provide a description of human sexual development in the full awareness that it may not be definite. In fact, in the future, it is likely to be amended, corrected or replaced by other descriptions following new and very different models. All we can do here is to summarize the sexological consensus as it exists today.

(1) For a detailed discussion, see the book by Thomas S. Kuhn (1922 - 1996), The Structure of Scientific Revolutions, 1962