Sexual orientation (homo- vs. heterosexuality) is one of many sex differences observed in humans. Sex differences can result from differential postnatal experiences (interaction with parents, environment) or from biological factors (hormones and genes) acting pre- or postnatally. The first option is often favored to explain sexual orientation although it is supported by little experimental evidence. In contrast, many sexually differentiated behaviors are organized during early life by an irreversible action of sex steroids. In particular, the preference for a male or female sex partner is largely determined in rodents by embryonic exposure to sex steroids.
The early action of these steroids also seems to affect sexual orientation in humans. Indeed, clinical conditions associated with major endocrine changes during embryonic life often result in an increased incidence of homosexuality. Furthermore, multiple sexually differentiated behavioral, physiological, or even morphological traits that are known to be organized by prenatal steroids, at least in animals, are significantly different in homo- and heterosexual populations. Thus, prenatal endocrine (or genetic) factors seem to influence significantly human sexual orientation even if a large fraction of the variance remains unexplained to date. The possible interaction between biological factors acting prenatally and postnatal social influences remains to be investigated.
Table of Contents
Introduction and Definitions
Sex Differences in Animals and Men
The Origins of Human Sex Differences
How Can Effects of Biological Factors on Sexual Orientation Be Studied in Humans?
Endocrine Control of Sex Differences
Organizational Effects of Steroids on Sex Partner Preference in Animals
An Endocrine Model of Human Homosexuality
Sex Steroids Modulate Sexual Motivation during Adulthood in Humans . . . But Not Sexual Orientation
The Organizational Role of Steroids on Sexual Orientation: Analysis of Clinical Cases
Sex Differences Not Related to Sexual Activity Suggest an Atypical Differentiation in Homosexual Subjects
Brain Differences Associated with Sexual Orientation
The Origin of Endocrine Differences Between Embryos
Is There a Role for Postnatal Experiential Factors?
About the Author(s)Jacques Balthazart
, University of Liege, Belgium
Jacques Balthazart (University of Liege, Belgium) has worked in the field of avian endocrinology since he initiated his thesis work at Liege in the 1970s on the endocrine control of reproductive behavior and reproductive cycles in the Rouen Duck. In his early studies, he was among the group that pioneered the use of radioimmunoassay methods to measure gonadotropins and steroid hormones in the plasma of birds. After a post-doctoral stay at the Institute of Animal Behavior at Rutgers University in the USA, he returned to Liege and established a long-term research program on the neuroendocrine control and sexual differentiation of male-typical reproductive behavior in Japanese quail asking mainly how testosterone is so effective in activating male-typical behavior in one sex but not the other. Overall, he has published more than 400 journal articles and review papers. Nearly all of them are on birds and relate to the action of steroids on brain and behavior. He has organized many conferences on the topic of hormones, brain and behavior, and he has served on the organizing committee of a large number of other meetings. He has and continues to serve on the editorial boards of many journals. He is currently the co-editor of Frontiers in Neuroendocrinology.