Reconsidering Prenatal Hormonal Influences on Human Sexual Orientation: Lessons from Animal Research

Gender-affirming hormone treatment - A unique approach to study the effects of sex hormones on brain structure and function

Investigating the effects of the gender-affirming hormone treatment of transgender people using neuroimaging provides a unique opportunity to study the impact of high dosages of sex hormones on human brain structure and function. This line of research is of relevance from a basic neuroscientific as well as from a psychiatric viewpoint. Prevalence rates, etiopathology, and disease course of many psychiatric disorders exhibit sex differences which are linked to differences in sex hormone levels. Here, we review recent neuroimaging studies from others and our group that investigate the effects of gender-affirming hormone treatment in a longitudinal design utilizing structural and functional magnetic resonance imaging and positron emission tomography. Studies point to a general anabolic and anticatabolic effect of testosterone on grey and white matter structure, whereas estradiol and antiandrogen treatment seems to have partly opposite effects. Moreover, preliminary research indicates that gender-affirming hormone treatment influences serotonergic neurotransmission, a finding that is especially interesting for psychiatry. A clear picture of a hormonal influence on brain activity has yet to emerge. In conclusion, the available evidence reviewed here clearly indicates that sex hormone applications influence brain structure and function in the adult human brain.

Sexual and pairing partner preference in birds and other animals

Research on hormonal involvement in animals' preferences for mating or pairing with same- or other-sex partners has been among the topics featured in Hormones and Behavior over the years. In several species of non-pair-forming mammals, there is good evidence that the early sex steroid environment has an organizational effect on later sexual partner preference. Research with zebra finches, a pair-forming species, shows a likely early estrogenic organizational hormone effect on pairing partner preference, an effect that can also interact with the early social environment to determine adult pairing preference. Experiments with two amphibian and fish species suggest that activational hormone effects (effects of the hormone milieu in adulthood) may regulate sexual partner preference. As a complement to the research on hormonal involvement, a growing body of theoretical and empirical research addresses the possible adaptive functions of the same-sex sexual and pairing behavior observed in many wild animals. Such advances have important implications for conceptualizing neuroendocrine mechanisms of partner preferences.

Interest, Personality, and Sexual Traits That Distinguish Heterosexual, Bisexual, and Homosexual Individuals: Are There Two Dimensions That Underlie Variations in Sexual Orientation?

A diverse U.S. sample comprising 1437 men and 1474 women was assessed on sexual orientation, masculinity-femininity of occupational preferences (MF-Occ), self-ascribed masculinity-femininity (Self-MF), Big Five personality traits, sex drive, and sociosexuality (positive attitudes toward uncommitted sex). Discriminant analyses explored which traits best distinguished self-identified heterosexual, bisexual, and homosexual individuals within each sex. These analyses correctly classified the sexual orientation of 55% of men and 60% of women, which was substantially better than a chance rate (33%) of assigning participants to one of three groups. For men, MF-Occ and Self-MF distinguished heterosexual, bisexual, and gay men, with heterosexual men most gender typical, gay men most gender atypical, and bisexual men intermediate. Independently, higher sex drive, sociosexuality, and neuroticism and lower conscientiousness distinguished bisexual men from other groups. For women, gender-related interests and Self-MF distinguished lesbians from other groups, with lesbians most gender atypical. Independently, higher sociosexuality, sex drive, and Self-MF distinguished non-heterosexual from heterosexual women. These findings suggest that variations in self-reported sexual orientation may be conceptualized in terms of two broad underlying individual difference dimensions, which differ somewhat for men and women: one linked to gender typicality versus gender atypicality and the other linked to sex drive, sociosexuality, and various personality traits.

A Life History Approach to the Female Sexual Orientation Spectrum: Evolution, Development, Causal Mechanisms, and Health

Women's capacity for sexual fluidity is at least as interesting a phenomenon from the point of view of evolutionary biology and behavioral endocrinology as exclusively homosexual orientation. Evolutionary hypotheses for female nonheterosexuality have failed to fully account for the existence of these different categories of nonheterosexual women, while also overlooking broader data on the causal mechanisms, physiology, ontogeny, and phylogeny of female nonheterosexuality. We review the evolutionary-developmental origins of various phenotypes in the female sexual orientation spectrum using the synergistic approach of Tinbergen's four questions. We also present femme-specific and butch-specific hypotheses at proximate and ultimate levels of analysis. This review article indicates that various nonheterosexual female phenotypes emerge from and contribute to hormonally mediated fast life history strategies. Life history theory provides a biobehavioral explanatory framework for nonheterosexual women's masculinized body morphology, psychological dispositions, and their elevated likelihood of experiencing violence, substance use, obesity, teenage pregnancy, and lower general health. This pattern of life outcomes can create a feedback loop of environmental unpredictability and harshness which destabilizes intrauterine hormonal conditions in mothers, leading to a greater likelihood of fast life history strategies, global health problems, and nonheterosexual preferences in female offspring. We further explore the potential of female nonheterosexuality to function as an alloparental buffer that enables masculinizing alleles to execute their characteristic fast life history strategies as they appear in the female and the male phenotype. Synthesizing life history theory with the female sexual orientation spectrum enriches existing scientific knowledge on the evolutionary-developmental mechanisms of human sex differences.