This Stanford University lecture explores human sexual behavior within a broader comparative framework of animal behavior. The lecture aims to explain sexual behavior by examining its proximate (immediate) and distal (evolutionary) causes, moving from observable behaviors to underlying neurobiology, endocrinology, and genetics.
The lecture touches on sexual orientation and gender identity primarily through the lens of neurobiological differences. Regarding sexual orientation, a landmark study by Simon LeVay found that a hypothalamic nucleus (INAH) was, on average, about half the size in gay men compared to heterosexual men, similar in size to heterosexual women. This finding, while debated due to potential confounds (e.g., subjects dying from AIDS), has been replicated and suggests a biological basis for sexual orientation. Other studies show correlations between sexual orientation and finger length ratios and autoacoustic reflexes, hinting at prenatal hormonal influences. The neurobiology of sexual orientation in women is less studied but shows similar correlations with finger length ratios and autoacoustic reflexes. Concerning gender identity, research on transsexual individuals reveals differences in the size of the bed nucleus of the stria terminalis, mirroring the gender identity rather than the assigned sex at birth. This suggests a biological basis for gender identity distinct from hormonal influences acquired later in life. The lecture emphasizes that while these studies indicate biological influences, they do not fully explain the complexities of sexual orientation and gender identity.
The lecture offers several interwoven lessons on sexual orientation and gender identity, primarily focusing on the neurobiological evidence challenging purely social or psychological explanations. Here's an organization by theme and detailed explanations:
I. Biological Basis of Sexual Orientation:
Neuroanatomical Differences: The lecture highlights a key study showing a difference in the size of the INAH (Interstitial Nucleus of the Anterior Hypothalamus) in the hypothalamus. Gay men, on average, had a smaller INAH, similar in size to heterosexual women, while heterosexual men had a significantly larger INAH. This suggests a potential neuroanatomical correlate of sexual orientation. However, the study's limitations are acknowledged, including the sample population (men who died of AIDS) and the possibility of confounds. Despite these limitations, the finding has been replicated, strengthening the argument for a biological component.
Prenatal Hormonal Influences: The lecture indirectly suggests a role for prenatal hormones in shaping sexual orientation. The correlations found between sexual orientation and seemingly unrelated traits like finger length ratios (2D:4D ratio) and autoacoustic reflexes hint at the influence of hormonal exposure during prenatal development. While not directly demonstrating causation, these correlations bolster the idea that biological factors play a role beyond solely genetic factors.
Limitations of Current Knowledge: The lecture emphasizes that while neurobiological differences are observed, these findings are not a complete explanation. The complexity of sexual orientation is acknowledged, and the current research only offers partial insights into the interplay of genetic, hormonal, and environmental influences.
II. Biological Basis of Gender Identity:
Brain Structure and Gender Identity: The lecture presents a study examining the bed nucleus of the stria terminalis (BNST). This brain region shows a significant size difference between men and women, with transsexual individuals exhibiting a BNST size consistent with their gender identity rather than their assigned sex at birth. This is crucial because it suggests that gender identity is not merely a social construct but has a neurobiological substrate that develops independent of later life hormonal treatments.
Robustness of Findings: The study on the BNST is highlighted for its robust methodology, including control groups of transsexual individuals who did not undergo gender-affirming surgery and individuals undergoing feminizing hormone treatments for testicular cancer. This strengthened the findings, showing the differences were not an effect of hormonal interventions.
"Wrong Body" Hypothesis: The BNST study supports the idea that some transgender individuals may have experienced a mismatch between their biological sex and their neurobiological gender identity—a "wrong body" perspective. This is a critical lesson presented as a biological challenge to traditional perspectives of gender identity.
III. Interplay of Biology and Social Factors:
Nature vs. Nurture: The lecture doesn't explicitly state it as a dichotomy but strongly implies that both biological factors (genetic, hormonal, and neurobiological) and social factors likely interact to shape both sexual orientation and gender identity.
Incomplete Understanding: The limitations of the existing research are highlighted repeatedly. More research is needed to fully elucidate the complex interplay of these factors and individual variation.
Caution Against Oversimplification: The lecture strongly cautions against simplistic interpretations of these neurobiological findings. These studies demonstrate correlations, and more research is necessary to understand causality and the full range of influences.
In summary, the lecture's main lessons challenge simplistic views of sexual orientation and gender identity by presenting compelling neurobiological evidence of their biological underpinnings. However, it emphasizes that the current understanding is partial, and future research needs to explore the complex interactions between biology and social factors.
