How Hormones Shape Sexual Orientation & Behavior | Dr. Marc Breedlove

Prenatal testosterone exposure significantly affects 2D:4D finger length ratios and is correlated with sexual orientation.

"the amount of testosterone that a fetus is exposed to while in the mother has a profound impact not only on the ratio of finger lengths, yes, you heard that right, but it also plays a meaningful role in sexual orientation. And in fact, there's a correlation there between finger length ratios and sexual orientation." (said at 0:00:30)

The speaker overstates the evidence by asserting that fetal testosterone exposure has a 'profound impact' on digit ratios and 'plays a meaningful role' in sexual orientation, alongside a definitive correlation between 2D:4D and sexual orientation. While 2D:4D is widely discussed as a putative indirect biomarker for prenatal androgen exposure, a 2024 meta-analysis of 54 studies (Kowal et al., PMID 37795916) found no consistent evidence connecting directly measured prenatal testosterone (e.g., in umbilical cord blood) to digit ratios, noting that conclusions assuming 2D:4D is a reliable proxy warrant great caution. Furthermore, while some meta-analyses report small statistical differences in 2D:4D by sexual orientation (e.g., Grimbos et al., 2010; Swift-Gallant et al., 2025), recent robust Bayesian meta-analyses accounting for publication bias (PMID 41861700) demonstrate that evidence for group differences in 2D:4D based on sexual orientation is weak and substantially diminished after bias adjustments.

• contradicts: Relationship between the 2D:4D and prenatal testosterone, adult level testosterone, and te… (American journal of biological anthropology 2024) · cited 21x in the literature

  "We found no evidence of the relationship between the above testosterone types and digit ratios. Furthermore, there was no relationship between testosterone and the right and left 2D:4D, male and female 2D:4D, and the 2D:4D and testosterone measurement (i.e., measured in blood or saliva)." (abstract, results, passage verified)

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• partial: Sexual orientation is associated with 2D:4D finger length ratios in both sexes: an updated… (Frontiers in psychology 2025) · cited 6x in the literature

  "Results indicate lower (more male-typical) digit ratios in homosexual women (right hand g = 0.26, left hand g = 0.16; both adjusted following trim-and-fill), and higher (more female-typical) ratios in homosexual men (right hand g = -0.17, left hand g = -0.20; both adjusted) compared to heterosexual same-sex counterparts." (abstract, results)

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• contradicts: Little evidence for group differences in 2D:4D ratios based on sexual orientation after ad… (Hormones and behavior 2026) · cited 1x in the literature

  "Our analyses showed little evidence for group differences in 2D:4D ratios by sexual orientation: RoBMA indicated moderate evidence against differences between homosexual and heterosexual women, with moderate to strong evidence of publication bias, and for men, moderate evidence against a right-hand difference but a small left-hand effect moderated by publication status." (abstract, results, passage verified)

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William Byne's group independently replicated the finding that the INAH3 nucleus is smaller in homosexual men compared to heterosexual men.

"So that was William Byne, who eventually—and there's an interesting aspect of it. It took him a long time to get a sample big enough, because, you know, Simon, it was such a horrible time in the AIDS epidemic... and it took longer for William Byne to gather the samples, but eventually did, and even though he was skeptical of it, he saw it, too." (said at 0:29:34)

William Byne and colleagues (2001) attempted to replicate Simon LeVay's 1991 findings regarding the size of the third interstitial nucleus of the anterior hypothalamus (INAH3) in relation to sexual orientation. While Byne's group did observe a trend toward smaller INAH3 volume in homosexual men compared to heterosexual men in the same directional pattern, the difference did not reach conventional statistical significance, and there was no significant difference in the total number of neurons within INAH3 based on sexual orientation. Describing this study as a straightforward replication that 'saw it, too' overstates a finding that was statistically non-significant.

• partial: The interstitial nuclei of the human anterior hypothalamus: an investigation of variation … (Hormones and behavior 2001) · cited 227x in the literature

  "Although there was a trend for INAH3 to occupy a smaller volume in homosexual men than in heterosexual men, there was no difference in the number of neurons within the nucleus based on sexual orientation." (abstract, results, passage verified)

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Studies show that testosterone levels rise in voters whose candidate wins a presidential election and fall in voters whose candidate loses.

"In elections, it's been shown that people whose candidate won the presidential election, their testosterone levels went up a little bit, and the people whose candidate lost went down a little." (said at 0:40:14)

A 2008 study examining voter salivary testosterone levels on election night (PMID 19844583, n=183) found that male voters supporting the winning candidate (Barack Obama) maintained stable testosterone levels across the evening (resisting the normal circadian decline), whereas male voters supporting losing candidates (John McCain and Robert Barr) experienced a decrease in testosterone levels. However, testosterone levels did not actually increase in winning voters, and no significant changes were observed in female voters.

• partial: Dominance, politics, and physiology: voters' testosterone changes on the night of the 2008… (PloS one 2009) · cited 137x in the literature

  "The results show that male Barack Obama voters (winners) had stable post-outcome testosterone levels, whereas testosterone levels dropped in male John McCain and Robert Barr voters (losers). There were no significant effects in female voters." (abstract, results, passage verified)

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