Parwy's Blog: How Hormones Shape Sexual Orientation & Behavior - Dr Andrew Huberman with Dr Marc Breedlove

Several biological factors and mechanisms significantly influence sexual orientation and behavior in humans and animals.

Finger Length Ratios (2D:4D): Men typically have a larger difference in length between their index and ring fingers compared to women. Studies show that lesbians, on average, have a more masculine digit ratio than straight women, suggesting they were exposed to higher levels of prenatal testosterone. However, this metric is only statistically significant across large populations and cannot predict an individual's sexual orientation.
Otoacoustic Emissions: Women generally produce more spontaneous popping sounds in their ears, known as otoacoustic emissions, than men. Research indicates that lesbians produce fewer of these emissions than straight women, providing further evidence of a connection to higher prenatal testosterone exposure.

Increased Probability: A highly consistent finding in human sexuality is that the more older brothers a male has, the higher his probability of being gay. Each older brother increases a male's odds of being gay by approximately one third.
Maternal Immune Hypothesis: This effect holds true even if the brothers are raised apart, indicating a biological rather than a social cause. Researchers theorize that a mother's immune system recognizes male-specific antigens during pregnancy and produces antibodies. With each subsequent male pregnancy, these antibodies cross the placenta and potentially alter the brain development of the fetus.

Hypothalamus Differences: Research demonstrated that a specific region in the hypothalamus, the sexually dimorphic nucleus of the preoptic area, is typically smaller in gay men than in straight men, closely resembling the size found in women.
Gay Rams: A small percentage of male sheep will exclusively mount other males and completely refuse to mount females, indicating a distinct sexual orientation. Dissections of these rams reveal differences in how their preoptic area processes testosterone compared to straight rams, mirroring the anatomical brain differences found in humans.
Aversive Pathways: The behavior of gay rams, which avoid mounting females even when isolated with them, suggests that sexual orientation involves not just an attraction to one sex, but a biological aversion to the other. This aversive pathway is proposed to play an important role in partner choice and sexual preferences.

Congenital Adrenal Hyperplasia (CAH): This condition causes a fetus to produce excessive androgens instead of necessary adrenal steroids. XX individuals with CAH are exposed to highly masculinizing testosterone levels prenatally, which can masculinize their genitalia, and these women report a statistically higher likelihood of identifying as lesbians as they age.
Androgen Insensitivity Syndrome (AIS): XY individuals with a dysfunctional androgen receptor gene produce testosterone but their bodies cannot respond to it. They are typically born with feminine external characteristics, are raised as girls, and overwhelmingly identify as straight women who are attracted to men.

Behavioral Feedback Loops: Hormone levels dictate behavior, but behavior and social environments also inversely influence hormones. For instance, winning competitive events increases testosterone, while losing decreases it.
Lifelong Plasticity: The human brain continues to grow at a rapid fetal rate well into early childhood, allowing for intense social learning. While the developing brain is highly plastic, adult brains also retain significant plasticity, and adult testosterone levels continue to actively impact mood, libido, and behavior.