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

In this discussion, Dr. Mark Breedlove, a professor of neuroscience at Michigan State University, explores the biological underpinnings of sexual orientation, emphasizing how hormones shape the developing brain. He highlights that the larger the number of older brothers a male has, the higher the probability he is gay, a robust finding in human sexuality. For a baby boy with no older brothers, the chance of being gay is about 2%, but this increases by a third with each additional older brother. This phenomenon, known as the fraternal birth order effect, is not socially mediated; rather, it appears to stem from a maternal immune response to male-specific antigens with each subsequent male pregnancy. The maternal immunization hypothesis suggests that a mother's immune system generates more antibodies to male-specific antigens with each male birth, and these antibodies can cross the placenta and affect the brain development of subsequent sons. Dr. Breedlove also delves into the relationship between prenatal testosterone exposure and sexual orientation, often using the 2D:4D digit ratio (the ratio of the length of the index finger to the ring finger) as a proxy for prenatal androgen exposure. Research indicates that this ratio tends to be smaller in men than in women, and this sex difference is present from birth, suggesting a prenatal hormonal influence. Studies have shown that lesbians, on average, tend to have more masculine digit ratios than heterosexual women, implying greater prenatal testosterone exposure. However, no significant difference in digit ratios has been found between gay and straight men, leading to the hypothesis that the difference lies not in the amount of prenatal testosterone, but in how their brains respond to it. He recounts the initial skepticism surrounding the idea that sexual orientation is not a choice, a view he has always held. He illustrates this with the example of having a first crush, which typically occurs before puberty and is not a conscious decision. He notes that early social influences, such as exposure to a largely heterosexual world, could explain why most people are straight, but this doesn't imply choice. Dr. Breedlove discusses pioneering work by Dennis McFaden on otoacoustic emissions and Simon LeVay on brain differences. McFaden's research revealed that lesbians have fewer otoacoustic emissions than straight women, a sex difference present at birth, suggesting higher prenatal testosterone exposure in lesbians. LeVay's highly controversial study found that a specific region in the hypothalamus, the preoptic area (specifically INAH3), was smaller in gay men, similar in size to that of women, and larger in straight men. While this finding was replicated, the direction of causation (whether a smaller INAH3 leads to being gay or vice versa) remains unclear due to the nature of post-mortem studies. The conversation also touches on the concept of an "aversive pathway" in sexual partner choice, particularly in males. Dr. Breedlove introduces the example of "gay rams," a small percentage of male sheep that exclusively mount other males and never females, even when receptive females are available. This behavior, coupled with observed differences in their preoptic areas compared to straight rams, suggests an active aversion to female partners, not just a preference for males. This "push-pull" dynamic, where attraction to one sex is accompanied by an aversion to the other, is a significant insight that could explain observed sex differences in sexual fluidity, with women generally exhibiting more plasticity in their sexual orientation than men. The discussion further explores conditions like Congenital Adrenal Hyperplasia (CAH), where XX individuals are exposed to higher prenatal testosterone due to an adrenal gland malfunction. While most women with CAH are heterosexual, they are more likely to be lesbian than the general population, and this likelihood increases with age, supporting the role of prenatal testosterone in biasing sexual orientation. Conversely, Androgen Insensitivity Syndrome (AIS), where XY individuals are genetically male but their bodies cannot respond to testosterone, results in individuals developing as phenotypic females who are overwhelmingly attracted to men, highlighting the complex interplay of genetics, hormones, and social rearing. Dr. Breedlove emphasizes the brain's lifelong plasticity, including the hypothalamus, which is traditionally considered hardwired. He notes that behavior can influence hormone levels, as seen in competition winners having higher testosterone. However, he stresses the difficulty in disentangling cause and effect in human behavior due to immense variability. He also shares a personal anecdote about his journey from a working-class background in the Ozarks to Yale, underscoring the role of luck and seizing opportunity in his scientific career. In concluding, Dr. Breedlove reinforces that there isn't a single cause for human behaviors like sexual orientation, but rather multiple developmental pathways. He acknowledges the persistent societal belief that certain activities can bias sexual orientation, but points out that data on social correlates are weak. While culture and family certainly influence the expression of masculine behaviors, fundamental sex differences like rough-and-tumble play are deeply rooted in biology.