Based on Chapters Six and Seven of Gay, Straight, and the Reason Why: The Science of Sexual Orientation by Simon LeVay
Chapter Six: The Role of Sex Hormones
There are no consistent differences in testosterone levels between gay and straight men, according to many studies. In contrast, up to one-third of lesbians (self-identified “butch”) may have higher testosterone levels in comparison to straight women. Markers of prenatal hormonal levels shows some differences that line up with sexual orientation.
Congenital Adrenal Hyperplasia (CAH) is a condition where a fetus is exposed to extremely high levels of male sex hormones. Women affected by CAH have been shown to be more masculine in mental and behavioral traits. Since homosexuality is linked to a variety of gender-atypical traits, it is unsurprising that studies have found CAH women to also be more homosexual, on average, than women not affected by CAH.
A similar, but opposite condition is androgen insensitivity syndrome (AIS), where the molecules that sense the presence of testosterone and other male hormones functions poorly or not at all. Genetic males with this condition grow up to look like women, identify as women, and most importantly, the majority of individuals with AIS are sexually attracted to men. This is consistent with the theory that sex hormones are a key determining factor in development of sexual orientation.
In genetically healthy individuals, similar findings have been presented. Sense of hearing differs between men and women and is affected by the greater prenatal exposure of men to testosterone and other male hormones. In studies, gay men have shown more female-like responses to sound and lesbians have shown more male-like responses. These findings are consistent with sexual orientation as an aspect of gendered traits.
Chapter Seven: The Role of Genes
Studies have found that family members of gay men and women are more likely to be gay themselves, even relatives who were brought up in the same environment, such as nieces, uncles, and cousins. The percentage of identical twins who are both gay has been found to be around 50 percent.
The results of twin and family studies have encouraged research to find genes that predispose to homosexuality. One study found that, among a sample of gay men, only maternal family members had increased rates of homosexuality. This suggests that “gay genes” may be on the X chromosome, as it is the only chromosome that men inherit only from their mother.
Gay men have more relatives, on average, than straight men. It’s possible that “gay genes” could be recessive, and two copies are needed for them to show their effect. This could explain why homosexuality hasn’t been eliminated by natural selection since gay people have few, if any, children of their own. The many relatives of gay persons could pass the genes to their children. It’s also possible that homosexuality is influenced not by “gay genes” but genes that predispose a person to more feminine or masculine traits. Such genes could be helpful in terms of reproducing if an individual receives only a few copies, but if they inherit several, it would result in homosexuality.
The latter theory is supported by a study which showed that greater femininity in straight men is associated with a larger number of female sexual partners, and greater masculinity in straight women is associated with a larger number of male sexual partners. In addition, heterosexual men and women with gay co-twins had more opposite sex partners, on average, than did heterosexual individuals without gay co-twins.
Gay, Straight, and the Reason Why Series:
- Introduction and Chapter One — What is Sexual Orientation?
- Chapters Two and Three — Why We Need Biology; Outline of a Theory
- Chapters Four and Five — Childhood; Characteristics of Gay and Straight Adults
- Chapters Eight and Nine — The Brain; The Body
- Chapters Ten and Eleven — The Older-Brother Effect; Conclusions