Based on Chapters Ten and Eleven of Gay, Straight, and the Reason Why: The Science of Sexual Orientation by Simon LeVay
Chapter Ten: The Older-Brother Effect
Gay men have more older brothers, on average, than do straight men. The reason for this is biological and not social in nature. It is theorized that the mother’s womb may build up certain factors with each male pregnancy, that predispose her sons to homosexuality.
With each older brother, the likelihood that a man will be gay is estimated to be 33 percent. This seems like a high percentage, but the rate of homosexuality among men with no older brothers is only 2 percent. Thus having one older brothers raises the likelihood of homosexuality to less than 3 percent.
The older-brother effect seem to only apply to right-handed men, according to several studies. This suggests that more than one mechanism may lead to homosexuality in men, and that they do so in a mutually antagonistic way.
Chapter Eleven: Conclusions
Sexual orientation is a part of a group of gendered traits. It results from the prenatal sexual differentiation of the brain, with the primary determinants being genes and sex hormones. Some characteristics of the bodies and minds of gay men are shifted in a female direction compared with straight men, and some traits of the bodies and minds of lesbians are shifted in a female direction compared with straight women.
Testosterone is the central hormone in determining whether the brain and body develop in a male or female fashion. If testosterone levels are high enough during a critical time before birth the brain is organized in a typically masculine manner — including a tendency to be attracted to women. If levels are low, the brain organizes in a female manner.
The estimated heritability of homosexuality ranges around 30-50 percent; similar to estimates for many other psychological traits. Few major genes have been identified that could be responsible for influencing homosexuality. It is likely that, like most heritable psychological traits, sexual orientation is influenced by multiple genes, each contributing a small effect. Mutual inhibition functions between brain centers that contribute to sexual orientation as shown by the occasional change in sexual orientation that results from brain damage or reduction of hormones.
Does the idea that sexual orientation is linked to gendered traits stigmatize gay people by reinforcing stereotypes? This is debatable, but the findings that gay and straight people tend to differ in gender-related traits is a valuable insight for understanding the origins of sexual orientation. The blend of gender-variant and gender-typical traits of gay people is what enables them to make their unique contributions to society. Acknowledging this should foster acceptance of gay people as they are, instead of encouraging them conform with straight majority.
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 Six and Seven — The Role of Sex Hormones; The Role of Genes
- Chapters Eight and Nine — The Brain; The Body