Based on Chapters Eight and Nine of Gay, Straight, and the Reason Why: The Science of Sexual Orientation by Simon LeVay
Chapter Eight: The Brain
All mental characteristics have some representation in the brain; sexual orientation should be no different. As mentioned in the Introduction, a region in the brain known to regulate male-typical sexual behavior –INAH3– was reported to be significantly smaller in gay men than in straight men. Other regions of the brain have been found to differ between gay and straight people.
A study was done on the brains of lesbians and straight women. The study found that lesbians had significantly less gray matter than straight women, especially in a site known to control processes that differ between men and women. In another study, gay men in a sample were found to have female-typical brain volumes and connections, while lesbians were found to have male-typical volumes and connections. The connections were found in the brain region known as the amygdala which, among other functions, controls processing of emotion and is involved in regulation of sexual functions.
Observations on human and non-human subjects suggest that male heterosexuality is not simply an attraction to the opposite sex, but active suppression of attraction to the same sex. Damage to or removal of the analogous region of INAH3 in male rats causes a change in their usual preference for female sex partners, to a preference for male partners. Homosexual attraction and/or behavior can occur in previously heterosexual men with Klüver-Bucy syndrome, caused by damage to the amygdala; the major source of input to INAH3 and nearby brain regions involved in sexuality. In addition, homosexual attraction can appear in a minority of men who are castrated for prostate cancer. These changes suggest that there are biological mechanisms for setting up sexual attraction to one sex that depends primarily on preventing attraction to the other sex.
Chapter Nine: The Body
Several studies done by researchers in Canada found that gay men are slightly shorter and lighter in weight, on average, than straight men. Lesbians, however, are slightly taller and heavier than straight women.
Due to the action of estrogen and testosterone before and after puberty, men tend to have longer limbs (arms and legs) and longer trunks than women. A large study found that gay men had significantly shorter limbs than straight men and lesbians had longer limbs than straight women. This suggests that gay men had less exposure than straight men to male sex hormones during development and lesbians had more exposure than straight women. This theory is supported by animal studies where female rats who are given testosterone while young have longer limbs than those who weren’t.
In the 20th century, biologist Alfred Kinsey and his colleagues conducted several “sex surveys”. Although they never parsed the data, their data on male penis size was later analyzed by separate researchers in the 21st century. The analysis found that gay men reported an average length that was one-third of an inch longer than those of straight men. Similar differences were found in circumference.
“Gaydar” –the ability to spot gay people without explicit information– has been confirmed to work by several studies, although it isn’t always correct. In studies, observers determined with better than 50 percent accuracy whether a person was gay or not. Their judgments were based primarily on gait (walking style), voice, and/or body shape. Gaydar appears to involve detection of mismatched gendered traits. The gaydar accuracy of gay and straight people is about the same.
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 Ten and Eleven — The Older-Brother Effect; Conclusions