Unexplainable.Net

Sexual Preference Directly Linked to Brain Features

Last Updated on June 9, 2020 by admin

This is the original article which caused all the furor, from Science
News, August 31, 1991. Vol. 140, No. 9, page 140.

==========================================
BRAIN FEATURE LINKED TO SEXUAL ORIENTATION
==========================================

A comparison of 41 autopsied brains has revealed a distinct difference
between homosexual and heterosexual men in the brain region that controls
sexual behavior. The finding supports a theory that biological factors
underlie sexual orientation, although it remains unclear whether the
anatomical variation represents a cause or result of homosexuality, says
neurobiologist Simon Levay, who describes the study in the Aug. 30
_Science_.

LeVay, of the Salk Institute for Biological Studies in San Diego, found
that a particular cluster of cells in the forefront of the hypothalamus
was, on average, less than half as large in the brains of homosexual men
as in their heterosexual counterparts. Although scientists have yet to
identify the precise function of the clump, called the interstitial nuclei
of the anterior hypothalamus 3 (INAH 3), the hypothalamus is known as the
seat of the emotions and sexual drives.

LeVay obtained brain tissue from autopsies performed at seven hospitals in
New York and California. His study included 19 homosexual men, 16 men
presumed heterosexual, and six women presumed heterosexual. All of the
homosexual men died of AIDS, as did six of the heterosexual men and one of
the heterosexual women.

As a group, the heterosexual men had larger INAH 3 regions than either the
homosexual men or the heterosexual women, LeVay reports. The size
difference remained statistically significant whether or not the subjects
died of AIDS, ruling out the possibility that it resulted from the
disease, he says.

“This proves that you can study sexual orientation at the biological
level,” LeVay asserts. “There are differences in the brains of adult gay
and straight men.” However, he warns, “my data don’t say how the
difference arose.”

Previous investigations have turned up other contrasts. In 1984,
scientists at the State University of New York at Stony Brook confirmed a
German study showing that male homosexuals differ from heterosexual males
or females in their response to injections of the sex hormone estrogen
(_Science News_: 9/29/84, p.198). And last year, researchers at the
Netherlands Institute for Brain Research reported that homosexual men had
a larger suprachiasmatic nucleus than heterosexual men. The
suprachiasmatic nucleus — which plays a role in day-night rhythms — also
resides in the hypothalamus but has no known part in sexual behavior.

Psychologist Sandra F. Witelson at McMaster University in Hamilton,
Ontario, reported last year that lesbians show a higher incidence of
left-handedness than the general population. Witelson, who studies
handedness as a measure of brain organization (_Science News_: 8/17/85,
p.102), told _Science News_ she has now found a similar incidence in
homosexual men.

Together, the studies conducted to date “really show that there’s
something different in the [brain] anatomies of homosexuals and
heterosexuals,” she says.

Witelson and LeVay speculate that atypical levels of sex hormones may
shape the brains of homosexuals in the womb or during childhood. This
explanation does not rule out environmental influences, Witelson notes.
“A certain brain structure could be a predisposition to homosexual
behavior that requires a certain environment to be expressed,” she says.