Brains, Genes, and Sexual Orientation in Men

Dipping my toes into controversial waters yet again...

One of the earliest studies is described in Science
News, August 31, 1991 :

A comparison of 41 autopsied brains has revealed a distinct difference
between homosexual and heterosexual men in the brain region that controls
sexual behaviour. 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.

Analysis presumed heterosexual... and only one female in the control group... sorry, indicative perhaps, but not ready for prime time.

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

Maybe. Certainly worth investigating in light of later data.

From Brightsurf, this year :

November 08, 2007 - Is sexual orientation something people are born with - like the colour of their skin and eyes - or a matter of choice"

Canadian scientists have uncovered new evidence which shows genetics has a role to play in determining whether an individual is homosexual or heterosexual.

The research was conducted by Dr. Sandra Witelson, a neuroscientist in the Michael G. DeGroote School of Medicine at McMaster University, and colleagues at Sunnybrook Health Sciences Centre in Toronto who studied the brains of healthy, right-handed, 18- to 35-year-old homosexual and heterosexual men using structural Magnetic Resonance Imaging (MRI).

About 10 years ago, Witelson and Dr. Cheryl McCormick, then a student of Witelson's, demonstrated there is a higher proportion of left-handers in the homosexual population than in the general population - a result replicated in subsequent studies which is now accepted as fact.

Handedness is a sign of how the brain is organized to represent different aspects of intelligence. Language, for example, is usually on the left - music on the right.

In other research, Witelson and research associate Debra Kigar, had found that left-handers have a larger region of the posterior corpus callosum - the thick band of nerve fibres connecting the two hemispheres of the brain - than right handers.

This raised the hypothesis for the current study - whether the anatomy of the brain of the sub-group of right-handed homosexual men is similar to that of left-handers.

They found that the posterior part of the corpus callosum is larger in homosexual than heterosexual men.

The size of the corpus callosum is largely inherited suggesting a genetic factor in sexual orientation, said Witelson "Our results do not mean that heredity is destiny but they do indicate that environment is not the only player in the field," she said.
...
The researchers also undertook a correlational analysis which included size of the corpus callosum, and test scores scores on language, visual spatial and finger dexterity tests. "By using all these variables, we were able to predict sexual orientation in 95 per cent of the cases," she said.

Note that it's not the Corpus Callosum alone, but a whole range of symoptoms indicative of brain structure. But 95%? Yes, Good Enough for me. And yet another case where MRI is proving an tool far more powerful than all of the strictly psychological tests that have been used in the past, put together.