Study Finds Structural Differences in Brains of Straight, Homosexual Men

Researchers have found a new anatomical difference in the brain structure of homosexual men and heterosexual men, a discovery that supports a theory that sexual orientation may be set by nature.

UCLA School of Medicine scientists Laura Allen and Roger Gorski report that in autopsy studies they found a brain structure called the anterior commissure to be 34% larger in homosexual males than in straight males.

The finding, Allen said Friday, adds weight to the theory that sexual orientation is not a matter of choice, but a function of biological design or development.

“It’s very supportive of the concept that (sexual orientation) is not an environmental factor, not something acting early in childhood, but probably something acting (before birth) or slightly afterward,” she said.

Allen said early studies have shown that hormones active just before and after birth can determine the sexual behavior and brain structure of laboratory rats.

“In the rat,” Gorski said, “we have good evidence that brain is basically female. To develop characteristics that are typically male, the rat brain has to be exposed to specific hormones around the time of birth.”