For example, the study found that straight men and gay women are both wired for a greater "fight or flight" response than gay men or straight women, the team reports this week in the journal Proceedings of the National Academy of Sciences.
Also, homosexual men and straight women showed significantly more neural connections across the two brain hemispheres than heterosexual men did.
Symmetrical or Asymmetrical
The two sides of the brain also changed in symmetry depending on the person's sexual orientation.
To determine brain volume, Savic and colleagues also compared the left and right cerebral hemispheres using magnetic resonance imaging (MRI) scans and previous research data.
The right hemisphere was found to be slightly larger than the left in heterosexual males and lesbians, whereas those of gay men and straight women were symmetrical.
The results compliment previous research that found differing brain reactions in homo- and heterosexuals in response, for instance, to sexual images and certain pheromones, the authors said.
The latest findings imply that "human sexuality has neurobiological underpinnings," the mechanisms of which are complex and "require humbleness and restraint from quick judgments," Savic said.
Sandra Witelson, a neuroscientist at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada, wasn't involved in the new study.
"This was a study that was waiting to be done, because it leads so clearly from the current literature," she said.
Recent work by Witelson and colleagues indicated that the corpus callosum, a long fiber tract that connects the two brain hemispheres, was larger in gay men than in heterosexual men.
Brain features such as the corpus callosum and amygdalae develop very early, suggesting they are primarily genetically determined, she said.
(Related: "Sexual Orientation Is Genetic in Worms, Study Says" [October 25, 2007].)
The latest findings "make it very hard to argue that these differences are a product of learning or environmental influences."
But it doesn't necessarily follow that a person's sexuality is determined at birth, Witelson cautioned.
"Sexual orientation has a large genetic component, but that doesn't mean environment is a hundred percent irrelevant," she said.
"For example, heterosexual men in prison [may] engage in homosexual behavior, but that doesn't mean that they are homosexual because, given a choice, they choose the opposite sex."
The next big research question, Witelson said, is to determine whether these brain differences influence behavior.
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