By David N. Leff

The chief of surgery at a top U.S. teaching hospital, half a century ago, refused to admit women to his specialty on the grounds that on average, female brains were smaller than males'.

That archaic — but then widely accepted — stance vis-a-vis gender identity is now politically incorrect, big time. Yet the underlying rationale still stirs conflict when applied to race and sexual orientation.

As regards homosexuals, the current argument runs around the question: Are gays and lesbians born, or made by their environment? Like Mark Twain's weather, everybody talks about this nature-vs.-nurture issue, but nobody does anything about resolving it.

Until now.

A one-page research report in today's Nature, dated Oct. 23, 1997, describes a controlled experiment that examines the morphological changes in the mammalian brain brought about by sexual intercourse. Its author is psychologist S. Marc Breedlove, of the University of California, in Berkeley. He titled his brief account: "Sex on the brain."

Breedlove began by castrating 20 healthy, adult male rats. Then, to restore their ability to copulate, he implanted slow-release capsules of testosterone under their skin. Next, he supplied each animal with a female cage-mate, whose ovaries he had removed, but rendered receptive to sex by implanting capsules of the hormone, estradiol, which supports estrus, the female sex drive. Some of the males were paired with treated females; others with a female cohort that received no estradiol.

Male rats promptly and frequently mounted the receptive females, whereas those caged with untreated, hence non-permissive, partners, never copulated.

After four weeks of this controlled cohabitation, Breedlove removed the spinal cords from his male rats and stained them with a purple dye similar to that used in human post-mortems.

This delineated the motor neurons in the neuromuscular connection at the base of the spinal cord that controls penile erection and ejaculation. It shrinks after castration, Breedlove pointed out, unless testosterone is provided. These size changes also affect the synapses serving those neurons.

The 10 copulating males had much smaller somatic cells and nuclei, as well as lighter penile muscles, than the nine non-copulators.

"Somehow," Breedlove suggested, "the extensive sexual experience affected the morphology of these neurons. One explanation is that because smaller neurons are more active, and fire more frequently, they were being primed for further action. But it is possible," he added, "that the neurons shrank because they were overworked.

"Copulatory experience can therefore alter the size of neurons," Breedlove pointed out. He added: "Whether the sensory experience or motor activity of copulation induced these morphological changes, interpretations of correlations between human behavior and neural morphology must acknowledge that the two are reciprocally related.

"These findings give us proof," Breedlove said in an interview, "for what we theoretically know to be the case — that sexual experience can alter the structure of the brain, just as genes can alter it.

"They also throw new light," he pointed out, "on beliefs that sexual orientation, particularly homosexuality, is inherited or under the exclusive control of genes. It doesn't nullify the notion of a hereditary component in homosexuality," he continued, "but suggests that sexual orientation may also be affected by experience, as scientists have always believed."

He then observed, "You can't assume that because you find a structural difference in the brain, that it was caused by genes. You don't know how the difference got there."

Alluding to the known observation that a part of the brain's hypothalamus, involved in sexual libido and gratification, is smaller in women and in homosexual men than in heterosexual men, Breedlove concluded that "this difference might be inborn, or could be the result of experience." *

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