While the study didn?t directly address sexual orientation, Breedlove says it may indicate that sexual experience plays an important role in developing a homosexual or heterosexual identity.

?The study demonstrated that sexual experience can alter neuronal structure,? said Breedlove. ?Therefore, differences in neuronal structure between straight and gay men, or even between men and women, could as easily be the result of their different sexual experiences as of any genetic or prenatal influence.? In the study, published in Nature, (Vol. 389, No. 6653, p. 801) Breedlove castrated adult male rats and dosed them with androgen to maintain their interest in sexual intercourse with females. They all received the same dose of androgen so that it could not explain variance in the results.

Breedlove placed 10 of the male rats in cages with females who?d been given extra estrogen to boost their sexual interest. He put the other nine male rats in cages with females who weren?t interested in mating because they?d had their ovaries removed and received no estrogen.

He allowed the rats to mate?or not mate?for 27 days. Afterward he examined the male rats? spinal-cord motor neurons, which mediate erection and ejaculation. He measured the size of neurons in both sexually active and inactive male rats.

Neurons were considerably larger in the sex-deprived rats, a size difference that mystifies Breedlove. Frequent sexual activity may have shrunk rats? neurons because they fired more often or were overworked, he speculates.