A NEW Northwestern University study of twins and adoptive brothers provides some of the strongest suggestions to date that sexual orientation is determined in large part by genetic factors.
Interviewing 56 identical male twins in which at least one twin was a homosexual, researchers found that in 52 percent of the cases, the other twin also was gay.
By contrast, such pairings were seen in 22 percent of 54 fraternal twins and 11 percent of 57 adoptive brothers.
Co-author J. Michael Bailey said the study, released yesterday, "suggests a genetic contribution to sexual orientation," since brothers with the most identical genetic material were most likely to be gay.
"This is the first study of this sort to be done in a carefully controlled way," said Dean Hamer, molecular biologist at the National Cancer Institute. Hamer is trying to determine whether there are genes that determine sexual orientation, and if so, trying to find them.
"Bailey is an emerging figure and one of only a few doing population genetics regarding sexual orientation," Hamer said. "I think his conclusion that there is probably a genetic component to homosexuality is undoubtedly right."
The debate over the relative roles of biology and environment in determining whether people are attracted to members of the same or the opposite sex has sparked controversy among scientists, politicians, religious leaders and gay activists for decades.
The debate received a push in the direction of biology in August when neuroscientist Simon LeVay reported that a part of the brain responsible for the male sex drive was smaller in homosexual men than in heterosexual men.
It was the second report of a difference in tiny brain structures. In 1990, a Netherlands researcher found that the part of the brain that governs daily rhythms is twice as large in homosexual men as in heterosexual men.
Bailey, a Northwestern assistant professor of psychology who conducted the twin study with Dr. Richard Pillard of the Boston University School of Medicine, said his study and LeVay's, while taking different approaches to the question, suggest the same answer.
"Both studies point to the likelihood that sexual orientation in men is an innate type of condition, that it develops early and that it is not socially influenced," Bailey said.
If that is true, he said, "One implication is that sexual orientation is not something people are easily converted into or out of. Straight people have no need to worry that homosexuality is contagious."
Conversely, he said, it is unlikely that gay people can choose to become straight.
Bailey and LeVay, who is at the Salk Institute for Biological Studies in La Jolla, Calif., have said they are conducting similar research on women.
Hamer, who stressed that his research was in the planning stages, said it was "intuitively obvious" that sexual orientation is genetically determined and that for most people, that genetic bent is toward members of the opposite sex.
"For our species to survive, there must be a genetic basis for heterosexuality," Hamer said. "Studies of other behaviors have shown that if something is absolutely essential for survival, like mating or eating, it is hard-wired and genetically determined. It isn't left to chance or environment."
Bailey said the next step was to "trace the pathway from the genes to the behavior. All we have said is that there are genes that affect this behavior, but we don't know anything about them."
The genes responsible for sexual orientation most likely exert their influence by determining whether the part of the brain that determines sex drive will be feminized and cause an attraction to men or masculinized and cause an attraction to women.
"One working hypothesis is that testosterone causes the part of brain that LeVay studied to differentiate into male or female," Bailey said.
LeVay studied the third interstitial nucleus of the anterior hypothalamus. In monkeys, if that part of the brain is destroyed, males no longer mate with females but continue other sexual behavior.
Gay and lesbian activists reacted with caution to Bailey's work, as they did to LeVay's.
"A study like this, if used ethically, not only sheds light on human sexuality but reinforces what many in the lesbian and gay community have said for years: that homosexuality is not a choice," said Ivy Young, director of the families project at the National Gay and Lesbian Task Force. "However, if it is used unethically, for repression and to 'cure' homosexuality, then that is regressive."