BOSTON -- Is homosexuality, at least in part, the product of biological causes? Researchers got somewhat closer to answering that furiously controversial question last week with a new study suggesting that genes may play a dominant role in determining whether a woman becomes a lesbian.

The findings, which parallel results of an earlier study of gay males, bolster the tentative but increasingly plausible theory that human sexual orientation -- heterosexual as well as homosexual -- is influenced to a significant degree by biology and is not exclusively a matter of upbringing or "lifestyle choice."

The study of lesbian twins "is the largest ever of female sexual orientation, and it suggests that genetic factors are as important in determining lesbianism as they are in influencing male homosexuality," said psychologist J. Michael Bailey of Northwestern University.

Bailey conducted the study, published in the Archives of General Psychiatry, with Richard Pillard of the Boston University School of Medicine. The men also collaborated on a similar 1991 study of gay male twins.

By using ads in the gay media, Bailey and Pillard located 147 lesbians, including 115 who had twin sisters and 32 with adoptive sisters. Each pair had been raised together in the same homes. The twins were separated into two groups, identical and fraternal, and sexual histories were gathered.

Among the identical twins, who share the same genetic makeup, 48 percent of the gay or bisexual subjects had a gay twin. Among the fraternal twins, who share no more genetic inheritance than ordinary sisters, the rate was 16 percent; and among the genetically unrelated adoptive sisters it was 6 percent.

If homosexuality were purely a matter of choice or upbringing, Bailey said, then the rates for the three groups should be about the same. If homosexuality were purely a matter of biology, then the identical twins should have a nearly 100 percent correspondence.

Genes 'at Least Half the Story'

"Based on our study, it looks as if genes are at least half the story. I think it's substantial. Before this study we didn't know any of the causes," Bailey said. But, he added, "We can't say what those genes are or what they are doing."

That is hardly surprising in view of the extremely recent nature of most research in the field. Homosexual behavior is probably as old as humanity, but "homosexuality" is a comparatively new idea in human thought (the word did not enter the English language until 1897); and it has been only 20 years since the American Psychiatric Association officially removed it from the list of diseases.

Moreover, although many scientists have argued since the mid-1960s that there are distinctive differences in the brain anatomy of males and females, it was not until the late 1980s that researchers began looking for signs of similar "dimorphism" between gays and heterosexuals. In 1991, Salk Institute neuroscientist Simon LeVay published autopsy results showing that at least in one region of the brain called the hypothalamus the structures of gay and straight appear to be dramatically different.

Since then, a growing body of evidence -- still far from conclusive and lacking widespread confirmation -- has arisen in support of the thesis that predisposition to homosexuality is largely an inborn trait. That work was the subject of a March 7 symposium at Harvard Medical School convened at the behest of gay medical students.

In opening remarks, Marshall Forstein, head of the American Psychiatric Association's gay caucus, emphasized that the subject has important cultural ramifications, and urged the scientists -- many of whom are gay -- to insist on the most rigorous standards of proof, despite possible personal interest in the outcome.

Several researchers argued that if they could prove convincingly that a predisposition to homosexuality is inborn, many critics might soften their opposition. Homosexuality could be seen as a natural trait, comparable to eye color or height, rather than a willful choice or "sin."

On the other hand, some researchers warned, if homosexuality is seen as having a physical "cause," then some people might decide to seek a physical "cure." LeVay of the Salk Institute recalled efforts in Nazi Germany to "reverse" homosexuality surgically.

Nonetheless, work is proceeding on many fronts. Dean Hamer of the National Cancer Institute's Laboratory of Biochemistry said his research team is searching for a gene that can be said to regulate sexual orientation. Using a group of 400 individuals, Hamer has compiled extensive family histories stretching back several generations.

His data confirm earlier studies indicating that homosexuality appears to run in families. Hamer said the gay men in his study were much more likely than average to have gay brothers. In addition, he found that gay men were more likely to have gay uncles and cousins, but predominantly on the maternal side. "There does appear to be, in at least a fraction of the population, a transmissible, presumably genetic, factor," Hamer said.

The identity of such a presumptive genetic factor (or factors) remains unknown. Hamer's team has already tested a "candidate gene" -- one involved with the body's response to male hormones -- but the results were negative. Hamer said the search continues and hinted that published findings might be imminent.

At the Harvard meeting, Pillard reviewed the landmark study he and Bailey did among gay male twins. As in the lesbian study, the researchers advertised in the gay media for gay male subjects -- a method that critics charge, and the authors agree, makes a truly random sample impossible. They produced a pool of 167 gay men who had identical or fraternal twins or adoptive brothers.

Pillard said he found "concordance" rates -- cases in which both brothers were gay -- of 52 percent for identical twins, 22 percent for fraternal twins and 10 percent for adoptive brothers.

Research, Politics and Ambiguity

In Pillard's view, sexual orientation clearly has a genetic basis. But because even identical twins raised together diverge in their ultimate sexual identity, some other factor seems to be almost equally powerful.

LeVay told the Harvard symposium that he has been exploring anatomical differences in another region of the brain where the two hemispheres of the cereberal cortex are connected. There, LeVay said he has found structures that are, on average, larger among gays.

In reviewing the evidence to date, Harvard biology professor Evan Balaban expressed skepticism and urged caution. He said nearly all working biologists assume that genes are involved in human behaviors, even those as complex as sex, but they disagree over the details.

In LeVay's work, for example, Balaban said the physical differences may be effects rather than causes, or they may be coincidental.

During a more freewheeling afternoon discussion among psychiatrists, many of the same issues were approached from a different angle.

Lawrence Hartmann, a past president of the American Psychiatric Association, said the search for the origins of homosexuality is one "drenched in politics" and steeped in ambiguity. He said genes are rarely the exclusive determinant of any behavior. Usually, he said, genes establish parameters, or a predisposition, leaving a large role for family upbringing, social and cultural influences and personal experience.

Some researchers are even beginning to question whether there is a single set of experiences that can be be called "homosexuality." April Martin, a clinical supervisor at Yeshiva University and a lesbian psychologist in New York City, noted that the division of society into homosexual and heterosexual is not only recent, but possibly misleading. "Nature did not create this dichotomy," she said. "People did."