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Male hormone levels in womb may affect sexual orientation, study says
SAN FRANCISCO (CNN) -- A California psychologist has found the level of male hormones a fetus is exposed to in the womb can influence future sexual orientation. The study, published in this week's journal Nature, is based an unusual research technique -- comparing the lengths of a person's index and ring fingers.
Marc Breedlove, a professor of psychology at University of California Berkeley, said that finger length is influenced by levels of male hormone, or androgen, in the womb. Thus, he used the finger comparisons as an approximate measure of fetal androgen levels.
In most people, the index finger is slightly shorter than the ring finger. But in the right hand, the difference is accentuated by higher levels of androgens during fetal development, according to the study. In women, the ring and the index finger tend to be almost the same size. In men, the index finger is usually shorter.
In his study of 720 people, Breedlove found higher levels of androgens can create a greater than normal tendency for both males and females to develop a homosexual orientation.
Breedlove found lesbians tended to have a more masculine hand pattern, with the index finger considerably shorter than the ring finger, when compared to heterosexual women as a group.
But he cautions, "There is no gene that forces a person to be straight or gay... I believe there are many social and psychological, as well as biological factors that make up sexual preference."
Breedlove says what his data does show is that some people are gay because of fetal androgen levels.
"We think that lesbians, as a group, were seeing slightly higher prenatal testosterone levels than were the heterosexual women," Breedlove said.
He said the pattern for men was more complicated. There did not appear to be a direct relationship between finger length and sexual orientation. Still, Breedlove found some gay men did appear, based on finger measurements, to have been exposed to higher levels of fetal androgens.
"This calls into question all of our cultural assumptions that gay men are feminine," said Breedlove. He says his findings point more toward gay men as hypermasculinized.
Breedlove conducted his research at three street fairs in the San Francisco Bay area in the fall of 1999. Each study participant had his or her hand photocopied on a portable copy machine to record finger length. Participants also filled out a questionnaire on sexual orientation and birth order.
Neuroscientist Simon Levay says Breedlove's work confirms his own views on the causes of homosexuality.
"I think it is one more contribution if you like to the idea that our personalities including our sexuality and our sexual orientation are influenced by things that happen when our brain first assembles itself before birth," Levay said.
But University of California Davis psychologist Gregory Herick says using finger ratios as a biological explanation for lesbianism is an over simplification.
"We're going to find there are many different ways people become heterosexual, homosexual or bisexual as an adult," Herick said. "I think one of the problems with interpreting findings of this sort, people have a tendency to say, 'Here's the answer. Now we know.' And they're eventually proven wrong."
CNN Correspondent Don Knapp contributed to this report.
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