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UC Berkeley psychologist finds evidence that male hormones in the womb affect sexual orientation
29 Mar 2000

By Patricia McBroom, Public Affairs

BERKELEY -- The level of male hormones in the womb can influence an unborn child's future sexual orientation, according to new research from a University of California, Berkeley, professor who used an unusual technique - measuring finger length - to gather evidence.

Marc Breedlove, professor of psychology, also found that higher levels of these male hormones, or androgens, can create a greater than normal tendency for both males and females to develop a homosexual orientation.

"There is no gene that forces a person to be straight or gay," said Breedlove, who studies the biology of sexual orientation. "I believe there are many social and psychological, as well as biological, factors that make up sexual preference.

"Having said that, these data do suggest that there are some people in the world who are gay because of fetal androgen levels."

Breedlove's findings appear in this week's issue of the journal Nature.

Breedlove looked at relative finger length because it is influenced by androgen levels in the womb and thus is an approximate measure of fetal androgen levels.

In most people, the index finger is very slightly shorter than the ring finger, but, at least in the right hand, the difference is accentuated by higher levels of androgens during fetal development. Typically, in women, the two fingers of the right hand are nearly the same length. In men, the index finger is obviously shorter.

Breedlove collected data on 720 people who attended three San Francisco Bay Area street fairs in the fall of 1999. Using a portable copy machine, his research assistants had subjects lay their hands flat on the machine to record finger lengths. Breedlove also administered a questionnaire that explored birth order and sexual orientation.

According to the data collected, homosexual women, on average, had a more masculine finger length pattern - an index finger considerably shorter than the ring finger on the right hand - than did heterosexual women.

"... this suggests that at least some lesbians were exposed to greater levels of fetal androgen than heterosexual women," Breedlove and his colleagues wrote.

Men had more a complicated pattern: There was no direct relationship between finger length and sexual orientation. However, some gay men did appear, based on their finger lengths, to have been exposed to greater than normal levels of fetal androgens before birth.

"This calls into question all of our cultural assumptions that gay men are feminine," said Breedlove. He said his findings are consistent with other, very sketchy indications that some gay men are hypermasculinized, having a greater average number of sexual partners in a lifetime than heterosexual men, higher than normal levels of testosterone circulating in the blood, and larger genitalia than heterosexual men.

Breedlove's findings, combined with other recent research, paint a complicated picture of the role played by fetal androgens in determining sexual orientation.

Past research by Ray Blanchard of the Clarke Psychiatric Institute in Toronto has found that the more older brothers a boy has, the more likely he is to be gay in adulthood. The UC Berkeley study confirmed this finding, in that gay men had a ratio of 140 brothers to 100 sisters among their older siblings. This is much higher that in the general population, where the ratio is 106 brothers to 100 sisters.

The UC Berkeley study also found that men with older brothers had a more masculine finger length pattern than men without older brothers. The number of older sisters was unrelated to finger measures in either men or women.

As indicated by the fingers measured in his study, Breedlove said that each subsequent son is exposed to higher levels of male hormones and that, while most later-born sons are straight, the increased androgen level slightly increases the probability of a male child developing a homosexual orientation.

"This means," said Breedlove, "that somehow the mother's body remembers how many sons she has had and exposes each successive male fetus to more androgen.

"It is just mindboggling to think that some men are gay because of the number of boys their mothers had before their own birth. These events must register in the woman's body before an individual is even conceived."

Still, cautioned Breedlove, biology does not determine sexual orientation. The findings are statistical relationships, which means that many men and women do not fit the pattern.

"There are plenty of gay men who are first-borns, many straight men with older brothers and many women whose fingers give no clue to their sexual orientation," he said. "This is not a test to be used on your friends and neighbors."


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