Berkeley psychologist finds evidence that male hormones in the
womb affect sexual orientation
Patricia McBroom, Public Affairs
-- 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.
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
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.
said that, these data do suggest that there are some people
in the world who are gay because of fetal androgen levels."
findings appear in this week's issue of the journal Nature.
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.
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.
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.
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
this suggests that at least some lesbians were exposed to greater
levels of fetal androgen than heterosexual women," Breedlove
and his colleagues wrote.
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
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.
findings, combined with other recent research, paint a complicated
picture of the role played by fetal androgens in determining
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.
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.
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.
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.
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."
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.
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."