Blanchard in 1997 which found that the chances of you being a homosexual rise by
about a third for each older brother you have.
This research, perhaps more than any other in this controversial area, has been
critically appraised. Blanchard, a psychiatrist at the University of Toronto, found that
the conclusions were the same in 14 different studies. The probability of this being
the result of chance alone is about 1 in 10,000. "These data therefore establish beyond
much doubt that homosexual males do, on average, have higher birth orders [more
elder brothers] than comparable heterosexuals," Blanchard wrote in 2001. "Because
the sexual orientation of a newborn boy cannot operate backward in time to affect his
older siblings, this finding implies that the number of older siblings, or some factor
associated with that, must affect the newborn boy's sexual orientation," he says. What
was intriguing about this study was that the effect was not seen in lesbians, and
neither did it matter how many older sisters were involved. It purely came down to
the number of elder brothers.
Homosexual males with older brothers weigh even less than heterosexual males with
older brothers, indicating that there is some sort of developmental battle taking place
in a womb carrying male foetuses for the second, third or fourth time. Marc
Breedlove, professor of psychology at the University of California, Berkeley, has also
found that gay men are more likely to have older brothers than straight men - gay
men had a ratio of 140 brothers to 100 sisters among their older siblings compared to
a general population ratio of 106 brothers to 100 sisters. But Breedlove suggests that
exposure to male hormones in the womb could be the cause. "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 foetal androgen [male sex hormone] levels," says
Breedlove.
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Foetal testosterone and the child systemizing quotient
Bonnie Auyeung, Simon Baron-Cohen, Emma Chapman, Rebecca Knickmeyer, Kevin
Taylor and Gerald Hackett^4
Department of Psychiatry, Autism Research Centre, University of Cambridge, Douglas House,
18B Trumpington Road, Cambridge CB2 2AH, UK, Department of Psychiatry, University of North
Carolina at Chapel Hill, CB# 7160, Chapel Hill, North Carolina 27599-7160, USA, Department of
Clinical Biochemistry, Addenbrooke’s Hospital, Cambridge, CB2 2QQ UK and Department of
Foetal Medicine, Rosie Maternity Hospital, Robinson Way, Cambridge CB2 2SW, UK