Yet another study providing evidence for a genetic cause for homosexuality was released today. It is all over the news, but here is one write up:
[quote]Boy’s odds of being gay traced to womb
Study looks anew at puzzling role of brothers’ birth order
- Sabin Russell, Chronicle Medical Writer
Tuesday, June 27, 2006
A boy’s chances of growing up gay increase with the number of older brothers
he has, and the Canadian researcher who spotted the trend a decade ago now
believes he is closer to explaining why: It all starts in the womb.
Brock University psychologist Anthony Bogaert first reported in 1996 the
startling finding that a boy’s probability of growing up gay increases by
about one-third with each older brother in his family. It’s a subtle
phenomenon – nearly all boys even in large families still grow up straight
– but subsequent research has affirmed that the “fraternal birth order
effect” is real.
Since that discovery, researchers have been trying to figure out what might
explain it. The most likely answer, they thought, had something to do with
how younger brothers are raised – perhaps having many older brothers drives
the youngest to adopt a different sex role.
But in a study released Monday by the journal Proceedings of the National
Academy of Sciences, Bogaert unearthed another surprise. The sexual
orientation of younger brothers appears to be established before birth.
"These results provide evidence that a prenatal mechanism … affects men’s
sexual orientation development,‘’ he wrote.
Bogaert came to his latest conclusion in a study involving the birth order
and family history of 944 men – about half identifying as straight and the
other half gay or bisexual.
In one analysis, he examined the fraternal birth order effect in families in
which unrelated older brothers were added to the mix through remarriage of
divorced couples or through adoption. In short, if there was something about
growing up with a lot of older brothers that raised the chances of a younger
boy being gay, it did not show up in these blended families of stepbrothers,
half-brothers and adoptees.
Bogaert also examined families in which biologically related brothers were
raised separately by families after divorce. No matter where the boys were
raised – in small or large families – the only factor that showed the
elevated chance of growing up gay was having older biological brothers.
By ruling out child-raising factors and ruling in biological factors,
Bogaert concluded that the results "support a prenatal origin to sexual
orientation development in men.‘’
The finding is consistent with – but does not prove – a theory that some
male homosexuality may be caused by exposure in the womb to maternal
antibodies created in the mother’s blood during previous delivery of male
children.
Although the placenta provides a barrier between the blood of mother and
fetus, during childbirth there is an “inevitable” mixing of maternal and
newborn blood, wrote Michigan State University neuroscientist Marc
Breedlove, in a commentary accompanying Bogaert’s article.
The theory suggests that mothers during childbirth may develop antibodies to
proteins made by their firstborn son’s Y chromosome, and subsequent
pregnancies may stir up those antibodies in an immune reaction that affects
the development of a male fetus. "Whether this is really what is happening
… remains to be seen, but it is provocative hypothesis,‘’ said Breedlove.
So far, scientists have found no similar relationship between birth order
and the probability that a girl will grow up to be lesbian. That could be
because a female baby has the same double X chromosomes as her mother and is
less likely to provoke an immune reaction during childbirth.
Breedlove stressed in an interview Monday that these biological
“perturbations” possibly affecting male fetuses should not be confused with
disease or a birth defect. They are simply biological effects that steer
development. "It just means there is a variation,‘’ he said.
Breedlove said that he is surprised that Bogaert’s original findings about
the fraternal birth order effect are not more widely known, because the work
is so interesting and has been replicated by other researchers. One reason
may be that homosexuality occurs in about 4 to 5 percent of the population,
so the increase noted among boys with several older brothers is a small
effect involving a small percentage of all people.
Similarly surprising research has found that the fraternal birth order
effect is limited to younger boys who are right-handed. In other words, if a
younger boy has many older brothers but is left-handed, he does not have an
elevated chance of being gay.
"We never dreamed of such an association,‘’ said Breedlove, a co-author of
that study.
The right-handed exception to the fraternal birth order effect was
particularly surprising because other research had previously uncovered
another puzzler: Both men and women who are left-handed are slightly more
likely to be gay.[/quote]
