The big brother effect
New Scientist, 29 March 2003
brothers owe a lot to their older male siblings. Historically, the
eldest have been saddled with the ageing family farm - not to mention
the ageing family - leaving younger boys free to pursue serious
commerce or adventure. Even today, younger brothers enjoy expert
skateboarding instruction, tips on how to use the family car undetected
and more generous curfews. Long before the younger ones appear, older
boys have broken their parents in, making life less miserable for those
who come after. But can boys really also credit their older brothers
with determining their sexual orientation?
The debate about
whether homosexual men are born or made is hotly contested. Puritans
still insist that homosexuality is a "lifestyle choice" (and a wrong
one at that). Many gay men will tell you they have been sexually
attracted to other men since they first noticed the difference between
the sexes. Meanwhile, biologists have been fascinated by the question,
not least because the observation that 2 per cent of men are attracted
to members of their own sex contradicts everything that Darwin would
have us believe. Science is not threatening to resolve this particular
debate any time soon. But the idea that sexual orientation is fixed
before birth has been strengthened with the intriguing finding that the
more male babies a woman has borne, the greater the likelihood that any
subsequent sons will be homosexual.
Over the decades, scientists
have found evidence to support both sides of the nature versus nurture
argument (New Scientist, 28 September 1996, p 32, and 28 November 1992,
supplement). Twin studies suggest that homosexuality isn't all down to
the genes, but they also show that there must be a genetic component,
as many more identical twins than fraternal twins share sexual
orientation. In 1993, Dean Hamer at the National Institutes of Health
in Bethesda, Maryland, even claimed to have found a region on the X
chromosome where a male gay gene might reside.
researchers have pointed to anatomical differences between gay and
straight men. In 1991, Simon LeVay, then at the Salk Institute in San
Diego, California, published evidence that a part of the brain known to
vary in size between men and women was unusually small in homosexual
men. In fact, in his post-mortem study, LeVay found that the region,
known as the third interstitial nucleus of the anterior hypothalamus,
or INAH3, was almost identical in size in women and gay men. LeVay
argued it was evidence that gay men are born, not made. "It's much more
than lifestyle or family processes," he insists.
LeVay's findings went unreplicated. Then, at last year's meeting of the
Society for Neuroscience in Orlando, Florida, Charles Roselli at the
Oregon Health and Science University in Portland presented supporting
evidence from work in animals. He and his colleagues studied 27 sheep -
10 ewes, 8 normal rams and 9 rams that exhibited homosexual behaviour.
In nature, about 8 per cent of rams are gay, says Roselli. For reasons
unknown, they never court or mate with females, preferring instead to
mount males. Like LeVay, Roselli found that the analogous part of the
hypothalamus in these rams resembled the female structure rather than
Those on the nurture side of the debate argue that
such anatomical differences are not the cause of homosexual behaviour,
but the result of it. "We don't know whether the [interstitial] nucleus
can be changed by experience," admits Roselli. But in other animals,
such as rats and ferrets, that particular brain region is shaped in
utero by the male hormones. "By inference, we'd think that differences
between male and female-oriented rams could be traced back there too,"
Other gay signatures have also been identified, but
their relevance to sexual preference remains unclear. Roger Gorski at
the University of California at Los Angeles, for instance, found that
the anterior commissure, a cable that connects the two hemispheres of
the brain, is larger in gay than straight men. Dick Swaab at the
Netherlands Institute for Brain Research in Amsterdam found that the
body's timekeeper, the suprachiasmatic nucleus, is larger in homosexual
men than in heterosexual men. And Doreen Kimura at Simon Fraser
University in Vancouver found that gay men's fingerprint-ridge
patterns, which are completely formed during gestation, were more like
those of heterosexual women than heterosexual men.
this, the notion that sexual preferences are hardwired and that people
are born with them remains extremely controversial. But the recent
findings have convinced at least some of the experts. All the evidence
points to the idea that male homosexuality is innate, says Ray
Blanchard, a psychologist at the Centre for Addiction and Mental Health
in Toronto. "The theory that early childhood experiences lead to
homosexuality has had 100 years, and they've come up with zip," he
says. Exactly what the biology behind homosexuality is remains a
puzzle, he admits. But after 15 years studying what he calls the
"fraternal birth order effect", he's convinced that he is onto
Blanchard stumbled into the arena while sifting
through some data for a colleague. In his cross-tabulations, he
couldn't help but notice the striking difference in sibling patterns
between heterosexual and homosexual men: so many gay men had older
brothers. "I had read somewhere about this. I thought it was absurd -
such egregious pseudoscience," Blanchard says. He is now the principal
proponent of the theory.
To test whether the apparent
correlation was anything more than anecdote, Blanchard began by
studying 302 gay men, all Caucasian non-twins who could identify every
child born to their mother, and the same number of heterosexual
age-matched controls. All the volunteers were given questionnaires
about their parents' age at their birth, as well as the genders and
numbers of older and younger siblings born to their mothers. The
homosexual males, Blanchard found, had significantly more older
brothers than the heterosexuals, 1.31 compared with 0.96. Parental age
and the interval between births played no role. He calculated that the
odds of being gay increased by about 33 per cent with each older male
Subsequent studies by Blanchard and other research
teams confirmed the finding. John Manning at the University of Central
Lancashire in Preston has found the effect in all his samples of gay
men. "It's one of the few reliable correlates of homosexuality that I
know of," he says. "Now, if you don't see it, you start thinking maybe
you've done something wrong." Marc Breedlove, at Michigan State
University in East Lansing, also found a striking preponderance of
older male siblings among his gay male volunteers compared with the
straight male volunteers, but no difference in the number of younger
The big brother effect has yet to be confirmed across
other racial groups, but it has proven to be robust in many contexts.
It seems to be true of people studied decades ago as well as today, of
those in Europe, Canada and the US, of psychiatric patients and
non-patient volunteers and of men who wish they were women as well as
men who are perfectly content with their orientation and anatomy.
Regardless of culture, demography or psychological state, having more
older brothers predisposes a man to being homosexual.
Roselli's rams seem to demonstrate the fraternal birth order effect.
"I'm seeing hints of it," he says. "But we're still collecting data."
The findings aren't completely comparable, since sheep tend to be born
in twins and triplets. But in the smallish sample Roselli has completed
so far, the rams classed as homosexual have significantly more older or
same-age male siblings than heterosexual ones. "The idea that it might
show up in an animal model is worth investigating," he says.
humans, the big questions these days are less about whether it's
happening, but rather how and why. Interestingly, female sexual
orientation seems to be completely unaffected by birth order. Neither
older sisters nor older brothers have an influence. Likewise in males,
older sisters are for all intents and purposes invisible. What is
pivotal is the number of older brothers. Some people have argued that
the big brother effect could be social - that having lots of older
brothers makes you more prone to sexual abuse as a child. But Blanchard
dismisses this, pointing out that there is no evidence that such abuse
is more common in boys with older brothers, or that there is a link
between abuse and sexual orientation.
But how might having older
brothers affect a boy's sexual orientation before birth? Blanchard
reasoned that a fetus can't "know" how many brothers have been gestated
in the womb before him; only a mother's body can. And the biological
system most capable of keeping track is probably the mother's immune
system. Breedlove suspects this is right: "That's what immune systems
are for - to remember who they've bumped into."
A mother could
somehow be taking note of something specifically male, such as a
Y-linked cell surface protein, Blanchard speculates. It's known that
fetal cells can leak into a mother's bloodstream during gestation as
well as at the time of birth. Some evidence even suggests these foreign
cells might lurk in her body for decades. Blanchard conjectures that it
is through just such a leakage that a male fetus might prompt his
mother's immune system to produce antibodies against the fetal cells.
The more male offspring she carries, the stronger her immunity and the
fiercer the attack.
A possible candidate is one of the
male-specific minor histocompatibility antigens, known collectively as
the H-Y antigen. They are present in cell membranes in male fetuses
from very early on. This means that even if the baby doesn't make it to
full term, in principle the mother's body could record its existence.
It also means that from before the time when sexual differentiation
starts to take place, around the eighth week of pregnancy, an immune
reaction can be triggered in the mother, possibly disrupting the
process and affecting sexuality in some way.
The H-Y antigen
almost certainly plays some role in sexual differentiation. It is
present only in males and has been highly conserved throughout
evolution. Blanchard notes that the antigen is strongly represented on
the surfaces of brain cells. "It's unlikely it's just there for the
hell of it," he says. That also makes it easy to understand how
maternal antibodies against the antigen could target the baby's brain,
he says, and affect its organisation. What's more, studies in mice have
shown that when females were actively immunised against fetal H-Y
antigen, 90 per cent of their male offspring showed "very poor
reproductive performance", despite being fertile.
No one knows
why some mothers might have a strong immune reaction while others mount
little or no response. But Blanchard believes having some kind of
reaction is much more common than we realise. The strongest evidence of
that is the fact that birthweight appears to be affected by older
siblings - even in straight men. A baby's weight at birth tends to vary
according to birth order in any case, with second-born children usually
heavier than firstborn, for example. But boys who have a lot of older
brothers tend to weigh less than would be expected for their place in
the family - less, for instance, than boys who have the same number of
This is true of both heterosexual and homosexual
men with older brothers, although the gay men tend to weigh even less.
In one study, Blanchard found that gay men with two or more older
brothers weighed on average 170 grams less at birth than heterosexual
men with older sisters but no older brothers. This is larger than the
typical difference between male and female birthweights, around 100 to
150 grams. But gay men who were firstborn weigh the same as straight
Also intriguing is the fact that, despite their
lower birthweights, later-born males tend to have larger than normal
placentas, according to a study of over 7500 placentas by Michel
Vernier at the University of California, Berkeley. Both Blanchard and
Breedlove think the fetus may be trying to fend off the attack from the
mother's immune system, and that the bigger placenta is compensating in
some way. Whatever the reason, the findings on birthweight and
placental size are important because they show that prior pregnancies
with male fetuses really do have an influence on the development of
subsequent fetuses. They also support the contention that the big
brother effect is not the result of living with older brothers. "It's
hard to see what this has to do with social dynamics," says Breedlove.
admits that the H-Y antigen is only one possible target. The effect
could be triggered by something else entirely - but he doesn't think
so. Manning, whose work concentrates mainly on hormonal effects in
utero, isn't yet persuaded by the immune explanation. "I would love to
see some direct evidence of it," he says. But Blanchard argues that his
findings aren't easily explained by the two main models of sexual
differentiation, which deal in either genes or hormones. He accepts
that androgens drive most of the sexual differentiation in the brain.
"But my data suggest it might not be 100 per cent," he says.
Blanchard and a colleague, James Cantor, have done some mathematical
gymnastics to show the importance of the fraternal birth order effect.
In a paper out last year, they calculate that about one gay man in
seven can attribute his sexual orientation to his older brothers
(Archives of Sexual Behavior, vol 31, p 63). Boys with 2.5 older
brothers are twice as likely to be gay as boys with no older brothers,
and a boy with four older brothers is three times as likely to be gay.
If Blanchard is right, then clearly, as average family size decreases,
so will the incidence of male homosexuality. It also follows that
historically there have been more gay men than there are today. But
even today, says Breedlove, you'd expect Catholics and Mormons to have
more homosexual sons. "Groups that encourage large families would have
more sons inclined to be gay than others," he says.
should this be? Why should mothers systematically tamper with the
sexuality of their later-born sons? There are no easy answers.
Breedlove warns that homosexuality might not be the selective
disadvantage that some people suppose, even when it comes to
reproduction. Gay men do father children. Other scientists have
suggested that homosexual males help promote their genes by being
unusually supportive uncles and brothers - a claim that, however
appealing, hasn't been carefully tested.
Manning dismisses this
idea. "It's tempting to look at the frequency of male homosexuality and
say maybe it's adaptive," he says. "But I actually don't think that
homosexuality has got anything to do with adaptation." Manning's own
research on hormonal effects in the womb suggests that homosexuality,
for whatever reason, might result in part from higher than normal
exposure to testosterone. Intriguingly, some of Breedlove's data
suggests the same for men who have older brothers in general,
regardless of their sexual orientation. High levels of testosterone
during development have been associated with greater muscle strength,
protection against heart disease and faster running speeds. "There
could be a need for a male who has older brothers to be more
competitive," says Manning. Perhaps in some cases that extra
testosterone also affects the sexuality of some men, he speculates.
Blanchard is hesitant to wade into this discussion, he raises the
possibility that there may be advantages to the male fetus that we
simply haven't thought to look at. Homosexuality could be only half the
picture. "And the other half might simply be survival of male fetuses."