WOMEN should forget their careers and stay at home having babies. That’s not
a chauvinistic male talking—it’s what our genes are telling us, according
to an international team of researchers. They have found that in industrialised
societies, nature is selecting genes for behavioural traits that encourage women
to have children earlier. Eventually, women’s biological urge to have children
could become so strong, it might override their desire to have a career.
The finding means humans are still evolving, because genes that boost the
number of children an individual has will tend to increase in frequency with
each generation. This goes against the prevailing view that human populations
are genetically stable. “There is this assumption that because we are all living
healthy, happy lives and we can have as many children as we want, then evolution
must have come to a stop,” says evolutionary biologist Christopher Wills of the
University of California in San Diego.
Ian Owens from Imperial College in London and his colleagues used data about
the lives of 2710 female twins in Australia to calculate the evolutionary
“fitness” of each woman—a measure of the number of descendants her lineage
would leave. The number of children they had, and when they had them, were
crucial to the calculation.
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Cultural factors had a big impact on the women’s fitness. More education
reduced their fitness, whereas Catholicism increased it—but these factors
couldn’t explain everything. “If you remove everything that’s cultural, there’s
still an enormous difference between women,” says Owens. He estimates that 50 to
60 per cent of fitness is environmentally determined, but 40 to 50 per cent is
genetic.
To find out which kinds of genes were inherited, the researchers looked for
traits that affected fitness in identical twins, who have all their DNA in
common, and non-identical twins, who on average share half of it.
Religion and education had virtually no genetic component. But in all social
groups, there was a strong genetic influence on the age at which women had their
first child. The earlier a woman had her first child, the fitter she was in
evolutionary terms, as she has had more time to have more children.
The researchers say that the genes involved probably determine psychological
or behavioural traits that make women more likely to have children younger.
“You’d think it was to do with your career, or what happens one particular
evening,” says Owens. “But genes affect behaviour, and behaviour partly
determines when you will have kids and how many you will have.”
To investigate further, the researchers looked at the results of personality
tests that the twins took. Their unpublished results suggest that psychological
traits such as extroversion and neuroticism affected the “fitness” score of
women. And some “social attitude dimensions” such as family values and
militarism boosted fitness. “These personality traits are about 50 per cent
heritable and could contribute quite a lot of the genetic variance in fitness,”
says team leader Nick Martin of the Queensland Institute of Medical Research in
Brisbane. Another possibility is that genes that cause a stronger biological
urge to have children are being selected for.
One of the driving forces of human evolution was infant mortality, which
weeded out genes for disease susceptibility in childhood, for example. But in
rich societies, modern medicine means that this has only a minor effect. More
recently, contraception has also enabled women to choose how many children they
have and when. The new research implies that controlling such forces doesn’t
mean evolution is no longer happening. Instead, new traits are being selected
for.
“What makes you fit now is whether you have more babies than the next
person,” says team member Simon Blomberg from the University of Wisconsin in
Madison. “But that’s not necessarily to do with health any more. It is more to
do with the decisions we make.”
But natural selection of this sort could have worrying implications. “If our
results are correct, one would predict steady selective pressure toward earlier
reproduction,” says Martin, “and selection against women who delay childbearing,
and the traits that currently drive women to professional success”.
So as society encourages women to have children later, the biological urge to
have kids early could discourage them from having careers. “The genes are
pushing in the other direction,” says Owens. “There’s a fierce conflict between
a career and wanting to reproduce.”
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More at:
Evolution (vol 55, p 423)