快猫短视频

WOMEN should forget their careers and stay at home having babies. That鈥檚 not
a chauvinistic male talking鈥攊t鈥檚 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鈥檚 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. 鈥淭here 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,鈥� 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
鈥渇itness鈥� of each woman鈥攁 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.

Cultural factors had a big impact on the women鈥檚 fitness. More education
reduced their fitness, whereas Catholicism increased it鈥攂ut these factors
couldn鈥檛 explain everything. 鈥淚f you remove everything that鈥檚 cultural, there鈥檚
still an enormous difference between women,鈥� 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.
鈥淵ou鈥檇 think it was to do with your career, or what happens one particular
evening,鈥� says Owens. 鈥淏ut genes affect behaviour, and behaviour partly
determines when you will have kids and how many you will have.鈥�

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 鈥渇itness鈥� score of
women. And some 鈥渟ocial attitude dimensions鈥� such as family values and
militarism boosted fitness. 鈥淭hese personality traits are about 50 per cent
heritable and could contribute quite a lot of the genetic variance in fitness,鈥�
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鈥檛
mean evolution is no longer happening. Instead, new traits are being selected
for.

鈥淲hat makes you fit now is whether you have more babies than the next
person,鈥� says team member Simon Blomberg from the University of Wisconsin in
Madison. 鈥淏ut that鈥檚 not necessarily to do with health any more. It is more to
do with the decisions we make.鈥�

But natural selection of this sort could have worrying implications. 鈥淚f our
results are correct, one would predict steady selective pressure toward earlier
reproduction,鈥� says Martin, 鈥渁nd selection against women who delay childbearing,
and the traits that currently drive women to professional success鈥�.

So as society encourages women to have children later, the biological urge to
have kids early could discourage them from having careers. 鈥淭he genes are
pushing in the other direction,鈥� says Owens. 鈥淭here鈥檚 a fierce conflict between
a career and wanting to reproduce.鈥�