HERE come the girls. Fewer boys than girls were born in the months after the huge earthquake struck Japan in March 2011.

Ralph Catalano of the University of California in Berkeley and colleagues examined hospital records of births in Japan between 2006 and the end of 2011. After the quake, births in areas closest to the epicentre were more likely to be girls, but provinces farther out showed no gender bias. About 2.2 per cent fewer boys were born in the most damaged areas than expected (American Journal of Human Biology, doi.org/nbj).

It is not the first time such a skew has been noted: fewer boys were born after the US stock market crash of 2008, for instance. The reason may be evolutionary, says Catalano. Boys are more likely to be premature and suffer problems associated with low birth weight than girls. In times of stress, it may therefore be beneficial for the mother to give birth to a girl.

However, it is unclear whether stress causes mothers to miscarry more males or whether fewer males are conceived. Earthquakes provide a natural test for this, Catalano says. If births immediately after show a gender skew, it would suggest that stress is triggering miscarriages. A bias nine months later indicates fewer male conceptions.


The team found evidence for both. Several mechanisms may be at work, says Catalano. Fetuses produce a hormone called human chorionic gonadotropin, which camouflages them from the mother’s immune system. Weak male fetuses make less of this hormone, meaning they may be at greater risk of attack.

William James of University College London believes the father’s testosterone level plays a role. During periods of high stress, he says, men produce less testosterone, which can reduce the number and quality of the “male” sperm, which carry a Y sex chromosome.

“During high stress, men produce less testosterone, which reduces the quality of male sperm”

This article appeared in print under the headline “Japan quake boosts number of female births”