Fewer boys were born nine months after the three worst earthquakes in Japan鈥檚 recent history, a study has revealed. The finding hints that stress makes it harder to conceive sons.
Misao Fukuda at M&K Health Institute in Japan and his colleagues studied the birth records of Japanese prefectures struck by the Kobe earthquake of 1995, the Tohoku earthquake responsible for the tsunami that hit the Fukushima Daichii nuclear plant in 2011, and the Kumamoto earthquake of 2016.
They found that the proportion of male babies born in affected prefectures dropped by 6 to 14 per cent nine months after each earthquake.
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For example, nine months after the Tohoku earthquake and tsunami killed more than 8000 people in Miyagi prefecture, the usual birth ratio of 108 boys per 100 girls dropped to 95 boys per 100 girls. This anomaly was not observed in other, more distant prefectures.
Vulnerable to stress
The finding suggests that fewer males were conceived directly after the earthquakes, since a nine-month gap separates conception and delivery, says Fukuda.
One possible explanation is that men鈥檚 sperm carrying the male Y chromosome were damaged more easily by the stress of the event, making them less likely to form embryos, says Fukuda. Another is that male embryos themselves were less resilient to stress, he says.
In line with this hypothesis, Fukuda previously found that .
However, it鈥檚 unclear why male sperm or embryos might be more vulnerable to stress, says at the University of California, Berkeley. 鈥淚t鈥檚 a nice theory but there鈥檚 no way of experimentally verifying it,鈥� he says.
Sex ratio mystery
Other studies have found that fewer male babies were born four months after other high-stress events like September 11, the Sandy Hook shooting, the Breivik shooting in Norway, and East Germany鈥檚 economic collapse.
These reflect the greater miscarriage risk for male fetuses exposed to stress during mid-pregnancy, says Catalano. Sons require more milk and energy and are more likely to die in infancy, making daughters a 鈥渟afer bet鈥� to give birth to in times of trouble, he says. In evolutionary terms, if the environment turns virulent, males become relatively poor investments, he says.
It makes sense that stressful events could affect the sex ratio at two different time points 鈥� four months and nine months later 鈥� because biology has multiple ways of protecting against risky births, says Catalano.
One enduring mystery is why, under normal conditions, more baby boys tend to be born than girls, says Catalano. In the UK, US and Australia, for example, 105 boys are born for every 100 girls.
IVF studies are now starting to offer some clues. We now know, for example, that female embryos are more likely to spontaneously abort in the first 4 to 6 weeks of pregnancy, potentially explaining why higher proportions of male babies are born overall.
Starting out with more boys at birth may offer an evolutionary advantage since they are less likely to make it to adulthood, says Catalano. By the age of 14, the sex ratio usually balances out to about one male for every female, he says. 鈥淚t might be nature鈥檚 way of evening things up.鈥�
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