Survival of the Smartest?

For years, scientists have known that large-scale stressors like famines and floods correlate with a drop in the male-to-female birth rate among survivors. Studies of Chilean and Japanese earthquake victims and Swedish moms who were pregnant during natural disasters have shown decreases in the percentage of male offspring to which they gave birth. In the U.S., a nationwide study in 2010 showed male fetal deaths in the month after 9/11 shot up 12 percent—a figure researchers said was probably an underestimate, since many miscarriages go unreported. Even economic troubles and wars lower the percentage of male births among the pregnant population. (Conversely, in times of prosperity, more male babies are born.)

Now, a new study of births in California shows that boys born in December of 2001 are smarter than boys or girls whose moms weren’t pregnant during the Twin Towers attack. What in the world is going on?

There are various theories as to why the stress of a disaster affects male fetuses more than female. Males in utero are generally considered less robust than females; boys born prematurely suffer more health problems and developmental delays. Some scientists believe a mother’s stress hormones affect males differently than they do females, weakening them. But an alternate theory holds that evolutionary coping mechanisms during disasters cause women to miscarry weaker fetuses—and more of those fetuses happen to be male.

The new California study showing that the smart survive would seem to point to such an explanation, as do Swedish results showing that males born to moms who endure severe stress live longer than other men.