Scientists have wondered for years why mothers in rich and peaceful countries are more likely to have baby boys, and new research suggests the answer may be stress, reports the Economist. Danish researchers have also found that stressed-out moms in the West are more apt to have baby girls—just like women in war zones and disaster areas.
A spike in the number of baby girl births in New York after 9/11 helped point scientists toward the stress theory. Opinions differ on why stress might increase the birth rate of baby girls. Some scientists believe that the adaptation evolved during hunter-gatherer times when girls may have been more likely to survive to find mates.