Hard times are especially hard on pregnant women. Miscarriages go up, as do premature births. The result: fewer baby boys. Economist Ralph Catalano, professor at the School of Public Health of UC Berkeley, showed it for the first time in a 2003 paper in the journal Human Reproduction.
Researchers have known, based on studies going back to the 1970s, that war and environmental disasters can affect the sex ratio, which normally sees to it that about half the babies born are boys and half girls. In times of environmental disaster, women are under stress, which can increase early release of hormones, including cortisol, thought to trigger labor.
Male fetuses are more likely to be miscarried or born prematurely than females, Catalano says, and premature babies are at higher risk of death. So more male fetuses don't make it through the pregnancy or die shortly after premature birth.
Stress affects men's reproductive systems, too. Some experts believe stress can slow down sperm motility. That, in turn, might result in fewer XY-chromosome-carrying sperm -- normally the faster swimmers that create male babies -- reaching the egg.