Populations of boys born during stressful times enjoy an advantage their whole lives, living longer, on average, than males born during periods of peace and prosperity, U.S. researchers reported on Monday.

The study adds to earlier findings that pregnant women are more likely to miscarry male fetuses than female fetuses during times of stress.

It shows that this tendency to miscarry males has a culling effect, said Ralph Catalano of the School of Public Health at the University of California, Berkeley, who led the study.

“The populations are hardier because they lost the weak ones earlier,” Catalano said in a telephone interview. “No individuals got stronger - it’s just that the weak ones aren’t there.”

The findings, published in the Proceedings of the National Academy of Sciences, also solidify what biologists have long known - that males are the weaker sex. “That’s one thing I can say,” Catalano said. “Statistically, it is clearly true. Compared to men, (women) are biological fortresses.”

Catalano and colleague Tim Bruckner followed up on earlier studies that showed fewer boys are born during times of stress, such as economic recessions, depression or natural disasters.

They used data from Sweden, which has a database of birth, life and death information dating back to 1751. Demographers have certified that the database can be extrapolated to the global population in the absence of more precise information from other regions.

EVEN RATIO

On average, worldwide about 105 boys are born for every 100 girls. But males are more likely to die young in general, and by the time couples are courting the ratio is fairly even - except after hard times.

There are two competing theories as to why, Catalano said. One is that a stressed pregnant woman produces more of a hormone called cortisol, which in turn damages fetuses.

Damaged fetuses are frequently miscarried. “Because male fetuses are more fragile than female fetuses, they are more likely to be damaged,” Catalano said.

Cortisol often makes a male fetus kick and squirm, and a second theory holds that a mother’s body will miscarry a presumably weak male fetus that does not kick or wiggle strongly.

“It’s not in her evolutionary interest to have a weak son in times of stress,” Catalano said. “He may not survive or may not be competitive for females.”

Both theories predict that fewer boys would be born, but they would have different long-term outcomes, Catalano said. Either all the male fetuses are damaged a little, and the boys who are born are weakened, or the miscarriage process culls the weak fetuses and leaves the strong ones.

In Sweden, after the most stressful times such as a famine, men’s lives were four months longer than in happier times, Catalano and colleagues found.

“The weak boys got culled out and those boys that survived are hardier on average. They live longer,” Catalano said.

Catalano said he sees the same effects in action today. “In California after 9-11 we reported that the sex ratio in California went down,” he said. “Many more males than you would expect died after September 11 in utero.”

Similar effects were seen after the collapse of East Germany in 1991, he said, when unemployment soared in the former socialist state.