Populations of boys born in stressful times enjoy an advantage their whole lives, living longer, on average, than males born in times of peace and prosperity, say US researchers.
The study adds to earlier findings that pregnant women are more likely to miscarry male foetuses than females foetuses 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," said Catalano.
"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 were following up on the earlier studies that showed fewer boys are born during times of stress, such as economic recessions or depressions and 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 absence of more precise information from other regions.
On average, around the world, 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 was that a stressed pregnant woman produces more of a hormone called cortisol, which in turn damages foetuses.
Damaged foetuses are frequently miscarried.
"Because male foetuses are more fragile than female foetuses, they are more likely to be damaged," said Catalano.
Cortisol often makes a male foetus kick and squirm. A second theory holds that a mother's body will miscarry a male foetus that does not kick or wiggle strongly enough and which is, presumably, weak.
"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 foetuses are damaged a little and the boys who are born are weakened or the miscarriage process culls the weak foetuses and leaves the strong ones.
So they looked at the data. In Sweden, after the most stressful times such as a famine, men's lives were four months longer than in happier times.
"The weak boys got culled out and those boys that survived are hardier on average. They live longer," Catalano said. For an individual, this might be a small difference but over a population it is significant, Catalano said.
Catalano said he has seen the same effects in action today.
"In California after 9/11 we reported that the sex ratio in California went down. 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.