Researchers have long observed that during times of social stress, there are fewer male births than female births. Now, a new study has found that boys born during tough times live longer than those born during periods of social stability.
Drops in male-to-female sex ratios are nothing new.
"During the Kobe earthquake in Japan, sex ratio drops," said Ralph Catalano, a professor public health at the University of California at Berkeley. "The collapse of the east German economy, sex ratio drops. The major smog in London in 1952, sex ratio dropped. Variation in the economy such that you get very bad economic times in Sweden over a century and a half, sex ratio drops."
Then, when environmental factors return to normal, experts say sex ratios even out.
Most scientists believe the dip in sex ratios is a stark example of evolution at work: through a process of natural selection, the theory goes, daughters are better able to propagate the species than weaker sons.
The mothers' bodies seem to spontaneously abort weak male fetuses more often by exposing them to high levels of the stress hormone cortisol in utero, while sparing the females.
Professor Catalano says cortisol acts as an evolutionary "testing mechanism" to weed out weaker male fetuses.
"And what we're interested in is whether the mechanism itself, the testing mechanism, of the fetuses actually damages those that survive, because the presumption has been that if you've been subjected to this kind of regimen of cortisol production in utero, that it has health consequences that affect you in life if you've made it to birth," he said.
That's one theory. Professor Catalano and colleague Tim Bruckner also wanted to explore another notion, that sons who survive end up living longer and healthier lives than males born during less stressful times.
They decided to look at cohort data in Sweden of babies born between 1751, the first year the Swedes began keeping birth records, and 1912.
Up until the mid-1800s, Sweden's history was marked by royal coups, wars, farming reform and religious intolerance.
Based on the male-to-female birth ratios over the 160-year period, the investigators found that the fewer males that survived the cortisol challenge in utero lived slightly longer than sons born during periods of relative calm.
"Our work suggests that at least historically, smaller cohorts of males than you would expect from the number of females does not mean that that cohort is a relatively fragile cohort," said Professor Catalano. "In fact, quite the contrary, those cohorts may be relatively hardy even though they may be smaller because the weakest of them are no longer there."
Professor Catalano doesn't think the findings are likely to have any impact on the way a doctor interacts with an expectant mother.
The study on sex ratios and lifespan were published in Proceedings of the National Academy of Sciences.