A team from the University of California, Berkeley, have found in a study that pregnant women are more likely to miscarry male fetuses than females fetuses during times of stress.
According to Ralph Catalano who led the study, this tendency to miscarry males has a culling effect and lends support to earlier findings that pregnant women are more likely to miscarry male fetuses than females fetuses during times of stress.
The researchers discovered that boys born in stressful times enjoy an advantage their whole lives, living longer, on average, than males born in times of peace and prosperity.
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.
Catalano believes the populations of boys are hardier because they lost the weak ones earlier.
The study also supports the theory that males are the weaker sex.
Catalano says 'statistically' speaking, when compared to men, women are 'biological fortresses'.
Catalano along with colleague Tim Bruckner were reviewing earlier studies that showed fewer boys are born during times of stress, such as economic recessions or depressions and natural disasters.
They used a Swedish database of birth, life and death information dating back to 1751.
Demographers have established that the database can be applied to the global population in the absence of more precise information from other regions.
It seems that on average, around the world, around 105 boys are born for every 100 girls, but boys are more likely to die young in general, and by the time couples are dating, the ratio is fairly even.
According to Catalano there are two opposing theories as to why this is so.
One was that a stressed pregnant woman produces more of a hormone called cortisol, which in turn damages fetuses, which are frequently miscarried.
Male fetuses are more fragile than female fetuses, they are therefore more likely to be damaged.
The second theory says that as cortisol often makes a male fetus kick and squirm, a mother's body will miscarry a male fetus that does not kick or wiggle strongly enough presuming it to be weak.
Catalano says in evolutionary terms it is not in a mother's interest to have a weak son in times of stress, as he may not survive or may not be competitive for females.
Both theories says Catalano, predict that fewer boys would be born, but they would have different long-term outcomes.
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.
When they looked at the Swedish data they found that after the most stressful times such as a famine, men's lives were four months longer than in happier times.
It seems the the weak boys were culled out and those boys that survived were on average stronger and they lived longer.
Whereas for an individual, the difference is small, over a population it is significant, says Catalano.
In California after 9-11 they found that the sex ratio in California went down, and many more males than you would expect died after Sept. 11 in utero.
He says similar effects were also seen after the collapse of East Germany in 1991, when unemployment soared in the former socialist state.
The findings are published in the Proceedings of the National Academy of Sciences.