The weaker male foetus can easily be destroyed by the stress that is caused by disasters like earthquakes and other natural calamities including severe famine. The ones that manage to survive will have a longer life than average, according to a study conducted by researchers belonging to the University of California. Female foetuses are generally stronger than that of males.
Mothers' bodies under stress have less tolerance for unhealthy embryos and foetuses and thus spontaneously abort the offspring that would have been otherwise carried to birth.
Writing in Proceedings of the National Academy of Sciences of the US, researchers concluded aborting boys might be a way to maximize the odds of survival of the bloodline.
According to them, only weak foetuses were selectively targeted by the mother's body, giving more robust males every opportunity to thrive. They also said that actively culling weak male foetuses might increase the overall chance of a woman's genetic line surviving down the generations, as weak individuals were more likely to produce fewer offspring.