China and India account for 37% of the global population

Nicholas Christakis:

Sex ratios are becoming increasingly unbalanced in many parts of the world, especially in China and India (which account for 37% of the global population). The normal sex ratio at birth is roughly 106 males for every 100 females, but it may presently be as high as 120 for young people in China, or as high as 111 in India. This shift has been much discussed, and may arise from preferential abortion, cessation of reproduction if the first child happens to be a boy (but not if the child is a girl), or neglect of baby girls relative to boys. Gender imbalance may also have other determinants, such as large-scale migration of one or the other sex in search of work. This shift has numerous implications. For example, given the historical role of females as caregivers to elderly parents, a shortage of woman to fill this role will induce large-scale social adjustments. Moreover, an excess of low-status men unable to find wives results in an easy (and large) pool of recruits for extremism and violence.

This shift in gender ratios may have other, less heralded implications, however. Some of our own work has suggested that this shift may actually shorten men’s lives, reversing some of the historic progress we have made. Across a range of species, skewed sex ratios result in intensified competition for sexual partners and this induces stress for the supernumerary sex. In humans, it seems, a 5% excess of males at the time of sexual maturity shortens the survival of men by about three months in late life, which is a very substantial loss.