In much of North Eastern India, there are 915 or fewer girls for every thousand boys - a sex ratio of more than 1.09 boys per girl. The persistence of such high sex ratios is puzzling, because it violates Fisher's Principle.
In most species, the ratio of males to females is approximately one to one. Fisher's principle explains why. In a society with fewer females than males, females achieve, on average, greater reproductive success than a typical male. Accordingly, any inheritable tendency that predisposes parents to producing females spreads. The number of females rises until, eventually, equality between males and females is restored.
So how can son preference persist?