The disastrous economic policies of Mao’s Great Leap Forward caused the single deadliest famine in the history of the world. Between 1958 and 1961, an estimated 15 to 45 million people died of malnutrition in China. And during this period, according to a new study, a strangely high proportion of babies born were female.
China has had a long cultural tradition of favoring sons over daughters, and boys outnumber girls every year in this data from 1940 to 1980. But from one year after the famine’s beginning to two years after its end, the proportion of males drops sharply, as you can see in the graph above.
This study may bolster the Trivers-Willard hypothesis, which suggests that poor maternal condition, such as famine, would favor giving birth to more girls. Since the reproductive success of males tend to be more variable—a reproductively successful male can father many children, whereas a unfit one fathers none—girls are a “safer” evolutionary investment in risky times. The birth sex ratios of mammals such as ground squirrels and red deer follow this pattern. Lab experiments where the male blastocytes of cows survive better in glucose-rich environments identified a possible mechanism.