In the late 1950s and early 1960s, China's Mao Zedong proposed the "Great Leap Forward," a campaign that was supposed to increase steel production and propel his country into the upper ranks of industrialized nations. But as leaders rushed to find ways to make more steel, grain production fell. Tens of millions of Chinese died. A new study finds that the famine also had demographic consequences: A significantly higher proportion of girls was born after the famine than in the years leading up to it.
Evolutionary biology predicts that when times are tough, it's advantageous to give birth to daughters. The reason may be that lower-quality males aren't as likely to get chances to mate as stronger males. Shige Song, a social demographer at Queens College of the City University of New York, wanted to see if this effect showed up among people suffering from a famine. Other researchers have looked for a change in sex ratio after two other famines, the 1944-45 Dutch hunger winter and the 1942 famine associated with the siege of Leningrad in the former Soviet Union, but they found conflicting results.
Song went after a bigger famine: the Great Leap Forward. Over 30 million people died in during this event. That's many times the number that perished in the two smaller European famines. The Chinese famine had many causes. Among them was that the focus on making more steel took labor away from agricultural tasks, like harvesting grain. The disaster started in the fall of 1958 in some regions and was widespread in China by January 1959.It lasted 3 years, until the end of 1961.
Song used data from a 1982 survey of 310,101 Chinese women aged 15 to 67, which collected information on all of their children. He found that, before the famine, the proportion of male births was slowly rising. (It's not clear why this was happening, but Song used the trend as a baseline to look at the changes in sex ratio associated with the famine.) The sex ratio reached a peak of "maleness" around 1958, then started to shift, falling from 521 males per thousand births in April 1960 to 510 males per thousand births in October 1963. The sex ratio didn't start to swing back toward a higher proportion of males until nearly 2 years after the famine ended, Song reports online today in the Proceedings of the Royal Society B.
Song thinks the long duration of the famine may explain why other studies have failed to show similar results. The Dutch and Leningrad famines may have just been too short, at 7 months and 6 months respectively, to affect the number of baby girls born. "It's not like you skip lunch for 1 day or 2 days, then you will influence the sex ratio," he says. "It's a cumulative process. The body has to learn that this is real."
Tessa Roseboom, a biologist at Academic Medical Center Amsterdam who studies the Dutch famine, agrees that seven months of famine may just not have been long enough to cause a change in sex ratio. She also points out that she has birth records for only 2500 babies, while Song's study covers more than 830,000. It's not surprising that other researchers have failed to find a consistent effect of sex ratio with the Dutch data, she says, "Just because the effect, if it's there, isn't very big."
But other research has found that even short-term fasting can influence sex ratios, says Douglas Almond, an economist at Columbia University who has also studied the Great Leap Forward famine. He has analyzed women who fast during Ramadan and found that babies who were conceived close to the time of the fast have a more female-biased sex ratio—and the change in the ratio is bigger than Song found. "I think there's an immediate effect," Almond says. He agrees with Song on the basic point, though. "Sex ratios respond to maternal condition, and nutrition is one of the big factors."