As a demographer, Shige Song was trained to focus on the social forces that change the proportion of males and females in a population. His studies of sex ratio in China, India and South Korea, where the birth of boys has significantly outpaced girls over the past few decades, had focused exclusively on the effect of cultural preferences for sons. Demographic studies since the 1980s have suggested son preference and sex-selective abortions of girls were the main causes of the skewed sex ratio in these countries.
What demographers have not been able to explain is why trends in sex ratio at birth also exist in societies that don’t value sons more than daughters. In the second half of the 1900s, more girls were born than usual in North America and most of Europe, while boy births significantly outpaced girls in Ireland, France, Italy and Spain. “It became clear to me that the standard social science model may not be adequate to explain sex ratio,” Song says. His controversial new theory has less to do with social forces among people, and more to do with simple biology.
