Viral Theory on China Gender Gap

If left entirely to nature, the number of newborn boys and girls in the world should be roughly equal. But in some cultures that balance has been artificially tipped in favor of boys.

In China, the problem is commonly attributed to gender discrimination, which leads to selective abortions and even female infanticide. But a new study suggests that disease may have as much to do with a baby’s gender as does preference.

In a soon-to-be-published paper in the Journal of Political Economy, Harvard economist Emily Oster contends that the gender disparity in some parts of Asia is linked to the prevalence of the hepatitis B virus.

Using recent vaccination data, Oster found that mothers infected with hepatitis B were 1.5 times more likely to give birth to a boy. Though that is not the sole explanation for the imbalance, Oster says, the liver disease may account for nearly half of the so-called missing girl population previously attributed primarily to gender discrimination.

The issue of sex ratio is so complex and intertwined with the social and economic context of a particular region that some experts are skeptical that it can be explained away by a simple virus.

But Oster’s findings, if substantiated, could help explain the growing gender disparity in China, where boys are prized for carrying on the family name and providing for aging parents.

The country’s strict one-child policy has led to the abortion of healthy female fetuses and the killing of baby girls in some areas, despite laws forbidding such practices. The shifting demographics also threaten to turn China into a country of surplus bachelors who desperately outnumber potential brides.

In 1982, China’s sex ratio was about 100 girls to 109 boys. By 2000, it was 100 girls to 117 boys. The current figure is closer to 100 to 119. In some rural areas, the ratio has hit 100 to 133.

Oster says as much as 75% of the disparity in China could be due to hepatitis B, which infects as much as 15% of the population.

In India and Pakistan, however, the liver disease accounted for less than 20% of gender imbalance, the study says.

Oster was inspired to examine the relationship between hepatitis and gender after reading a book by Baruch Blumberg, who discovered the hepatitis B virus. The book explored the link in Greece and Greenland. Oster then looked at vaccination records in Alaska and found that the ratio of boys to girls was higher before the availability of hepatitis vaccine and near normal after the shots were introduced.

Oster studied vaccination records in other countries, primarily in Asia, where the birthrate of boys is unusually high. Her research suggests that the virus is linked to the poor survival rate of female embryos.

Siri Tellier, a United Nations Population Fund representative in China, pointed to two factors that undercut the findings: In Tibet, where hepatitis B is widespread, the birthrate of males is relatively low. And in China, the gender ratio among firstborns is close to normal, but the gap widens for subsequent children. Tellier says the girl-boy ratio for second births is 100 to 152.

“This makes it very unlikely that the sex ratio is caused by hepatitis B -- otherwise why would it go up by birth order?” Tellier said. “I’m not saying it’s impossible, but you definitely need further discussion on this before you decide that’s the final causal mechanism.”

Observers say that even if hepatitis is proved to be a key culprit in the imbalance, it does not change the fact that gender bias exists and will continue to influence reproductive choices.

“The only way to solve the problem of sex ratio differences is to get rid of gender discrimination in the system and guarantee gender equality,” said Lu Jiehua, a population studies expert at Peking University. “That’s the only way to reduce the pressure on farmers and low-income people to pick boys over girls.”