Millions of women are “missing” from such far-flung places as Greece, China, Greenland and Taiwan, and Emily Oster, Ph.D., thinks she knows why.
Oster, an economist at the University of Chicago, has teamed with Nobel Prize-winning scientist Baruch Blumberg, M.D., Ph.D., to try to better understand the mystery behind one of Blumberg’s seminal observations in the 1970s: Parents who carried HBV, he found in repeated studies in several countries, tended to have more boys than girls.
When he made the discovery, “not a great deal generally was made about it,” recalled Blumberg, who is the Hepatitis B Foundation Distinguished Scholar. Many of these cases were in cultures and societies that favored male births, he pointed out, and where girls were often neglected and women frequently lived in poorer conditions.
But Oster paid attention. Several years ago, when she was an economics graduate student at Harvard University, she was given a copy of Blumberg’s memoirs – Hepatitis B: The Hunt for a Killer Virus. She was intrigued by his observation and the potential connection.
According to Oster – now an assistant professor of economics – a number of countries such as China, India and Pakistan had many more men than would normally be expected. Some economists attributed the disparity to poor female care and living conditions, and even infanticide and selective abortions. Some authors contended that there were between 60 million and 100 million “missing women.”
Oster thought that there was more to the story. China, for example, had a particularly high rate of HBV – 10 to 15 percent of the population was infected. She wondered if this might be connected to the skewed sex ratio. Using available data, Oster examined HBV carrier rates in various populations, as well as the number of boys born relative to the number of girls in China and in populations with high rates of HBV infection. In fact, her numbers showed a male to female birth ratio of as much as 1.5 to 1 in countries where HBV was endemic, in contrast to the general 1-to-1 ratio.
She also found dramatic evidence when she examined the gender ratios before and after an HBV vaccination program in Alaska. Before vaccination, native Alaskans had a high incidence of HBV infection and a high ratio of male to female births. White Alaskans, in contrast, had little HBV and a normal boy-girl ratio. After universal vaccination, the native Alaskans’ birth ratio dropped to normal; the white Alaskans did not change. A subsequent study of a vaccination program in Taiwan showed similar results.
Using information on the prevalence of the virus by country and estimates on the effect of the virus on male-female ratios, Oster concluded that HBV could account for some 45 percent of the “missing” – or unborn – females. Reporting in the Journal of Political Economy in 2005, she showed that what Blumberg saw in his original observation “really was a relationship between HBV carrier status and offspring gender.” The work became her doctoral thesis.
“Up until Oster published her findings in 2005, the thinking had been that it would be very difficult if not impossible to study this if it were only behavioral,” said Alison Evans, Sc.D., director of Public Health Research at the Hepatitis B Foundation. “She really challenged the notion that the high rate of male births was attributed to female infanticide and more recently selective abortions of female fetuses.”
“The implications [of these findings] for the populations are potentially important,” Oster said. “An argument could be made that there is slightly less gender selection abortions and infanticide – and possibly less gender bias – than previously thought in many countries.”
“At a demographic level, there is some sort of biological effect,” said Blumberg. Yet, “there are so many other factors that could influence gender ratio in a population that a biological reason might be very difficult to pinpoint.”
The work has continued to take surprising turns. In 2006, evidence from a study of 3 million births in Taiwan by other researchers showed that a mother’s HBV infection had only a small effect on children’s gender ratios. More recently, Oster and Blumberg, in still unpublished work, used data on HBV infection and offspring sex ratios from his original studies of countries such as Greece and the Philippines to suggest that a father’s HBV infection significantly increases the male-female birth ratio.
“I’m seeing evidence from the original Blumberg paper that the uneven sex ratios are because the fathers are HBV carriers,” Oster said. She currently is surveying HBV carriers in China and asking about children’s genders. “If the data show similar effects from fathers, then a potential mechanism to explore could be the relative concentrations or health of sperm in those men who are infected versus those who are not.”
Steve Benowitz, science writer, Philadelphia
