In a deeply disturbing essay in The New York Review of Books in 1990, the economist and future Nobel laureate Amartya Sen laid bare some brutal math. Because of biological advantages in fighting disease, women typically outnumbered men in fully developed countries, with about 105 women for every 100 men. And yet in developing countries like China and India, there were only about 94 women for every 100 men. The women seemed to have vanished into thin air. What was happening? As a first step toward unraveling the mystery, Sen decided to compute how many women would have been alive in parts of Asia and North Africa had their countries' sex ratios matched those of the developed West. The math shocked the world: more than 100 million women were missing. Skip to next paragraph Enlarge

Sen did not tell sensational stories about newborn girls being left out in the cold or of freezing buckets of water by the birthing bed. He simply wrote that his numbers told "a terrible story of inequality and neglect leading to the excess mortality of women." But his calculations helped shape a widespread assumption that misogynistic abuses were rife in Eastern societies — ranging from infanticide to fatally neglectful health and dietary care. While the 100 million figure was subsequently lowered to 60 million by the Princeton demographer Ansley J. Coale, Sen's findings continue to guide the work of governments and NGO's to this day.

But what if something else, beyond a preference for sons, was responsible for the "missing women"? In the summer of 2004, a 24-year-old Harvard graduate student in economics named Emily Oster was reading Baruch S. Blumberg's book "Hepatitis B: The Hunt for a Killer Virus." Oster, a tireless number-cruncher, has published research on everything from the decision-making of Powerball players to the correlation between witch trials and rotten weather in Medieval Europe. She was intrigued by several small-scale studies in the Blumberg book that suggested that if either parent was a carrier of the hepatitis B virus, the couple were more likely to have male children. What if nature, wondered Oster, and not the lack of nurture, was behind Sen's 100 million missing women?

At the National Bureau of Economic Research in Cambridge, Oster began searching for data on hepatitis B and sex ratios. She found that among Alaska natives in the 1970's, there were both a high rate of hepatitis B and an overwhelming number of boys born. With the help of statisticians, she confirmed that when the hepatitis B vaccination became available in Alaska in the early 80's, the gender balance was restored. Inspired, Oster crunched the data available for hepatitis B in China, India, Pakistan, Egypt and Bangladesh. Those countries with higher rates of hepatitis B in the 80's, she found, also tended to be the countries with the highest number of missing women. She decided that she had the fuel for what was bound to be a controversial paper.

In December, Oster unveiled her research in The Journal of Political Economy. Working under the assumption that carriers of hepatitis B had 1.5 boys for every 1 girl, she concluded that "hepatitis B can account for about 45 percent of the 'missing women': around 75 percent in China, between 20 and 50 percent in Egypt and western Asia, and under 20 percent in India, Bangladesh, Pakistan and Nepal."

Her figures, intriguing as they are, still don't solve the entire mystery: even accounting for hepatitis B, 33 million female births did not occur. Clearly, there are other explanations for the missing women. Several economists have suggested that Oster's research illustrates how seemingly tedious number-crunching can significantly challenge cultural myths. Yet others fear that such findings are based on flimsy data and could help governments turn a blind eye to gender discrimination. Amartya Sen argues that when it comes to hepatitis B and the gender gap, Oster is "saying that a connection exists, rather than testing it"; he suggests a more thorough statistical analysis that would take closer account of regional differences and variation over time as well as the notable fact that when families already have a child, their additional children are especially likely to be boys. Siri Tellier, the United Nations Population Fund representative in China, warns, "We should not draw shaky conclusions, which could lead to ill-advised action."

It's true that Oster's research might lead some to play down the extent of Asia's gender troubles. Yet she stresses that her work applies only to the 1980's — when the initial missing-women calculations were made and before the hepatitis B vaccine was widely distributed. You might expect that once it was distributed, the vaccine would have reversed the sex ratio imbalance — as it did in Alaska. But in fact, the male-female imbalance in Asia has only become worse in recent years. The chief reason, it seems, is ultrasound: if a fetus is female, she is more likely to be sex-selectively aborted. In a 2003 publication, Sen himself draws attention to a large shift since his 1990 essay; while the female disadvantage in mortality has in fact been reduced, "this has been counterbalanced by a new female disadvantage — that in natality — through sex-specific abortions aimed against the female fetus." Though India has made efforts to ban such abortions, they remain common. Ena Singh, the assistant representative of the U.N. Population Fund in that country, describes the misuse of ultrasound as "an unholy alliance between tradition and technology." If Oster's virus theory is correct, then the impact of ultrasound throughout the region may have been even greater than previously thought.

The numbers are striking. A 2005 article in The Journal of the American Medical Association says that more than two decades of China's one-child policy has led to a current sex ratio imbalance of 117 boys born for every 100 girls. While Japan breaks the trend with signs of girl preference, countries like Vietnam are joining the ranks of the girl avoiders: in one province there are 128 boys for every 100 girls. Such numbers have led analysts to worry about rising violence, more trafficking of women for marriage and more prostitution. Public health agencies are striving to avert demographic disaster, pushing for a more effective regulation of ultrasound and reducing school fees for girls, among other measures. The Chinese province of Hainan has gone so far as to provide larger housing subsidies to families with a daughter rather than a son. Still, population experts caution that valuing girls may take a cultural sea change. Perhaps the more than 20 million lonely, surplus men predicted in China in 2020 will be the ones to bring it about. Eve Conant has written about foreign policy, science and health for Newsweek and other magazines.

Copyright 2006 The New York Times Company. Reprinted from The New York Times Magazine of Sunday, February 12, 2006.