Over and over again, through the course of my work and my education, I have heard Amartya Sen's charge that 100 million women are missing in the world, primarily in Asia.  This came up frequently, usually to illustrate the health impacts of discrimination against women.  In his initial publication on the "missing women", Sen proposed that the conventional wisdom that women make up the majority of the population of the world is erroneous, based on the high ratio of women to men in the populations of Europe and North America (1.06 or higher).  In other parts of the world, particularly South and West Asia, North Africa, and China, this ratio is lower (.94 or less).  Sen argued that, "[i]n these places the failure to give women medical care similar to what men get and to provide them with comparable food and social services results in fewer women surviving than would be the case if they had equal care."

Revisiting the problem of missing women in a 2003 publication, Sen made the case that improvements in healthcare in many of these societies had reduced women's mortality rates, but that ratios of women to men remainded largely unchanged because of an increasing incidence of sex-selective abortions.  India outlawed the practice of fetal sex identification, except for cases in which determining sex would be medically relevant, in an attempt to prevent sex-selective abortions from occurring.  Sen extrapolates the incidence of sex-selective abortion in Indian states from birth statistics, comparing the rate of female to male births in various states to that found in Germany, and attributing the difference to the abortion of female fetuses. 

And this is what I have heard for the past 10 years or so: that women are missing - in vast numbers in some societies - and that they are missing because women and girls in some societies receive less food and less nutritious food than men and boys, they are less likely to receive adequate medical care, and that as technology makes fetal sex determination easier and more wide-spread, female fetuses are more likely to be aborted than male fetuses. 

I just learned that a new factor has been introduced into the equation.  In 2005, Emily Oster proposed that the prevalence of Hepatitis B in a country's population must be taken into account when considering a gender gap that favours men.  According to this Slate article, Oster decided to examine the problem of the missing women after learning about research that indicated that women who carry Hepatitis B are significantly more likely to give birth to boys than to girls.  Oster's research indicates that approximately half of the 100 million "missing women" can be attributed to a high rate of Hepatitis B infection in a country's population.  Further, she discovered that as much as 75% of sex ratio difference in China can likely be explained by the effects of Hepatitis B.  Interestingly, less than 20% of the difference can be attributed to Hepatitis B in India, Pakistan and Nepal.  I have often heard India and China lumped together in discussions of attitudes toward girl children and women, based largely on their high rates of "missing women."  Obviously, Oster's research undermines the basis on which those comparisons rest.

50 million "missing women" is still a high number, and as Oster herself points out, her research likely somewhat overstates the impact of Hepatitis B on the absolute number of female infants born.  Behavioural factors and social values are still important - plenty of research has documented that discrimination against women and girls with respect to food and health care takes place in many societies and has short and long-term impacts on women's well being.  Oster's research does not indicate that preference for men and boys is not still a significant problem in many societies.  But it does add another layer to the problem (and suggests a possible partial solution - universal vaccination against Hepatitis B), and I'm interested to see how long it takes her research to be incorporated into the "conventional wisdom" about the problem of missing women.