Is hepatitis B the source of China’s gender imbalance?

Harvard Economist Emily Oster makes an intriguing and well-argued “case”:http://www.people.fas.harvard.edu/~eoster/hepb.pdf that Hepatitis B infection offers an explanation of what happened to about half of the ‘misssing’ 100 million girl babies in Asia and the middle-East: they weren’t born. Buther own argument makes me wonder whether the explanation fits China’s “disturbing demographic”:http://www.inquit.com/article/278/demographic-consequences-of-choosing-male-children Slate magazine carries a report recalling Amartya Sen’s initial essay “arguing”:http://ucatlas.ucsc.edu/gender/Sen100M.html that “more than one hundred milllion women are miissing” in Asia and pointing to Oster’s paper: bq. If you believe Oster’s numbers‚Äîand as they are presented in a soon-to-be-published paper, they are extremely compelling‚Äîthen her detective work has established the fate of roughly 50 million of Amartya Sen’s missing women. Her discovery hardly means that Sen was wrong to cry misogyny, at least in some parts of the world: While Oster found, for instance, that Hepatitis B can account for roughly 75 percent of the missing women in China, it can account for less than 20 percent of the boy-girl gap in Sen’s native India.”Slate”:http://www.slate.com/id/2119402/ The “paper”:http://www.people.fas.harvard.edu/~eoster/hepb.pdf itself is highly readable and tightly-reasoned: a fascinating example of a clever economist following a hunch. I think Oster has made a convincing case that puts a sharp focus on the practices and policies of India and Pakistan, especially, where Hep.B. infection rates do not seem to explain much of the fall in female birth ratio. But I’m still troubled by her conclusion on China. Briefly, she argues that known Hep.B. infection rates in China can explain “about 75%” of the very high male/female birth ratio in China which she quotes at an average of 1.07:1. But the marginal ratio is much higher than this long-run average: Chinese census data (reported in the “Economist”:http://economist.com/displayStory.cfm?story_id=179826 magazine) put even higher at 1.13:1 in 1989. The marginal rate has exceeded the average rate since the late 1980s, according to the Economist’s data. Oster acknowledges in a footnote (p 3 of her paper) that the one-child policy—and a preference for male children—probably increases the ratio of male children at birth above the rate that she partly explains as the expected impact of the known rate of Hep.B. infection in China. But she gives this no further consideration in her paper. What makes the observation more crucial, however, to an evaluation of her argument for the impact of Hep.B. is an important ‘bonus’ observation that she saves for her conclusion. Here, I need to quote a couple of paragraphs from her paper:

It is interesting to consider… that there may be scope for interactions between parental preferences and HBV. In particular, the combination of a gender-biased “stopping rule” for fertility and high prevalence of HBV could have important implications for population growth and gender balance within families. A gender-biased stopping rule in a homogenous population will not alter the overall population sex ratio. However, the same stopping rule in a population affected by HBV can both mitigate the effect of HBV on sex ratios and affect population growth. Imagine that 20% of the population are carriers of HBV and parents use a male-biased stopping rule in which they stop after the first boy. Then, in the first birth, 60% of the HBV carriers and 51% of the non-carriers will give birth to boys. This implies that more people stop having children after the first birth than would be implied by a non-HBV model. However, the population of people going on to have a second birth will contain only 16% HBV carriers. This means that, on average, later births will be more female biased. The overall interaction has two effects: slower population growth rate and more all-female families than would be predicted by the overall sex ratio at birth. In addition, if this is an important effect, the overall sex ratio in the population will be lower than would be produced naturally by HBV, since fertility responds endogenously.

In other words, the male bias introduced into the distribution by the Hep.B. infection is self righting where there is a gender-preference that leads to a ‘stopping rule’ after one male child is born. Over a relatively short period, when the first-cohort Hep.B. infected parents have produced the required male child, a female bias in subsequent births should ‘mitigate’ the average outcome. But this ‘mitigation’ hasn’t happened in China, as Oster’s own graphs indicate (even among 35 year-olds, the male/female ratio is almost 1.05).

There are two potential explanations: # Oster is wrong about Hep.B. and, hence, there is no self-righting effect
# Something else is affecting the outcome I am convinced that Oster is onto something in her paper: her arguments and cases look plausible to me. I think that (2) is more likely. The “something else” is probably that the ‘non-infected’ parents also have the same ‘stopping rule’ as the infected parents, but aren’t able to fulfill their preferences quite as quickly. That, in turn, would mean that the numbers of girl babies ‘missing’ in China is, indeed, much bigger than Oster’s explanation allows.

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