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

Har­vard Econ­o­mist Emi­ly Oster makes an intrigu­ing and well-argued “case”: 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”: Slate mag­a­zine car­ries a report recall­ing Amartya Sen’s ini­tial essay “arguing”: 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”: The “paper”: 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”: 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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