Explaining the Explanations of 100 Million Missing Women - Mechanism and Causality in Biology and Economics - page 219

families (either parent had anti-HBs) had the lowest sex ratio. Moreover, Blumberg
found that HBV carrier mothers have fewer female births and consequently
conjectured that HBV may cause a high sex ratio at birth.
The observation of the relation between the response of a parent to infection with
HBV and the gender of his or her offspring is Blumberg's “strangest” one, because
it means that HBV is gender biased (Blumberg 2002 , p. 182). In a sense, there is
biological evidence supporting Blumberg's observation. Before Blumberg pro-
posed his hypothesis, scientists had already known that HBV infections are gender
sensitive. For instance, males are more likely than females to become HBV carriers,
that is, more likely to be HBsAg(+), whereas more females than males develop anti-
HBs. As a result, Blumberg thought that he had provided a “biological explanation”
for the issue of the missing women, an issue he had learned about from a prominent
demographist, Ansley J. Coale:
[Coale] published a paper concerning the high sex ratios that have been observed in China.
He proposed that, because there is no other biological explanation, the apparent deficit in
female births in China could be a consequence of female infanticide. As might be expected,
his findings and conjectures had a big play in the media. I pointed out to him that there
might be a biological explanation. China, and particularly South China, has some of the
highest frequencies of HBV carriers in the world. If our observations on the relation
between carriers and gender of offspring in Greece and elsewhere were also valid in
China, then this might provide a biological explanation for apparent loss of female children.
(Blumberg 2002 , p. 185)
Carrier families are more likely than antibody families to have male children.
Consider the countries where the incidence of both HBV and son preference is high.
If families would like to have children until they reach the desired number of boys,
then antibody families, who are more likely to have girls, would have a lower sex
ratio and a larger family size than carrier families. The hepatitis B hypothesis hints
at one intriguing policy implication: if an HBV vaccination program were success-
ful, then there would be fewer carriers and hence a lower sex ratio at birth, with the
consequent effect on family size and the sex ratio of the population.
Drew et al. ( 1978 ) first offered a theory on the way HBV affects sex ratio. Based
on the facts that males are more likely to be HBsAg(+) and females are more likely
to be anti-HBs(+) and the statistical facts that parents with HBsAg(+) have a higher
sex ratio (more boys) and parents with anti-HBs(+) have lower sex ratio (more
girls), Drew et al. ( 1978 , p. 691) conjecture that males would recognize HBsAg as
“self” and remain HBsAg(+) persistently (i.e., become carriers). By contrast,
females would be more likely to recognize HBsAg as “foreign” and thus produce
anti-HBs. Hence, HBsAg(+) is associated with the possibility of giving birth to
more boys, and anti-HB(+) is associated with giving birth to more girls. Drew et al.
(p. 691) provided some possible explanations: either HBsAg protects male fetuses
or anti-HBs hinders fertilization by a Y-bearing sperm. But the proposed mecha-
nism is still a conjecture, as Blumberg recently admitted that there is no evidence
for such effects of HBV in utero (2007, p. 229).