Biology Reference
In-Depth Information
salient” (Oster
2006
, p. 324). To reconcile her biological explanation with the
cultural one, Oster, with her coauthor, proposed a further hypothesis: it is possible
that HBV interacts in a complicated way with birth order and the sex of previous
children (Oster et al.
2008
).
Against this background, a pair of Taiwanese economists—Ming-Jen Lin and
Ming-Ching Luoh—conducted a study that was designed mainly to test whether
Oster's further hypothesis is sustainable (Lin and Luoh
2008
). They acquired a
unique dataset of Taiwan's nationwide hepatitis B vaccination program that was
launched in July 1984. The data, more substantial than Oster's Taiwanese data in
size, are the comprehensive historical data obtained from the Hepatitis B Mass
Immunization national databank of Taiwan, which includes gender, year and month
of birth, birthplace, mother's age, birth order of the child, unique ID of the mother,
and mother's HBV status at the time of pregnancy. Lin and Luoh conducted a
regression analysis to see whether there was any significant correlation among
gender, birth order of the child, and mother's HBV status. The specification of
their regression is as follows:
Boy
¼ α þ β
1
HBsAg
þ β
2
Birth Order 2
ð Þ
þ β
3
Birth Order 3 and higher
ð
Þ
þ β
4
HBsAg
ð
Birth Order 2
Þ
þ β
5
HBsAg
ð
Birth Order 3 and higher
Þ
þ β
6
Mother Age Dummies
ð Þ
þ β
7
Child Birth Year Dummies
ð
Þ
þ β
8
Birth Township Dummies
ð
Þ
þ
ε
ð
Þ
12
:
1
Parameter
β
5
investigate whether the effect of HBV differs among different birth orders. Lin and
Luoh's argument appeals to both substantive and statistical significance. If the
testing results are that the estimated value of
β
1
measures the effect of HBV on sex ratio at birth; parameters
β
4
and
β
1
is small and statistically insignifi-
cant and if there is no significant difference between
β
5
, it then suggests not
only that HBV plays no role in determining sex ratio at birth but also that some part
of the further hypothesis—that is, the hypothesis that there are complex biological
mechanisms in operation between HBV and birth order—should be ruled out. On
the other hand, the magnitude of the values of
β
4
and
β
3
indicates whether the
offspring in higher birth orders are more likely to be male; if the magnitudes are
large enough to be significant, then the result supports the son-preference explana-
tion. The end result of Lin and Luoh's testing is that
β
2
and
β
1
is small and sometimes
insignificant; measuring the effect of HBV on sex ratio at birth, parameters
β
4
and
β
5
are small and insignificant, confirming that birth order does not amplify the
effect of HBV. Together, these two results show that the biological explanation is
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