China has a very large deficit of females,<sup>1</sup> and public policies have sought hard to reduce the son preference which is widely believed to cause this. The Chinese government has sought to increase gender equity through a wide range of interventions aimed at changing people’s perception that girls are less desirable than boys, as well as to bring women firmly into public life. These interventions include, _inter alia_ , vigorous media campaigns to change ideas about gender roles and equity; legislation to enhance gender equity; and sponsoring grassroots women’s organizations.<sup>2</sup> Financial incentives to parents to raise daughters are yet another tool used to this end: they were used by some Chinese clans even in the nineteenth century (Hsu 1948), and revived recently by the Chinese government (Xinhua 2006).

The assumption underlying such policies is that a strong preference for sons is driving the imbalance in sex ratios. Recently, Emily Oster has strongly challenged this assumption, arguing that up to 75 percent of the female deficit in China can be accounted for by the high prevalence of hepatitis B infection (Oster 2005). This would suggest that instead of focusing primarily on the difficult task of altering parental preference for sons, the first plank of policy interventions should instead be the much simpler task of immunizing people against hepatitis B.

There is indeed much evidence that biological factors drive gender differentials in health outcomes.<sup>3</sup> Studies have also shown that the overall sex ratio at birth in Sweden rose by a couple of percentage points over some centuries as women’s nutritional status improved,<sup>4</sup> and it is possible that some biological factors could have a much larger impact on sex ratios at birth. We therefore consider Oster’s hypothesis carefully, reviewing the demographic and medical evidence for and against it. We conclude that the data indicate that hepatitis B infection has a very marginal effect on the proportion of “missing girls” in China. Further, we conclude that the patterns of sex ratio at birth in China (as also in other Asian countries) are difficult to reconcile with a large role of biological factors of any kind.

# **I Studies of the relationship between HBV status and sex ratios at birth**

# **_1. Evidence from micro-studies_**

Six micro-studies carried out by medical scientists researching hepatitis B found a relationship between this disease and the sex ratio of births. Blumberg (2006a) summarizes these results: “In a study in Greece, families in which either parent was a carrier of HBV had a higher ratio of boys to girls than in families where the parents (particularly the mother) had antiHBs (Table 1).... Similar studies were done in five other populations; they were consistent with the initial results.” Chahnazarian et al. (1988) summarized the results of these studies. They noted that the populations studied were very small and therefore the sex ratios fluctuate between the studies. Thus the precise magnitude of the effect is unclear, although the pattern is clear. They tried hard to dissect the data from three studies which included data on the hepatitis B infection status (HBV status) of both parents, to see whether the father’s or the mother’s carrier status had a larger effect, but concluded that “It is not clear whether hepatitis B antigen (carrier status) in the father and in the mother has differential effects on the sex ratio at birth.” Such questions have a potential relevance for the medical sciences which goes far beyond that of sex ratios at birth. For example, Blumberg (1976) points out that “This connection of HBV with sex selection may also 1