which have been extensively documented. While the disease is fairly uncommon in US and Europe, it is endemic in mainland China and very common in other parts of Asia. Oster argued that this difference in disease prevalence could account for about 45% of the supposed "missing women", and even as high as 75% of the ones in mainland China. Furthermore, Oster showed that the introduction of a Hepatitis B vaccine had a lagged effect of equalizing the gender ratio towards what one would expect if other factors did not play a role. Subsequent research Oster's challenge was met with counter arguments of its own as researchers tried to sort out the available data and control for other possible confounding factors. Avraham Ebenstein questioned Oster's conclusion based on the fact that among first born children the sex ratio is close to the natural one. It is the skewed female-male ratios among second and third born children that account for the bulk of the disparity. In other words, if Hepatitis B was responsible for the skewed ratio then one would expect it to be true among all children, regardless of birth order. However, the fact that the skewness arose less among the later born than among the first born children, suggested that factors other than the disease were involved. Das Gupta pointed out that the female-male ratio changed in relation to average household income in a way that was consistent with Sen's hypothesis but not Oster's. In particular, lower household income eventually leads to a higher boy/girl ratio. Furthermore, Das Gupta documented that the gender birth order was significantly different conditional on the sex of the first child. If the first child was male, then the sex of the subsequent children tended to follow the regular, biologically determined sex pattern (boys born with probability 0.512, girls born with probability 0.488). However, if the first child was female, the subsequent children had a much higher probability of being male, indicating that conscious parental choice was involved in determining the sex of the child. Neither of these phenomena can be explained by the prevalence of hepatitis B. They are, however, consistent with Sen's contention that it is purposeful human action – in the form of selective abortion and perhaps even infanticide and female infant neglect – that is the cause of the skewed gender ratio. Oster's theory refuted Part of the difficulty in discerning