One of the most interesting facts reported was on the gender skew based on birth order:
The sex ratio at birth for first order births was slightly high in cities and towns but was within normal limits in rural areas. However, the ratio rose very steeply for second and higher order births in cities 138 (132 to 144), towns 137 (131 to 143), and rural areas 146 (143 to 149), although the numbers of second order births in cities were low. These rises were consistent across all provinces, except Tibet, with very high figures for second births in Anhui (190, 176 to 205) and Jiangsu (192, 174 to 212). For third births, the sex ratio rose to over 200 in four provinces, although third births accounted for only 4.3% of the total.
The researchers have concluded tentatively that sex-selective abortion, though illegal in China, is likely the cause of the sex imbalance. They think that it is sex-selective abortion, rather than unregistered girls, that skew the figures, and that sex-selective abortion is also the culprit in the skew in second births:
The researchers also conclude that provinces which allow couples a second child after a girl -- rather than provinces that strictly limit families to one child or provinces that don't enforce the one child policy -- result in the highest sex ratios for second order births and the overall highest sex ratios.
[T]he dramatic increase in sex ratio with second births that our data document, shows that couples are selecting to ensure a boy, the so called "at least one son practice." In urban areas where few couples are allowed a second child, the high sex ratio for first order births (110, 95% confidence interval 107 to 113) suggests some sex selection occurring with the only child. This pattern of dramatic increases in sex ratios for second children is not unique to China. In both South Korea and parts of India, where overall sex ratios are high, the sex ratio increases dramatically for second and higher order births, which has been attributed to sex selective abortion, as couples try to ensure the birth of male offspring while limiting their family size.
Interesting to see hard figures to support what many have reported anecdotally. And it's an important part of the story for abandoned Chinese girls, since second daughters are more likely to be abandoned than are first daughters. It looks like they are far less likely to be born in the first place.
I've long wondered whether the reported gender skews in China were taking into account unregistered girls, so it was good to see a study that corrected for that phenomenon, but it's still unclear whether children in SWIs are counted in census surveys. Are they "unregistered?" Anyone have any specific information about whether they are being counted?