China has more boy babies, compared to girls, than would be expected from the usual biological sex ratio. Monica Das Gupta has written a quick summary of her research explaining why she attributes this to preference to sons (resulting in differential rates of abortion and, possibly, infanticide, by sex).
Link from Marshall Jevons, also some earlier discussion by me here in the context of Emily Oster, an economist who’s looked at this problem from a different perspective. Oster attributes the sex ratio to hepatitis infections but that doesn’t seem so plausible given this graph from Das Gupta:
P.S. There’s an interesting history underlying Oster’s claims and their refutations; see this post by Stephan Klasen.