Interesting story in Slate (a good site if you have the time). Basically one economist in the 80s postulated that the gender gap (more boys than girls) in Asia is due to maltreatment of girls. Another economist, Emily Oster, found that about half of the 'missing' women were never born, due to the surprising ability of pregnant women who have hepatitis B to give birth to boys rather than girls (a 7 or 8 to 1 preference).

Then there's the story of Baby Emily, who's parents taped her babblings in the crib, and gave the tapes to friends who are psychologists. They found these babblings were much more complex than her normal speech to her parents. It illustrates a problem of changing big theories based on studies of one child. Guess who the child was? Emily Oster.