Since the introduction of the ultrasound in Asia, in the early 1980s, it's often been used to determine the gender of a fetus -- and, if it's female -- have an abortion. In a part of the world with big populations, these sex selection abortions have had a big, unintended consequence.The hypothesis that increasing the ratio of men to women would produce "more sex-trafficking, more AIDS, and a higher crime rate" is entirely reasonable, but like so much observational data there's a big self-selection factor here. Families and women not involved in the sex trade tend to avoid rough neighborhoods and red light districts. There's also a question about outliers -- a few very bad areas with very high male to female ratios.
Hvistendahl: I mean there are over 160 million females missing from the population in Asia, and to put that in perspective, it's more than the entire female population of the United States.
So, what happens in a world with too many men? For starters, there's more sex-trafficking, more AIDS, and a higher crime rate. In fact, if you want to know the crime rate in a given part of India, one surefire indicator is the gender ratio. The more men, the more crime. Now, the ultrasound machine didn't create these problems, but it did enable them. So, you have to wonder. What's next?
Once again, the suggestion that changing gender ratios would have significant social consequences makes perfect sense, but if you want to go from sensible suggestion to well-supported hypothesis, it's not enough to mention a fact that points in the right direction; you also have to show how other explanations (self-selection, sampling bias, etc.) can't explain away your fact. That, unfortunately, is where Freakonomics and many other economists-explain-the-world books and articles fail to make the grade.