So here's the deal : In "The Tipping Point" Galdwell says the fall in the levels of criminality in New York in the 1990s is due to an "anti-crime behavior virus" that spread through the community and lead to the miraculously fast decrease in crime

Levitt however, in his and Dubner's book "Freakonomics", says it is due to a pro-contraception law that was passed in the state 20 years ago causing all potential criminals of the 1990s never to be born in the first place in precarious conditions. Hence, crime having no labor, it decreased dramatically.

This is one of the issues where you can't answer saying that both are right. I'm voting for Levitt to be honest. I think the "epidemics" approach doesn't work in all cases. I'm still waiting to see how Galdwell digs into details to explain this in "The Tipping Point" but I'm going for Levitt right now.