It was Freakonomics that theorized that abortion was the cause of the drop in crime (which occurred nationwide as it was happening in NYC, though not to the same degree). Gladwell has debated with the authors of Freakonomics (primarily the economist Steven Levitt) about this.

See http://gladwell.typepad.com/gladwellcom/2006/03/thoughts_on_..., http://www.freakonomics.com/2006/03/09/malcolm-gladwell-on-t..., http://gladwell.typepad.com/gladwellcom/2006/03/levitt_and_d...