Why Airlines Overbook Flights

Airlines long ago figured out that they can maximize revenue by overbooking flights. Yeah, they have to pay people when they bump them, but that expense is nothing compared to the cost of just selling the number of tickets that corresponds to the actual number of seats on the airplane. After all, they make money from no-shows. Who these people are that buy airline tickets and then blow off their plans at the last minute? That's what I don't understand. (via Digg)