Child car seats no help for kids over 2?

The guys who wrote Freakonomics have studied the data, and they don't think child safety seats or booster seats save lives of kids over two. Seat belts, they say, are just as safe:

In recent crashes and old ones, in big vehicles and small, in one-car crashes and multiple-vehicle crashes, there is no evidence that car seats do a better job than seat belts in saving the lives of children older than 2.

The whole article is in the New York Times Magazine.