A friend asked at lunch the other day what I thought accounted for the phenomenal success of Freakonomics. A good question. The book appeared in April 2005; only last week did it fall off The New York Times best seller list for the first time. It has sold more the 2 million copies in the United States alone (there are 30 foreign editions published or in the works). That's three times as many copies sold in 18 months as Milton Friedman's