In 2000, James Heckman, along with Daniel McFadden, received the Nobel Prize in economics. Heckman won the prize for "his development of theory and methods for analyzing selective samples," highly technical work that it is difficult to explain to the layman. Nevertheless, the work rewarded by the Nobel committee has been valuable for economists' studies of many issues that laymen do care about. The main technical problem on which Heckman has spent much of his professional life involves self-selection. An economist who wants to know, for example, how male workers will respond to a higher wage rate can take microdata on wages and hours worked and find a relationship. This approach has a problem that economists have long recognized: some men will not work at all and, therefore, will not be in the data set. And, presumably, these men who do not work will be disproportionately from the group that, had they worked, would have earned low wages. They have "self-selected" out of the workforce. So the economist's estimates on the effect of wages on hours worked will be biased. How is the economist to deal with this fact if he wants to generalize from his sample to the male population in general?... MORE