The economic analysis of crime starts with one simple assumption: Criminals are rational. A mugger is a mugger for the same reason I am a professor — because that profession makes him better off, by his own standards, than any other alternative available to him. Here, as elsewhere in economics, the assumption of rationality does not imply that muggers (or economics professors) calculate the costs and benefits of available alternatives to seventeen decimal places — merely that they tend to choose the one that best achieves their objectives.

If muggers are rational, we do not have to make mugging impossible in order to prevent it, merely unprofitable. If the benefits of a profession decrease or its costs increase, fewer people will enter it — whether the profession is plumbing or burglary. If little old ladies start carrying pistols in their purses, so that one mugging in ten puts the mugger in the hospital or the morgue, the number of muggers will decrease drastically — not because they have all been shot but because most will have switched to safer ways of making a living.