Suppose we know someone's objective and also know that half the time that person correctly figures out how to achieve it and half the time he acts at random. Since there is generally only one right way of doing things (or perhaps a few) but very many wrong ways, the "rational" behavior can be predicted but the "irrational" behavior cannot. If we predict the person's behavior on the assumption that he is rational, we will be right half the time. If we assume he is irrational, we will almost never be right, since we still have to guess which irrational thing he will do. We are better off assuming he is rational and recognizing that we will sometimes be wrong. To put the argument more generally, the tendency to be rational is the consistent (and hence predictable) element in human behavior. The only alternative to assuming rationality (other than giving up and assuming that human behavior cannot be understood and predicted) would be a theory of irrational behavior - a theory that told us not only that someone would not always do the rational thing but also which particular irrational thing he would do. So far as I know, no satisfactory theory of that sort exists.

David Friedman, Price Theory, An Intermediate Text