From Harvard Magazine:
Like all revolutions in thought, this one began with anomalies, strange facts, odd observations that the prevailing wisdom could not explain. Casino gamblers, for instance, are willing to keep betting even while expecting to lose. People say they want to save for retirement, eat better, start exercising, quit smoking—and they mean it—but they do no such things. Victims who feel they’ve been treated poorly exact their revenge, though doing so hurts their own interests.
Such perverse facts are a direct affront to the standard model of the human actor—Economic Man—that classical and neoclassical economics have used as a foundation for decades, if not centuries. Economic Man makes logical, rational, self-interested decisions that weigh costs against benefits and maximize value and profit to himself. Economic Man is an intelligent, analytic, selfish creature who has perfect self-regulation in pursuit of his future goals and is unswayed by bodily states and feelings. And Economic Man is a marvelously convenient pawn for building academic theories. But Economic Man has one fatal flaw: he does not exist.
When we turn to actual human beings, we find, instead of robot-like logic, all manner of irrational, self-sabotaging, and even altruistic behavior.