The widespread effects of irrationality have been greatly underestimated

In the social sciences, particularly economics but also political science, there has been a "myth of rationality," a belief that individuals make choices that are at least quite good (if not necessarily absolutely maximal). For themselves, and hopefully also for society at large.

This is, however, almost completely untrue. People often don't know what is good for them, and even if they had any such conception, often don't act on it, but do completely otherwise.  Many difficulties (individual and societal) stem from this fact.