If we define irrationality as thought, emotion, or behavior that leads to self-defeating consequences or that significantly interferes with the survival and happiness of the organism, we find that literally hundreds of major irrationalities exist in all societies and in virtually all humans in those societies. These irrationalities persist despite people's conscious determination to change; many of them oppose almost all the teachings of the individuals who follow them; they persist among highly intelligent, educated, and relatively undisturbed individuals; when people give them up, they usually replace them with other, sometimes just as extreme, irrationalities; people who strongly oppose them in principle nonetheless perpetuate them in practice; sharp insight into them or their origin hardly removes them; many of them appear to stem from autistic invention; they often seem to flow from deepseated and almost ineradicable tendencies toward human fallibility, overgeneralization, wishful thinking, gullibility, prejudice, and short-range hedonism; and they appear at least in part tied up with physiological, hereditary, and constitutional processes. Although we can as yet make no certain or unqualified claim for the biological basis of human irrationality, such a claim now has enough evidence behind it to merit serious consideration. (Author)