Every basketball fan knows that the ball should go to the "hot hand," the player who is on a roll, who has knocked down two or three baskets in a row. Everyone knows this, that is, except Thomas Gilovich. In an analysis of Philadelphia 76er shooting records, Gilovich, an associate professor of psychology at Cornell University, discovered that a player on a hot streak is no more likely to hit the next shot than a player who has been missing. Why we are inclined to cling to unsubstantiated beliefs is the subject of his recent book, How We Know What Isn't So: The Fallibility of Human Reason in Everyday Life (Free Press).

"People misperceive randomness," says Gilovich, 38. "They tend to spot patterns where none exist. "From his home in Ithaca, N.Y., he discussed this and other mental sand traps with reporter Veronica Burns.

Are you sure there is no hot hand?

Players and coaches challenged our data, so we did a follow-up study of foul shots. In an analysis of Boston Celtic free throws over two seasons, we found that players made 75 percent of their second foul shots after making the first—and 75 percent after missing the first.

Why do people believe in the hoi hand when it doesn't exist?

Psychologists have discovered that people have faulty intuitions about what chance sequences look like. The average pro basketball player makes nearly 50 percent of his shots. If he takes 20 shots in a game, he has close to a 50-50 chance of hitting four in a row. It's the same as flipping a coin: Heads and tails even out, but there may be clusters.

The popularity of the movie JFK suggests that legions of people still believe that President Kennedy was the victim of a conspiracy. Do you dispute that on statistical grounds?

People want to believe that big events have big causes; dramatic events have to have dramatic causes. The death of a charismatic President seems to call for something more than one obscure, minor individual. So the findings of the Warren Commission just don't fit with how our minds work. For such a major event we want a major cause.

How about the belief that appearing on SPORTS ILLUSTRATED's cover is bad luck?

The SI cover jinx is an instance where we magnify a grain of truth. Typically, athletes are put on SI's cover when they have performed exceptionally. At the same time they are subject to a phenomenon we call regression to the mean, which says that extreme scores are likely to be followed by less extreme scores. But fans don't care about regression to the mean; they call it a jinx.

What causes this faulty reasoning?

People often reach a conclusion based on a single observation. Take the instance in which a previously infertile couple conceive their own biological child shortly after adopting one. This prompts people to say, "You see, it never fails." They believe that people are anxious because they can't have children, they adopt, they begin to relax, and then they have a biological child of their own. But notice that you never hear about the times when no pregnancy followed an adoption.

Is there an upside to our tendency to embrace myths and fallacies?

Not for the gambler or stock trader. But the tendency to try to find order in things can be enormously advantageous. We do not like to think that elements of our life are beyond our control, so this tendency makes the world seem a more secure place. What's more, our need for order can lead to discoveries, the advancement of knowledge, as we search for and spot patterns in things. I only ask that we remain skeptics—that when a claim is made we examine it, inspect the facts and the assumptions. The world is a complex and interesting place, all the more so if people think about it in an inquiring way.