Consider how easy it is to draw the bull’s eye after the arrow strikes. After the stock market drops, people say it was “due for a correction.” After the football game, we credit the coach if a “gutsy play” wins the game, and fault the coach for the “stupid play” if it doesn’t. After a war or an election, its outcome usually seems obvious. Although history may therefore seem like a series of inevitable events, the actual future is seldom foreseen. No one’s diary recorded, “Today the Hundred Years War began.”

This hindsight bias (also known as the I-knew-it-all-along phenomenon) is easy to demonstrate: Give half the members of a group some purported psychological finding, and give the other half an opposite result. Tell the first group, “Psychologists have found that separation weakens romantic attraction. As the saying goes, “Out of sight, out of mind.” Ask them to imagine why this might be true. Most people can, and nearly all will then view this true finding as unsurprising.

Tell the second group the opposite, “Psychologists have found that separation strengthens romantic attraction. As the saying goes, “Absence makes the heart grow fonder.” People given this untrue result can also easily imagine it, and most will also see it as unsurprising. When opposite findings both seem like common sense, there is a problem.

Such errors in our recollections and explanations show why we need psychological research. Just asking people how and why they felt or acted as they did can sometimes be misleading—not because common sense is usually wrong, but because common sense more easily describes what has happened than what will happen. As physicist Niels Bohr reportedly jested, “Prediction is very difficult, especially about the future.”

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More than 800 scholarly papers have shown hindsight bias in people young and old from across the world (Roese & Vohs, 2012). Nevertheless, Grandma’s intuition is often right. As baseball great Yogi Berra once said, “You can observe a lot by watching.” (We have Berra to thank for other gems, such as “Nobody ever comes here—it’s too crowded,” and “If the people don’t want to come out to the ballpark, nobody’s gonna stop ’em.”) Because we’re all behavior watchers, it would be surprising if many of psychology’s findings had not been foreseen. Many people believe that love breeds happiness, and they are right (we have what researchers identify as a deep “need to belong”).

Indeed, noted Daniel Gilbert, Brett Pelham, and Douglas Krull (2003), “good ideas in psychology usually have an oddly familiar quality, and the moment we encounter them we feel certain that we once came close to thinking the same thing ourselves and simply failed to write it down.” Good ideas are like good inventions: Once created, they seem obvious. (Why did it take so long for someone to invent suitcases on wheels and Post-it Notes?)

But sometimes Grandma’s intuition, informed by countless casual observations, is wrong. In other modules, we will see how research has overturned popular ideas—that familiarity breeds contempt, that dreams predict the future, and that most of us use only 10 percent of our brain. We will also see how it has surprised us with discoveries about how the brain’s chemical messengers control our moods and memories, about other animals’ abilities, and about the effects of stress on our capacity to fight disease.