Modern psychologists use the word temperament to refer to a person's predisposition to behave in a certain way. Temperament refers to the style rather than to the content of behavior. It concerns whether one generally seeks excitement or sits alone whether one is usually highly expressive or inhibited, joyous or sullen.

Personality, on the other hand, is the full-blown complex set of traits that distinguish an individual. We would need to list hundreds of particulars in order to describe a person we know well: slow to anger, tough when provoked, generally calm, reads the sports pages, generous, athletic.

Each of us forms our own theories of individuality. We use these ideas not only to characterize other people ("Joe is an honest person.") but also to predict ("Morgan is generous, so I'll ask her if I can borrow a ten."). Belief in the importance of personality traits rests on the assumption that knowing a person's characteristics will tell us something about how that individual will behave. Trying to predict how anyone will act, however, is a bit like trying to predict the weather in some random month from now in a random place at a random time.

We tend to see how a person behaves and then attribute that behavior to a basic personality trait. Most of us assume that these traits consistently influence others' behavior. We think that someone who is honest never lies to friends, does not cheat, and doesn't steal. But we are often wrong. Some psychologists question whether people really are consistent enough across situations to make knowledge of personality useful in predicting behavior. This very complexity, however, is probably what keeps most of us interested in each other, as we puzzle over how to piece together an accurate picture of another person.