Why do people act the way they do? Many of us intuitively gravitate toward explaining human behavior in terms of personality traits: characteristic patterns of thinking, feeling and behaving that tend to be stable over time and consistent across situations.
This intuition has been a topic of fierce scientific debate since the 1960s, with some psychologists arguing that situations — not traits — are the most important causes of behavior. Some have even argued that personality traits are figments of our imagination that don't exist at all.
But in the past two decades, a large and still-growing body of research has established that personality traits are very much real, and that how people describe someone's personality accurately predicts that person's actual behavior.
The effects of personality traits on behavior are easiest to see when people are observed repeatedly across a variety of situations. On any one occasion, a person's behavior is influenced by both their personality and the situation, as well as other factors such as their current thoughts, feelings and goals. But when someone is observed in many different situations, the influence of personality on behavior is hard to miss. For example, you probably know some people who consistently (but not always) show up on time, and others who consistently run late.
We've also gained a clear sense of which personality traits are most generally useful for understanding behavior. The world's languages include many thousands of words for describing personality, but most of these can be organized in terms of the "Big Five" trait dimensions: extraversion (characterized by adjectives like outgoing, assertive and energetic vs. quiet and reserved); agreeableness (compassionate, respectful and trusting vs. uncaring and argumentative); conscientiousness (orderly, hard-working and responsible vs. disorganized and distractible); negative emotionality (prone to worry, sadness and mood swings vs. calm and emotionally resilient); and open-mindedness (intellectually curious, artistic and imaginative vs. disinterested in art, beauty and abstract ideas).