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There are so many common misconceptions of what having a personality actually entails. In his controversial 1968 book, Personality and Assessment, Walter Mischel, the social psychologist best known for leading the famous Stanford marshmallow experiment, argued that any notion of consistency among personalities is largely a myth. A person’s actions in a given moment depend more on their situation, he contended, than on some enduring essence of who that person is. His research suggested a correlation about .30 (out of 1.0) between one behavior and the next.

In 1979, the psychologist Seymour Epstein conducted a series of studies in which he observed people’s behaviors on several occasions—from impulsivity to happiness, to nurturance, to problem solving. He found that Mischel was right that understanding someone’s behavior at any given moment requires accounting for the situation they’re in more than anything else. But what Mischel didn’t consider, Epstein contended, was that beyond individual moments, a person’s general character still could be gleaned from the average of their many behaviors over time. In four studies, Epstein showed that when comparing behaviors over the course of two weeks, the stability of personality shattered the .30 barrier—sometimes reaching .90.

More recent research has confirmed Epstein’s findings. The best way to think about personality traits, it seems, is as various “density distributions”: Throughout the course of the day, everyone fluctuates in their “true” selves quite a bit. Acting out of character is more the rule than the exception. Yet at the same time, it still makes sense to talk about differences in personality between people, because when whole distributions of behavior are considered, there are very consistent individual differences. For instance, almost everyone craves at least some solitude throughout the day, but some need a lot more than others.

What this new understanding of personality means is that people are only introverted, agreeable, conscientious, emotionally stable, and open to new experiences to the extent that their repeated patterns say they are. Genes certainly influence patterns of behavior (we have what Brian Little refers to as a “biogenic” nature), but there is nothing sacrosanct about being a certain way. With enough adjustments to these patterns over time, it seems that people can change who they are.

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This is all good in theory, but what does it mean in practice? It’s worth noting that, even without any conscious effort, personality does appear to evolve quite a bit throughout a person’s life. Research shows that people tend to become more mature and well-adjusted as they age: The typical 65-year-old is more self-disciplined than about 85 percent of early adolescents, and more agreeable than about 75 percent of them. Social roles matter, too. As someone becomes more invested in a job, they often become more conscientious; likewise, when someone becomes more invested in a long-term relationship, they tend to become more emotionally stable and have higher self-esteem. In fact, the more the commitment is part of a person’s identity in any context, the more it appears to cause personality change.