Personality is the pattern of thoughts, feelings and behaviors unique to a person. People tend to think of personality as fixed. But according to psychologists, that's not how it works. "Personality is a developmental phenomenon. It's not just a static thing that you're stuck with and can't get over," said Brent Roberts, a psychologist at the University of Illinois at Urbana-Champaign.

That's not to say that you're a different person each day you wake up. In the short term, change can be nearly imperceptible, Roberts told Live Science. Longitudinal studies, in which researchers survey the personalities of participants regularly over many years, suggest that our personality is actually stable on shorter time scales.

In one study, published in 2000 in the journal Psychological Bulletin, researchers analyzed the results of 152 longitudinal studies on personality, which followed participants ranging in age from childhood to their early 70s. Each of these studies measured trends in the Big Five personality traits. This cluster of traits, which include extroversion, agreeableness, conscientiousness, openness to experience, and neuroticism, are a mainstay of personality research. The researchers found that individuals' levels of each personality trait, relative to other participants, tended to stay consistent within each decade of life.

That pattern of consistency begins around age 3, and perhaps even earlier, said Brent Donnellan, professor and chair of psychology at Michigan State University. When psychologists study children, they don't measure personality traits in the same way they do for adults. Instead, they look at temperament — the intensity of a person's reactions to the world. We come into the world with unique temperaments, and research suggests that our temperaments as children — for example, whether we're easy going or prone to temper tantrums, eager or more reluctant to approach strangers — correspond to adult personality traits.

But those decades add up. Throughout all those years, our personality is still changing, but slowly, Roberts said. "It's something that's subtle," he added. You don't notice it on that five-to-10-year time scale, but in the long term, it becomes pronounced. In 1960, psychologists surveyed over 440,000 high school students. The students answered questions about everything from how they reacted to emotional situations to how efficiently they got work done. Fifty years later, researchers tracked down 1,952 of these former students and gave them the same survey. The results, published in 2018 in the Journal of Personality and Social Psychology, found that in their 60s, participants scored much higher than they had as teenagers on questions measuring calmness, self-confidence, leadership and social sensitivity.

Again and again, longitudinal studies have found similar results. Personality tends to get "better" over time. Psychologists call it "the maturity principle." People become more extraverted, emotionally stable, agreeable and conscientious as they grow older. Over the long haul, these changes are often pronounced.

Some individuals might change less than others, but in general, the maturity principle applies to everyone. That makes personality change even harder to recognize in ourselves — how your personality compares with that of your peers doesn't change as much as our overall change in personality, because everyone else is changing right along with you. "There's good evidence that the average self-control of a 30-year-old is higher than a 20-year-old," Donnellan said. "At the same time, people who are relatively self-controlled at 18 also tend to be relatively self-controlled at age 30."

So why do we change so much? Evidence suggests it's not dramatic life events, such as marriage, the birth of a child or loss of a loved one. Some psychologists actually suggest these events reinforce your personality as you bring your characteristics with you to that particular situation, Donnellan said.

Instead, changing expectations placed on us — as we adjust to university, the work force, starting a family — slowly wears us in, almost like a pair of shoes, Roberts said. "Over time you are asked in many contexts across life to do things a bit differently," he said. "There's not a user manual for how to act, but there's very clear implicit norms for how we should behave in these situations." So we adapt."