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I am turning 30 in March, and so I was a little unnerved when I recently came across a question that researchers who study personality have apparently been trying to answer for more than a century: How much can a person really change after 30?

“You’re doomed! What you’ve got now — that’s it,” is the answer Brian Little — a lecturer in psychology at the University of Cambridge, whose new book, Me, Myself, And Us: The Science of Personality and the Art of Well-Being touches on this question — gave me, and he was only half-joking. “At least, William James would’ve said that.”

James is the groundbreaking Harvard psychologist whose 1890 text The Principles of Psychology is thought to be the first time modern psychology observed the idea that personality settles down, or stabilizes, in adulthood. “In most of us, by the age of thirty, the character has set like plaster, and will never soften again,” reads one of its most quoted lines.

In the century since James wrote these words, a bulk of empirical evidence has proven him right — to a point. As Little likes to phrase it, it’s more like our personalities are “half-plastered” by the time we enter our fourth decade: Yes, much of the way we behave is influenced by our core personality traits, which, research has shown, have a rather strong genetic component and therefore are pretty stable throughout our lives. And most research, not to mention common sense, suggests that though we change a lot in adolescence and our early twenties, these changes slow down once we enter adulthood But, Little argues, we can also choose to act against our natures. Our basic personality traits don’t really change. But that doesn’t mean we can’t change and behave in ways that are opposite to our true selves, when the situation calls for it.

When psychologists talk about personality, they are usually referring to what are called the Big Five traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism. These are our core characteristics, which generally don’t fluctuate depending on the particular mood we’re in. Some newer research in the emerging field of personality neuroscience suggests that these traits are biogenic, stemming from our genes, which helps explain why so many studies have found personality to be relatively stable. Research on identical twins, for example, shows that these five traits are largely heritable, with about 40 to 50 percent of our personality coming from our genes.

Some aspects of our personalities start to show up when we’re just days old, as Little writes in his book:

Such features of personality can be detected in the neonatal ward. If you make a loud noise near the newborns, what will they do? Some will orient toward the noise, and others will turn away. Those who are attracted to the noise end up being extraverts later in development; those who turn away are more likely to end up being introverts.

As we grow older, our personalities do evolve, of course; throughout adolescence and early adulthood, we change rapidly. One review of 152 longitudinal studies found the biggest changes in personality traits occur from childhood through the 20s. In the 30s, 40s, and 50s, we can and do still change, but these changes come more slowly, and require more effort, said Paul T. Costa Jr., scientist emeritus at the laboratory of behavioral science at the National Institutes of Health.

“If we do the proper longitudinal research — we follow the same people over time — what do we see? We see that the very big changes you see from early adolescence to early adulthood are greatly muted after 30, 35,” said Costa, whose research with National Institute of Aging psychologist Robert McCrae has lent support to the idea of personality stability. “There are still changes in personality after that, but they’re very, very modest compared to earlier phases in the life span.”

Costa and McCrae’s work has found that from about age 18 to 30, people tend to become more neurotic, more introverted, and less open to new experiences; they also tend to become more agreeable and more conscientious. After age 30, these same trends are seen, but the rate of change dips. “It’s not that personality is fixed and can’t change,” Costa said. “But it’s relatively stable and consistent. What you see at 35, 40 is what you’re going to see at 85, 90.”

This makes intuitive sense: It’s maturity he’s speaking of, really. In the body, physical maturity happens rapidly throughout childhood and adolescence, and then stabilizes once you’ve reached your adult height, for example. If at least half of personality has a biological basis, it makes sense that it would follow that developmental arc, too. And if many of our character traits are also influenced by our environment, well, think of all the changes that occur in adolescence and early adulthood: college, first jobs, first loves, frequent moves. Speaking (very) broadly, life tends to settle down in the 30s, so it makes sense that our personalities do, too. “There’s nothing magical about age 30,” Costa said. “But if you look at it from a developmental view, you can see the wisdom of [William James’s provocative statement].” In adulthood, as our lives become more constant, “it’ll take some relatively powerful change in the environment” to change our behavior.

So, decades’ worth of psychological research mostly confirms the conventional wisdom: We get old, and we get set in our ways. (As New Girl’s Nick Miller phrased it, “I like getting older. I feel like I’m finally aging into my personality.”) But let’s return to Little’s theory of the half-plastered personality (a phrase, by the way, he mischievously uses because in England, where he teaches, it’s slang for “totally drunk”). Little argues in his book that while there are certain aspects of our personality that we really can’t change — as in, if you are introverted when you are a newborn, you’ll be introverted as an 80-year-old — you can control your behavior, even if it is at odds to your core, genetically given traits. “The general idea is that, no, we are not victims of our circumstances, of our genes — we can freely choose how we behave, to a certain extent,” Little said.

We colloquially call this acting out of character, and most of the research in this area has been done on introversion and extroversion. Introverts can act like extroverts when the situation calls for it — Little himself says he’s a biogenic introvert, but from his animated demeanor in the classroom, his students would never know it. There’s not much of a trick to acting out of character; you simply act the part for a while, as long as you need to. Little regularly turns himself “on” and acts like a loud, attention-loving public speaker in front of the classroom, when his natural instinct is to be much more reserved.

But there’s a cost to pay for acting against your true nature in this way. “The autonomic nervous system gets compromised, and it can have a depleting effect on us,” Little said. I asked him to elaborate on those physiological effects for a bit. “It’s anxiety, really. Some of the indicators of autonomic arousal are, your heart starts pounding, and you have muscle tension — all the signs that would be regarded as a stress reaction.” To counteract that stress, you’ll need to revert back to your real self for a bit: To use the classic example, when introverts are forced to act extroverted at a party for a few hours, they’ll often need to “recharge” with some alone time afterwards. Similarly, naturally disagreeable people can pretend to be nicer to get along with people at work; after a while, however, “they’re going to need their restorative time, too — maybe by kickboxing in the gym or something,” Little said.

“So how long can we act out of character? We don’t know; that has not been studied yet,” he said. It’s possible that with a lot — as in, many years — of practice, people can retrain themselves to behave opposite to their true natures, with fewer of the costs. (A good therapist doesn’t hurt, either.) “There is increasing evidence for neuroplasticity in human brains, such that it is possible that changes in the neural mechanism underlying [personality] traits may be rewired with sufficient practice,” Little said later, in an email exchange. “But I don’t think we are totally malleable, and that even with years of practice of acting out of character, there may still be costs associated … They just become less ‘costly’ the more experience we have enacting them.”

As an adult, then, it seems you can change who you are — kind of. You are mostly stuck with your basic personality traits. But you can, to a certain degree, control how you express those traits. It takes practice, and may never feel automatic — it will still likely require those periods of restoration — but you can indeed train yourself to become more conscientious, more agreeable, more (or less) of whatever it is that you currently are not, at age 30 and well beyond.