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.