Scientists Once Thought Personality Was Set in Stone. They Were Wrong

This article appeared in the September/October 2021 issue of Discover magazine as "Not Set in Stone." Become a subscriber for unlimited access to our archive.

If you were a rat, the smell of cat urine would likely scare you as much as a rattlesnake’s hiss. But the rat navigating a maze for Joanne Webster, pathobiologist at London’s Royal Veterinary College, was no ordinary specimen. As it explored the labyrinth that stretched ahead — cautiously, at first, then more boldly — the rat stumbled upon a strange smell wafting from one of the corners. The scent seemed to draw it in.

What the rat found so tempting, as did many of his labmates tested in the maze, was indeed cat urine — turned, somehow, from rodent repellent to love potion. The rats had previously been infected with Toxoplasma gondii, a brain parasite that, as the scientists running the trials were learning, considerably altered the way the rats behaved. Not only did they display this potentially fatal, certainly ill-advised feline attraction, but they were also more eager to explore the maze than uninfected animals were. For better or for worse, their rat “personality” had changed towards a higher level of openness to experience.

Research shows that human personality can change, too, sometimes even dramatically — and not just because of a parasitic infection. In fact, personalities can fluctuate with age, shift because of life events and even change due to things we ingest. What’s more, we can actually alter our personalities with our own efforts, and making those changes may be simpler than we think.

Back in the 1990s, it was generally believed that adult personality was fixed. (At least, so long as the brain remains intact.) In other words, it doesn’t change much once we hit our 30s. Thanks to an influx of new studies, however, researchers are now discovering that personality is actually quite malleable over time.

If you test and re-test someone over a period of 10 years, there’s roughly a 25 percent chance that their personality traits will change, according to a study co-authored by Brent Roberts, a personality researcher at the University of Illinois Urbana-Champaign. And the longer you wait between tests, the higher the likelihood that the person’s agreeableness (being polite, kind and friendly), neuroticism (a tendency towards anxiety, self-doubt and low emotional stability) or conscientiousness (being responsible, reliable and hard-working) will undergo considerable shifts.

Old views die hard. “Even some academics hold the belief that personality doesn’t change,” says Roberts. “I’ve found this attitude to be immune to the data.”

Sometimes, though, it’s easy to overlook the various ways our personalities change over the years. Why? Because similar shifts might be happening to our peers, too.

Many of us look at our early youth with dismay, criticizing how reckless and self-centered we were. Research shows that as years pass, most people’s personality changes towards more conscientious, less neurotic and more agreeable behavior — which basically means getting along well with others. Similar patterns have been found all across the planet, from Germany and Italy to Estonia, South Korea and Japan. When you compare yourself to your peers, “simultaneously you can look to be the same person, but also be changing, because everybody is getting more conscientious or everybody is getting less neurotic,” says Roberts. Scientists call this “the maturity principle,” since most people would agree that becoming more emotionally stable and conscientious — specifically, more diligent and organized — is a good thing.

Plus, some degree of change happens to almost everyone. In one 2019 study, as many as 97.9 percent of people showed shifts in personality traits across a 50-year period. For most participants, the changes were modest. But for a few they were large (by psychological standards). “We find that some people really seem to undergo dramatic changes,” says Wiebke Bleidorn, a psychologist at University of California, Davis.

A growing batch of recent studies suggests that adult personality can be altered at any age – though, as Bleidorn and her co-authors show in a 2017 study, the magnitude of such shifts is by far the greatest for people in their 20s. And after we hit our 80s, the general pattern of change is no longer for the better. In one Scottish study which followed people for six years after the age of 81, their extroversion, agreeableness and conscientiousness all declined significantly as they grew older. “As a system, we tend to deteriorate as we get closer to dying, and personality traits are a kind of indirect indicator of our overall functioning,” says Roberts.

Some researchers argue that such personality developments — first becoming nicer and more responsible with age, then more grumpy and fickle — may be biologically built into our species, a bit like children’s developmental milestones. What seems to support this theory is the fact that personality maturation is universal across cultures. This holds true as well in observations of chimpanzees, who turn more agreeable and conscientious as they age. “It seems that there is also something like a genetic program that drives personality maturation,” says Bleidorn.

Most scientists today agree that the way our personality traits shift with time is not only simply biological, though; our environment likely plays a role, too. These changes may follow predictable life events and common experiences, like falling in love, graduating, becoming a parent and, later, retiring.

Personality tends to be at its most stable in midlife. Bleidorn believes this may be due to the fact that this is also the period when someone’s environment is more consistent. “Most people are done with having children, live in the same house, have a stable social network, maybe the same romantic partner — which may have to do with their stability in personality,” she says.

On the other hand, in early youth we experience both a lot of novelty and a lot of personality change. For example, researchers have studied how people’s first serious relationships can trigger such shifts. In a German study published in Journal of Personality and Social Psychology, adults who started their first romantic partnerships between the ages of 18 and 30 became considerably less neurotic and more conscientious compared to those who remained single. What’s more, as Bleidorn notes, “these positive effects remain even if people break up.” Getting your first job also seems to predictably change people for the better. Another 2011 German study found that entering the labor market boosts conscientiousness — we become more likely to show up on time, pay our bills, and work hard.

Meanwhile, if you happen to be a married woman, and you’ve noticed that since the wedding your husband is happy to trade his party outfits for slippers, you are not alone. One study done on more than 300 heterosexual spouses — specifically, 169 married pairs — living in Florida showed that over the first 18 months of marriage, new husbands became much less extroverted. For those who find this disappointing, the upside is that said husbands also became more conscientious — that is, more likely to deal with dirty dishes in the sink and pay bills on time. The personality of the wives, on the other hand, showed an uptick in emotional stability after their vows. For instance, they became less easily upset and less prone to worrying.

Admittedly, the evidence on how marriage changes personality is not yet as strong as it is with first jobs or first relationships. The same goes for the transition to parenthood or divorce. While some researchers have found that becoming a mom or a dad makes people more neurotic (think of all the new reasons to worry), and less conscientious (which makes sense given all the new demands on their time), most studies find no differences in personality between parents and people without kids.

Retirement, meanwhile, can sometimes cause increases in agreeableness, but not all studies show these effects. Divorce is a mixed bag, too. For some it means more extroversion; for others, less. Roberts has an explanation for such confusing results: “It’s not the event per se, but your experience of it,” he says. “It’s not the divorce, but whether it was a good or a bad divorce. That’s far more important when it comes to personality change.”

Major life events, like divorce or retirement, likely affect our personality in the first place because they change our everyday behaviors and thought patterns. If sustained, these changes in behaviors can lead to deeper, more substantial shifts in personality traits. After all, personality traits are nothing more than a sum of personality states: moment-to-moment feelings, actions and beliefs. When you work on these, you can alter your personality. Practice makes perfect, as the saying goes.

If you’d like to change at least some part of your personality, you’re in good company. A staggering 87 to 97 percent of people said they would too, in a 2014 survey with 200-plus participants published in the Journal of Research in Personality. Conscientiousness topped the wish list of desired traits. Luckily, given the proper tools, says Roberts, people can indeed alter their personality.

In one experiment, university students entered a 15-week program designed to help them change their personality traits. The whole idea was based on the oft-referenced adage to “fake it till you make it.” Each week, the students received a list of challenges through a computer program. These ranged from very easy, like saying “hello” to a cashier at a store to boost extroversion, to more difficult — say, arriving five minutes early for appointments to bolster conscientiousness. The students had to choose several tasks off a list, with the program suggesting more and more demanding options as the trial progressed. It all worked quite well. For example, those who wanted to increase their extroversion — and completed two challenges per week — managed to reliably increase that particular personality dimension over the semester.

While these changes don’t take a lifetime to accomplish, they don’t happen overnight, either. Personality interventions, on average, require 24 weeks to bring out marked changes, according to a recent analysis of over 200 studies. Neuroticism appears to be relatively malleable; in these experiments, people became considerably more emotionally stable in just four weeks.

One question that remains unanswered is how long such effects last. Roberts suspects that the best analogy here is physical exercise. “If I only try a little bit, it’s probably not going to have a lasting effect,” he says.

There are some things, however, that can change your personality with almost zero effort on your part — but with some potentially nasty side effects.

Humans are no rodents, but research indicates that T. gondii infection can affect our personalities, too. Studies show that infected people are less conscientious and more extroverted than those who are T. gondii-free. What’s more, at least 40 studies have by now linked toxoplasmosis with schizophrenia. “Almost everything we’ve seen in rats [has been] seen in people too, right down to this bizarre attraction to the smell of cat urine,” says pathobiologist Webster, one of the authors of the study on the infected rats.

T. gondii is a tiny parasite that you can pick up from eating undercooked meat or by changing a cat’s litter box. These single-celled organisms impact behavior because they’re “trying to maximize transmission from an intermediate host — such as a rodent — to its definitive host, which is the feline,” Webster says. Experiments on rodents suggest that cysts of the parasite present in the brain can alter levels of dopamine, a neurotransmitter that plays a role in reward and motivation. When a rodent is more motivated to explore and more attracted to cat urine, it’s more likely to be gobbled up by a cat, thereby spreading the parasite. Webster believes that T. gondii’s impacts on humans are just a “knock-on effect” that wasn’t intended for our species. Yet the parasite doesn’t know what host species it’s in, and will influence the behavior of rodents and humans alike.

Eating undercooked meat is probably not the smartest way to change your personality. But for many people, altering certain traits could have a considerable payoff. High levels of conscientiousness, openness to experience, emotional stability and agreeableness have all been linked to relationship satisfaction, academic achievement, job performance, and even how long we live. Being very conscientious can lower a person’s mortality risk by 44 percent — more than even the best Mediterranean diet. 

“If we could put conscientiousness into a pill, it would be the most successful pharmaceutical in history,” says Roberts.  

Getting older, going through major life milestones or building new habits aren’t the only things that can alter your personality. “Magic mushrooms” might be particularly effective at shifting certain traits. In a 2011 study published in the Journal of Psychopharmacology, their active compound, psilocybin, has been shown to make people more open to experience, with effects remaining even after a year. However, that particular study was small and has yet to be replicated. Another hallucinogenic drug, LSD, can also change brain dynamics and make people more open to experience, as demonstrated in a 2016 experiment using fMRI scans. But while research and development in psychedelic therapy has accelerated in recent years, these substances still remain largely illegal in the U.S. and most countries. — M.Z.

Marta Zaraska is a science journalist and author. Her most recent book is Growing Young: How Friendship, Optimism and Kindness Can Help You Live to 100.