If you’ve ever wondered whether psychotherapy achieves meaningful, long-term change in a person’s life, wonder no more: combined evidence from multiple studies suggests that it does. A meta-analysis published recently in Psychological Bulletin reports that a variety of different therapeutic techniques result in positive changes to personality, especially when it comes to neuroticism, that last over a considerable period of time.

Personality is, as your intuition might tell you, relatively stable—people who start out gregarious and adventurous tend to stay gregarious and adventurous throughout their lives. Assessments of people’s personality traits taken at different times tend to agree pretty well with each other. But that doesn’t mean personality is static: personal growth, life experiences, and age all play their part, and people’s personalities do change somewhat throughout their lives—usually for the better.

An OCEAN of change

But it can be tricky to work out precisely what is being evaluated in measures of personality like the “Big Five” of Openness to experience, Conscientiousness, Extraversion, Agreeableness, and Neuroticism ("OCEAN"). Any personality questionnaire will come up with metrics that capture both someone's stable, long-term tendencies (their traits), as well as how they are feeling in a given moment or phase in their life (their state). So, it’s not enough to find that therapy brings about personality changes—it’s also necessary to figure out how deep those changes go.

Quite a few studies have found the positive effects of therapy, but the state of the art in evidence is the meta-analysis, which combines data from multiple studies on the same topic. Brent Roberts, a researcher at the University of Illinois, Urbana-Champaign, led a team that used data from 207 studies and more than 20,000 participants to explore the effects of therapy on personality.

They found that personality change occurred and was robust for different therapy types (including cognitive behavioral therapy, psychodynamic therapy, and other kinds). It didn’t differ much between techniques, although patients who were hospitalized showed the least amount of change. Patients with anxiety disorders saw the greatest improvement, while those with eating disorders or substance abuse saw the smallest changes.

Studying the effects of therapy isn’t quite like a pharmaceutical trial—you can’t really give someone a therapy “placebo.” But you can compare people who are receiving treatment to people who are on a waiting list: waitlisted people will have similar levels of motivation to seek therapy but are missing out on the treatment itself. That makes it possible to track whether people are just getting better over time, regardless of head shrinkage, or whether it’s the therapy that’s making a difference. When the meta-analysis explored this question, it found that the therapy really was doing the work—any treatment longer than four weeks resulted in change, although there was no real added benefit after eight weeks.

Not all traits are equal

Could it be that people, jubilant and feeling good about themselves after weeks of therapy, are just more positive about everything in their life, nudging their positive personality scores higher? It’s possible, but not all personality traits saw the same effect. Any change in openness to experience wasn’t reliable across studies, while conscientiousness and agreeableness showed only small changes. Extraversion showed a larger change, and the effect on neuroticism was dramatic. Neuroticism tends to decrease from young adulthood into middle age, and “therapy lasting four or more weeks achieves half that amount of change,” the authors write. And “personality levels remained altered after more than a full year,” they note.

Aside from the evidence this provides for the merits of therapy, the results also help with our understanding of personality change. The accepted idea, the researchers write, is that personalities “typically change slowly and incrementally.” But this evidence suggests that personality change could be quite fast at times. It’s possible that people undergo periods of more dramatic change, with stasis in between, rather than experience a gradual drift over time. That’s a question that future research will need to answer, because studies looking at personality change typically haven’t surveyed people often enough to provide data on this.

Although a meta-analysis provides far more robust evidence than any study on its own, Roberts and his colleagues are quick to point out that there are still pieces of evidence missing. For instance, it’s possible that people seeking therapy are at a low point in their lives and that therapy just helps them bounce back; the evidence from the meta-analysis suggests this possibility isn’t the case, but more studies are needed to be sure.

And although Roberts and his colleagues corrected for publication bias in their analyses, there was “pervasive evidence” of this being a problem, they note. This “indicates a need for preregistered, controlled studies,” they write, conducted “by individuals who are not motivated to show the effectiveness of any given therapy.”

Psychological Bulletin, 2016. DOI: 10.1037/bul0000088 (About DOIs).