The study challenges the idea that personality traits are established at birth or in childhood and remain static ever after, said Brent Roberts, psychology professor at University of Illinois in the US.
"We are not saying personality dramatically reorganises itself. You are not taking an introvert and making them into an extrovert. But this reveals that personality does develop and it can be developed," Roberts added.
A review of 207 studies involving more than 20,000 people found that those who engaged in therapeutic interventions were, on average, significantly less neurotic and a bit more extroverted after the interventions than they were beforehand.
People who are high in neuroticism tend to be more anxious, moody and depressed than others, and are more likely to perceive events as threatening, Roberts said.
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"Some clinical psychologists see neuroticism at the core of every form of psychopathology, whether it is drug and alcohol abuse, psychopathy, depression or panic disorder," he said.
"The fact that we saw the most change in neuroticism is not surprising because, for the most part, that is what therapists are there to treat," he added.
"It is very common for individuals to think of personality as that part of them that is really distinct and enduring in a way that is recognisable," he said.
While there is a lot of evidence that personality is relatively stable over the lifespan, "there never has been any evidence that people are perfectly unchanging, perfectly stable," he said.
The impetus for the new analysis was the realisation that many clinical studies assessed participants' personality traits at the beginning and end of treatment.
The study involved interventions such as supportive or psychotherapeutic counselling, cognitive-behavioural therapy, pharmacological treatment (with antidepressants, for example), hospitalisation or a combination of approaches.
Patients with anxiety disorders changed the most, researchers found and those with substance abuse problems changed the least.
The study appears in the journal Psychological Bulletin.