Learn to leave that evil twin behind

Want to be happier and more successful? Then change your personality, says Carol Dweck, and defy the genetic lottery

James Springer and James Lewis were identical twins separated shortly after birth and reared apart. Both married and divorced women named Linda. Both then married women named Betty. One James enjoyed carpentry and the other enjoyed mechanical drawing. Both showed similar levels of sociability, flexibility and self-control on personality tests.

When Barbara and Daphne, also identical twins, were reunited at the age of 39, each arrived wearing a beige dress and a brown velvet jacket. Each had the eccentric habit of pushing up her nose, and each giggled more than anyone else she knew.

Such dramatic examples might lead us to believe that personality is encoded in our genes and impossible to change. If velvet jackets and sociability are pre-programmed, doesn't that imply that everything in between is too?

This is not the case. Far from being simply encoded in the genes, much of personality is a fluid, flexible and dynamic thing that changes over our life span and is shaped by experience. What's more, we know how to alter it.

There are two important points that studies of twins often don't focus on.

First, there's another significant part of who we are, aside from the kinds of habit, preference and temperament that the two Jameses and Barbara and Daphne shared. From infancy, humans develop beliefs about themselves and their world, and these beliefs contribute a tremendous amount to their personality. What's more, these beliefs are moulded and re-moulded by experience and can be changed by interventions.

Second, there's a part of life, perhaps the most important, concerning how well people function. Do they know how to sustain satisfying relationships, have productive careers and be the moral people they want to be? Are they developing their skills and fulfilling their potential?

My research shows that acquired beliefs play a critical role in how well people function. There is a key one that I call a person's "mind-set". Some people have a fixed mind-set, believing that their qualities, such as their intelligence, are simply permanent traits. Others have a growth mind-set, believing that their most basic qualities can be developed through their efforts and education. People with the latter are more open to learning, willing to confront challenges, able to stick to difficult tasks and bounce back from failure.

Those who possess a growth mind-set do better in school. They are better in business - they make better managers and negotiators. And they are more successful in their relationships.

However, a growth mind-set can be taught. When it is, students show increased motivation to learn and they earn higher grades. This is true in secondary school as well as at university. Managers become more able to take (and act on) negative feedback from their employees and more willing and able to foster their employees' job skills. They become better able to learn during the negotiation process and to surmount the setbacks that occur. And they become better at resolving conflicts in their personal relationships and recovering from rejections.

To acquire a growth mind-set, students can be taught that the brain is like a muscle that gets stronger with use and can be shown how to apply this idea to their schoolwork. Managers go through a workshop that teaches them that people can develop their job skills and abilities. People in relationships learn that social skills can be developed.

Such mind-sets can be learnt through the kind of praise people receive.

Ironically, when they are lauded for their intelligence they acquire a fixed mind-set. Far from raising their self-esteem, this praise makes them vulnerable. I've received many letters from people who were constantly told how clever they were as youngsters. Here is one example:

"I read about your research on the effects of over and wrongly praising children... and it was like looking through a time machine at my childhood. I was one of those oh-so-precocious children: IQ in the 180s, sensitive, addicted to praise and totally unwilling to do work if I didn't find it to my liking... Fast forward to the present me. Early thirties. Never finished college... I rarely stretch myself to try new things, and even when I do I tend to drop them within a month, about the time I've reached the limits of my natural talents."

Yet when students are praised for their effort or strategies, they take on a growth mind-set - they are eager to learn and highly resilient in the face of difficulty.

Changing other beliefs can make people less aggressive, less anxious and less prone to depression.

Perhaps it was inevitable that James Springer and James Lewis would love carpentry and mechanical drawing or that Barbara and Daphne would giggle.

But it is not inevitable that people will function poorly in important areas of their lives. Beliefs matter and can be changed. And when they are, so too is personality.

Carol Dweck is professor of psychology at Stanford University and author of Mindset , published in the US by Random House.