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AS A child, Wendy Johnson was extremely shy. 鈥淥ne of my report cards said: 鈥榃endy is so shy, it鈥檚 painful to watch!鈥� She鈥檚 not like that now. 鈥淚 am definitely a person who learned to overcome overt shyness,鈥� says Johnson, a psychologist at the University of Edinburgh, UK. She says shyness is an indicator of a low level of extroversion, a key measure of personality, which she studies. So does this mean Johnson has changed her personality? Undoubtedly, she says.

That answer might surprise you. Most of us consider our personality to be an integral and unchanging part of who we are 鈥� perhaps the essence of that thing we call the self. In 1887, psychologist William James went so far as to argue that it becomes 鈥渟et like plaster鈥� by the age of 30. His idea stuck. Psychologists have long debated how to measure personality, settling eventually on the 鈥渂ig five鈥� traits (see 鈥�What are you like?鈥�). But at least they were able to agree on a definition: personality refers to an individual鈥檚 thought patterns and behaviours, which tend to persist over time. Now mounting evidence is undermining that notion. Personality is far more mutable than we thought. That may be a little unsettling. But it鈥檚 also good news for the almost 90 per cent of us who .

There鈥檚 no doubt that personality is partly genetic. . Newborn babies don鈥檛 have personalities as such, but do have characteristic ways of behaving and reacting, something psychologists call 鈥渢emperament鈥�. This includes persistence in the face of setbacks, and 鈥渞eactivity鈥�. Very reactive babies are shy and avoid novel situations. Temperament is often viewed as the biological basis of personality, but it is far from innate. Genes and environment interact to influence it even before birth. For example, there鈥檚 evidence that mothers who are stressed during pregnancy are more likely to have an anxious child.

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Experiences in childhood also shape our personalities. Research shows that when surrounded by other kids with these traits. Parental behaviour has an impact too. Jerome Kagan at Harvard University, who pioneered research on reactivity, found that if parents encourage reactive infants to be sociable and bold, they grow up to be less shy and fearful. This might help explain why temperament doesn鈥檛 always predict later ratings on big five traits. Smiley babies don鈥檛 necessarily go on to be extrovert, for example. And Kagan found that only 25 per cent of highly reactive infants were extremely .

, genes seem to account for about 40 per cent of the variation in each of the big five traits 鈥� at the level of the general population, rather than for any individual. But it would be wrong to assume that genes and the environment are acting independently to influence personality. 鈥淭hey never are,鈥� says Johnson. Another way to consider the genetic influence is to ask how many genes have been identified that are clearly linked with any one of the five traits. 鈥淶ero,鈥� says Johnson. 鈥淓very time we find something that looks like it鈥檚 associated with one, well, it鈥檚 associated with a whole pile of other traits, too.鈥�

In other words, genes and environment interact in complex ways to shape our personality. But where鈥檚 the evidence that this process stops when we reach 30? Well, there isn鈥檛 any. In fact, once psychologists got over the intuitive appeal of the idea, they began finding plenty to contradict it.

The first direct challenge came from studies following adults over long periods. Research published in 2003 revealed that , we tend to become significantly more agreeable, conscientious and emotionally stable. An intriguing link with physical changes in our ageing brains has emerged recently. Roberta Riccelli at Magna Graecia University in Catanzaro, Italy, and her colleagues found an association between greater neuroticism and a thicker, less wrinkly cortex. With age, the cortex becomes thinner and folds more, the team notes. It remains to be seen whether there鈥檚 a causal connection.

This year, Petar Milojev at Massey University in New Zealand and his colleagues published a study refining the picture of . They looked at data on nearly 4000 people aged 20 to 80 and found that personality is least stable in young adulthood, and also after about age 60. This makes sense if changes in the environment can influence personality, says Milojev, because young and older adulthood are periods when people tend to experience maximum change.

What鈥檚 becoming clear is that major life events can have long-lasting impacts on personality at any age. Embarking on . Going through a divorce makes , while men become less conscientious, according to one US study. Although a German study has found that .

The effects of becoming unemployed can be more dramatic. We already know that it has a major effect on well-being. 鈥淚t鈥檚 probably the life event that has the biggest impact,鈥� says Christopher Boyce at the University of Stirling, UK. And he has found that the impact of unemployment on personality is twice as big, with . . Nathan Hudson, now at Michigan State University in East Lansing, found that people who have invested heavily in their jobs tend to show an increase in conscientiousness. Even moving to a new town or country might influence your personality 鈥� , for example, while .

鈥淧ersonality is least stable in young adulthood, and also after about age 60鈥�

Other factors can lead to more transient shifts in personality. For example, Sebastian Schindler at the University of Bielefeld in Germany has found that inducing a sad mood in volunteers in the lab caused a 10 per cent increase in their neuroticism ratings, and a decline in extroversion of between 2 and 4 per cent. 鈥淭hese figures seem low, but remember we are talking about personality, which is defined as 鈥榮table and long-lasting鈥�,鈥� he says.

It鈥檚 not entirely surprising that your personality 鈥� your general patterns of thought and behaviour 鈥� is influenced by your emotions. However, the extent to which environmental factors shape our dispositions over a lifetime is remarkable. In work published last year, Johnson and her colleagues compared results from personality tests taken by people when they were aged 14 and again at 77. 鈥淭hough measured crudely, we couldn鈥檛 find any evidence for stability in individual personality characteristics,鈥� she says.

That鈥檚 not to say we are at the mercy of capricious environmental influences. There is also growing evidence that we can actively shape our own personality 鈥� and would benefit from doing so. In particular, there鈥檚 plenty of research linking low levels of neuroticism and high levels of extroversion with better health and subjective well-being. What鈥檚 more, Hudson has found that 87 per cent of people would in their ratings on each of the big five traits. 鈥淭he data are compatible with the intuitive notion that most people would ideally like to be a little more energetic and at ease socially, kinder and more loving towards others, more responsible and productive, less stressed and irritable, or more creative and thoughtful,鈥� he says.

. In January, Brent Roberts at the University of Illinois and his colleagues reported that four to eight weeks of psychotherapy (the specific type didn鈥檛 seem to matter) can bring about changes in personality, most notably an increase in extroversion and a substantial decrease in neuroticism.

Then there are psychedelic substances. Research at Johns Hopkins University School of Medicine has shown that can have a big effect on personality. Even a single session increased the openness scores of volunteers. One year on, their heightened openness remained.

But magic mushrooms are illegal in many countries and psychotherapy is not for everyone. So what to do? Is there a DIY route to a different you? To explore this question, Hudson and Chris Fraley at the University of Illinois at Urbana-Champaign asked a group of student volunteers to write down the traits they would like to alter and then helped them identify specific behavioural changes that would help them achieve their goals. Broad goals are not very effective, Hudson notes.

Four months later, the including increases in extroversion and conscientiousness and lower neuroticism. What鈥檚 more, there was a virtuous circle in which alterations in behaviour led to changes in traits, which in turn led to alterations in behaviour.

Psychologists continue to debate the extent to which personality is plastic in adulthood, but there is now no doubt that it can and does change. And that鈥檚 good news for all of us. Knowing that you are not 鈥渟et鈥� by 30 is empowering. 鈥淵ou can think: I鈥檓 not stuck with who I am. I can change,鈥� says Boyce.

This article appeared in print under the headline 鈥淢ercurial you鈥�