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What makes one person different from another? In everyday life, personality is one of the most important ways we have of distinguishing Sue from Sarah, or Bob from Bill. But what makes Sue Sue, and Bob Bob, with all their individual quirks? There are enough theories and research projects to fill a small library.

Enter Judith Rich Harris. Her first book, The Nurture Assumption, caused an international storm in 1998 when she claimed that, barring downright abuse or neglect, parents have no direct long-term effect on the personality, intelligence or mental health of their children, and that children鈥檚 peer groups are much more likely to influence how kids turn out than their parents.

The storm rages to this day. But despite chronic poor health as a result of two autoimmune diseases, systemic sclerosis and lupus, Harris has continued battling for her theories and working on new ones.

As that rare and exotic beast, an independent investigator, she has acquired influential friends (the evolutionary psychologist Steven Pinker among them), rather than formal affiliations.

In her latest book, No Two Alike, Harris has moved on logically from what shapes children to one of the biggest mysteries in psychology 鈥� what makes our personalities? Once again, she looks likely to generate a lot of heat with her ideas about how we become not like our parents, or our peers, or even our identical twin, but like, well, ourselves.

Liz Else talked to her recently.

What鈥檚 different about your theory of personality?

Previously, researchers tried to solve the mystery of personality development with simple theories: it all boils down to conditioning, or how your mother treated you while you were a baby, or whether you were the first or second child in your family. Simple theories simply couldn鈥檛 do the job, however. An adequate theory of personality development has to take account of the complexity of the human mind.

My theory is the first to take account of the surprising findings by two separate, and sometimes mutually antagonistic, sets of researchers: evolutionary psychologists and behavioural geneticists. The behavioural geneticists discovered that though genes work pretty much as expected, the environment works in an altogether unforeseen and mysterious way. Evolutionary psychologists, on the other hand, have shown that introspection is not a reliable guide to how the mind works. Introspection makes us think that there is a unitary 鈥渕e鈥� in there, peering out through the windows of the eyes and issuing orders, but research has shown that the mind is not unitary: different tasks are handled by different modules or mechanisms or systems.

Where do we start trying to unravel personality?

Identical twins are a perfect test case for theories of personality development. If a theory can鈥檛 explain the differences between identical twins, then it cannot explain environmental effects on personality. Even identical twins brought up in the same home have different personalities.

Take Ladan and Laleh Bijani, conjoined twins from Iran who died in 2003 during an attempt to separate them. They were identical twins who had spent their entire 29 years joined at the head. And yet Ladan, the more outspoken of the pair, told journalists before the surgery: 鈥淲e are two completely separate individuals who are stuck to each other. We have different world views, we have different lifestyles, we think very differently about issues.鈥� Why did Ladan and Laleh have different personalities? We know the answer can鈥檛 be in their genes because they had the same genes.

鈥淭he striking thing is how much human behaviour varies鈥�

So where did you start looking for inspiration?

One of my inspirations was an article in Science about self-organised systems in insects. A colony of ants, for example, can be seen as a self-organised system. No overseer tells the ants what to do, and yet all the jobs get done. The larvae get cared for, the damaged nest gets repaired. The system works in such a way that if one ant carries out a particular job, it becomes less likely that another ant will attempt that job because it no longer needs doing. The result is what economists call 鈥渄ivision of labour鈥�.

What about humans?

Self-organisation also produces division of labour in human groups. Each individual looks for something to specialise in, his or her own niche in the group. If one niche is occupied, the individual will seek another. This process increases the differences even between identical twins, because once they鈥檝e chosen different specialities, a circular mechanism (not unlike that proposed by James Flynn and William Dickens to account for increases in intelligence or talent during childhood) kicks in and causes small initial differences between them to widen.

Although identical twins look very much alike, people who know them well will distinguish between them. They might, for example, address more questions and comments to one twin than the other 鈥� perhaps by chance at first. But the consequence of this small asymmetry is that the twin who is addressed more often will do more talking than the other twin, which will cause people who know them to address still more of their questions and comments to that twin. The result, over time, will be one outspoken twin and one quieter one 鈥� like Ladan and Laleh Bijani.

What role do the various systems or mechanisms of the mind play in this?

Separate systems of the mind are responsible for carrying out the three major social tasks of childhood. First, children have to form and maintain satisfactory relationships with the various people in their lives: the relationship system. Second, they have to adapt to the group or society to which they belong, which means learning how to behave in a way that is acceptable to the other members: the socialisation system. And third, they have to develop a workable strategy for competing with the others who will be their rivals in adulthood: the status system.

In humans, competition for status is multidimensional: someone who cannot be the strongest or the prettiest might still have a chance of being the smartest or the funniest. Children have to learn while they are growing up not to place their bets on contests they have no hope of winning but to direct their efforts towards more promising alternatives. This means they have to learn something about their own assets and liabilities. The acquisition of self-knowledge is one of the functions of the status system.

That sounds like you back the status system as making the biggest contribution to individual differences.

Yes. The socialisation system has the opposite effect: it makes children more similar in behaviour to their same-sex peers. Together the status system and the socialisation system explain why children become more like their peers in some ways and less like them in other ways. Children want to do what their peers do: they conform. At the same time, they want to be better than their peers: they compete. The existence of these two conflicting motives is another clue that two different mental systems are involved. As for the relationship system, it produces no long-term changes in behaviour, only temporary modifications, tailored to the situation.

Why do we need theories about personality?

We hear a lot about 鈥渉uman nature鈥�, a phrase popularised by evolutionary psychologists. But the striking thing about humans is how much their behaviour varies. Even in a situation that demands a certain amount of conformity, like a classroom, some people sit quietly while others fidget; some are eager to answer questions, others stammer and blush when quizzed. So it isn鈥檛 enough to know about human nature, about the behaviours and propensities that humans have in common. If we want to understand and ultimately to predict behaviour, we also need to know something about the particular person whose behaviour we wish to predict. We need to know something about his or her personality.