ONE of the most striking features of life is how differently each of us approaches it. Some people love bright lights and fast cars, others will go to any length to avoid such things. A novel social situation that is a treat for one individual is a cause of nights of worry for another. Some people fret about saving for their pensions whereas others spend money as soon as, if not before, they have it. Where do such differences come from?

Circumstances undoubtedly play a part. Some human behaviour is easily explained by social context - for example, people living in a dangerous environment generally think in a more short-term way. Other variations are simply quirks. What interests psychologists such as myself most, though, are the systematic ways in which individuals differ, otherwise known personality traits.

Why do we all have different personalities? Why hasn't natural selection homed in on optimum character traits instead of allowing so much variability? As the study of personality moves to a more scientific footing, we are starting to understand the underlying neurobiology, and to see that each personality trait is beneficial in certain circumstances and costly in others. We may tend to think that some personality types are more desirable than others, but these new insights make it clear that there is no "best" personality. It really does take all sorts to make a world.

Psychologists think of personality traits as thermostats within the brain, each regulating a range of behaviours and attitudes. Some of these behaviours and attitudes seem to be linked. For example, people who are highly competitive and like loud music and travel tend also to have high sex drives. People who have a specific phobia tend to worry a lot about other things too, and they are more prone to depression. The same people who have trouble resisting the temptation to take drugs have an increased likelihood of developing problems with gambling and antisocial behaviour in general. From such correlations, we infer that there are a limited number of thermostats, each working independently. Your thermostat setting represents where you stand on a continuous scale for each personality trait.

In the past couple of decades a consensus has emerged that five key thermostats account for most of the variation in personality. These big five - extroversion, neuroticism, conscientiousness, openness and agreeableness - define five axes along which all individuals fall. Your personality, as measured by one of the many available questionnaires, consists of five different scores. Since each axis is continuous and they are independent of one another, there are countless unique personality configurations.

Where you stand on any one of the big five will show up in a particular type of situation. A person's level of extroversion manifests itself in response to pleasure-seeking or risk-taking activities, with the most introverted people seeming to find these least rewarding. When things get threatening or dangerous, or are perceived to be so, neuroticism is key - highly neurotic people will become anxious or stressed, whereas low scorers will be less affected. Conscientiousness is all about goal orientation. High scorers stick rigidly to plans or principles while low scorers are more spontaneous. Agreeableness comes to the fore in our personal relationships. Highly agreeable people pay attention to the needs and emotions of others. Low scorers are not so oriented to these cues. Openness determines a person's response to ideas. High scorers like to entertain aesthetic, metaphorical or esoteric ideas; low scorers avoid them.

Neuroscientists are now beginning to relate the big five to the brain. Each trait represents the strength or threshold of a certain type of psychological response, and that response in turn depends upon a particular, specialised neurobiological mechanism in the brain. Take neuroticism. Neuroscientists know which parts of the brain are involved in the response to threats: there's a circuit involving a structure called the amygdala. Magnetic resonance imaging scans reveal that the amygdalae of individuals who score high in neuroticism have a higher baseline level of metabolic activity compared with those of low scorers. High scorers also show greater activity in response to distressing stimuli. There is even evidence that the size of the amygdala is proportional to a person's neuroticism score (Neuroscience and Biobehavioral Reviews, vol 30, p 511). Amazingly, the simple, self-rating questionnaires used by personality psychologists actually turn out to measure something about the nervous system that can be verified through objective scientific techniques.

We can tell a very similar story for extroversion. A suite of structures in the midbrain is responsive to stimuli that betoken rewards: sweet foods, money, pictures of members of the opposite sex, addictive drugs and so on. These reward centres depend on the neurotransmitter dopamine. There is a linear relation between extroversion score and a person's physiological response to a dopamine-like drug called bromocriptine - high scorers secrete more of the hormone prolactin, among other things. This strongly suggests that extroversion is the outward manifestation of the responsiveness of the brain's reward system (Behavioral and Brain Sciences, vol 22, p 491). The reward structures could even be larger in high extroversion scorers, though we do not yet know.

Conscientiousness involves the ability to control immediate responses in the service of some longer-term goal or plan. Parts of the prefrontal cortex are implicated. We know this from studies of brain damage to this part of the brain, which can lead to previously conscientious people becoming unable to control their impulses. Brain imaging reveals that people with impulse-control problems have less activity in part of the right prefrontal cortex than others. Also, one study of boys with attention-deficit hyperactivity disorder found they had a smaller volume of this part of the prefrontal cortex than normal (Journal of the American Academy of Child and Adolescent Psychiatry, vol 43, p 332).
Complex interplay

We do not yet know which brain mechanisms underlie agreeableness, but we do know that there is a correlation between agreeableness and theory of mind - essentially, the propensity to analyse the mental states of others. There is some neurobiological evidence for the involvement of specific brain regions in theory of mind, so it should not be long before researchers begin to unpick agreeableness.

The fifth factor, openness, is the least understood. It seems to involve the ability to make distant or divergent mental associations. As yet, we have little idea of the brain mechanisms involved.

So, for each of the big five, we can tentatively identify a type of psychological response linked to a brain mechanism (see Illustration). The way these various systems are set up explains people's tendencies to behave in different ways.

Of course our thermostat settings alone do not determine our behaviour - that depends on a complex interplay between our brain and our environment - but they do influence it to the extent that if we know someone's personality, we can make a pretty good prediction of how they will respond even in a situation we have never seen them in before. What's more, these settings seem to be quite stable across an individual's lifespan, reflecting the discovery that there is a substantial genetic component to personality. This genetic element also explains why people sharing similar social and cultural backgrounds can be so different.

Intriguing and sometimes controversial research carried out in the past few years has implicated several genes in personality differences (New Scientist, 13 September 2003, p 30). For example, there are two common forms of the serotonin transporter gene which codes for the substance that removes the mood-affecting neurotransmitter serotonin from the gaps between neurons. People with at least one copy of the short form of the gene have higher neuroticism scores than people with two copies of the long form. The genetic findings also correlate with the neuroscientific ones. In the brain scanner, individuals with one or more short forms of the gene show a greater spike of amygdala activity when they are presented with a fearful face than do individuals with two long forms.

We are making great progress in understanding the mechanisms that give rise to behaviour but until recently, relatively little attention had been paid to why evolution left us with such a variety of personalities. Usually, natural selection acts to reduce variation, with the overall genetic variation in a population the result of an interplay between mutation and selection. Mutation keeps adding more variation through random alterations to genes, while selection removes variation through increased mortality or decreased reproduction of individuals with low fitness. In this evolutionary tug of war, selection should win, as long as the differences in fitness between individuals with different versions of various genes are substantial and consistent. So why is there so much variation in personality?

The key to this conundrum is that natural selection is strong, but not always consistent. How this influences the evolution of personality has become clear in recent years as researchers begin to consider rudimentary personality traits in wild animals.

Niels Dingemanse at the University of Groningen in the Netherlands is a pioneer of this approach. He and his colleagues have established that there is genetic variation underlying exploration behaviour in small birds called great tits - some individuals inherit a highly exploratory personality and others a more cautious one. The researchers measured this trait in wild great tits and related it to their survival over three years. For females, the higher the exploration score, the more likely they were to survive in 1999 and 2001. These were years when food was relatively scarce and it paid the birds to disperse further. However, in 2000, when resources were abundant, low-scoring females were more likely to survive. With food plentiful, dispersing too far was an unnecessary hazard. For males this pattern was reversed, reflecting the different survival pressures that they face.

This study and others like it make a powerful point: the optimum level of a personality trait depends on the details of the local ecology. When these vary dramatically across space and time, natural selection cannot home in on a single optimum way of being. That explains why the population of great tits includes both exploratory and cautious individuals.

It also explains a discovery made by Alison Bell at the University of Illinois at Urbana-Champaign. She looked at the feistiness of sticklebacks in various contexts - their aggressiveness towards each other, boldness towards predators and activity in unfamiliar settings - and found that these behaviours were linked and expressed consistently in individuals living in waters where they were under threat of predation. In a second population upstream, which lacked predators, no systematic character traits were detectable. In other words, a personality trait had emerged in the fish in the unpredictable environment but not in those for whom life was more stable (Journal of Evolutionary Biology, vol 18, p 464).

Such findings have obvious extensions to humans. If our range of personalities has evolved as a consequence of a changeable world, then we should expect that each trait is advantageous under some circumstances and costly under others. To explore this idea, I studied 545 British adults with a range of extroversion scores. High scorers had more sexual partners, and we know that they also do better in economic and career terms on average. But those in my sample were also more likely to be hospitalised as a result of an accident or illness, and their family lives were less stable. Since they were more likely to divorce, the men often ended up not living with their children (Evolution and Human Behavior, vol 26, p 363). It is tempting to think of extroversion as an unalloyed blessing, but it is not. Being highly extrovert will attract you to certain types of situation, bringing certain types of life opportunity - you will do well in some settings. However, your personality will entail risks, and some alternative pathways might be closed to you. There will also be situations for which your personality is too risk-prone.

We can see this principle at work with another of the big five, agreeableness. Highly agreeable individuals have good social networks and support. This is because they give consideration to the mental states of those around them, and so attract and retain friends and allies. In many ancestral environments, not to mention the present day, this would have served them well. However, in devoting time and effort to the needs of others, they incur costs in terms of their own agendas. Agreeableness is a negative predictor of success in the worlds of executives and artists, where people need to put themselves first and focus on what they want. What is the optimum level of agreeableness? Apart from at the extremes of the distribution - psychopaths at one end and sufferers of something called dependent personality disorder at the other - there is no correct answer. Whatever your level of agreeableness, there will be benefits and costs.

Likewise, high conscientiousness may help you get the job done, but could also blinker you to other possibilities that a more easily distracted person, or someone whose approach to life is more flexible might benefit from. High openness, meanwhile, has been linked with increased social and sexual success. These advantages, however, tend to accrue mostly in historical eras and geopolitical contexts in which artistic types are highly valued. At other times and places, particularly when a population is struggling for survival, practical and capable characters will be more in demand. Even highly neurotic people can console themselves that when the threat of danger is real, their vigilance may serve them better than the heedless attitude of more laid-back individuals.

The better you know your own personality, the more aware you will be of these costs and benefits. This puts a new spin on the old question of whether you can change your personality. To some extent we can alter who we are, but we might be better off recognising that for almost any personality profile, there is an optimal environment. So if your personality causes you grief, why not try changing the niche you occupy in this complex ecosystem that is modern life?

[New Scientist 09 February 2008, page 36-39]