We are all familiar with the idea that different people have different personalities, but what does this actually mean? It implies that different people behave in different ways, but it must be more than that. After all, different people find themselves in different circumstances, and much of their behaviour follows from this fact.

However, our common experience reveals that different people respond in quite remarkably different ways even when faced with roughly the same circumstances. Abbey might be happy to live alone in a quiet and orderly cottage, go out once a week, and stay in the same job for thirty years, whilst Beth longs for exotic travel and needs to be surrounded by vivacious friends and loud music. Charlie goes through a string of divorces from marriages that seemed solid, whilst Derek stays in one that seems unlikely for most of his life. Erica loves walking and landscape paintings, whilst Fran likes abstract art, punk rock and bungee jumping.

In all of these cases, we feel that it cannot be just the situation which is producing the differences in behaviour. Something about the way the person is ‘wired up’ seems to be at work, determining how people react to situations, and, more than that, the kind of situations they get themselves into in the first place. This is why personality seems to become stronger as we get older; when we are young, our situation reflects external factors such as the social and family environment we were born into. As we grow older, we are more and more reaping the consequences of our own choices (living in places we ourselves have chosen, doing jobs that we were drawn to, surrounded by people like us whom we have sought out). Thus, personality differences that might have been very slight at birth become dramatic in later adulthood.

Personality, then, seems to be the set of enduring and stable dispositions that characterise a person. These dispositions come partly from the expression of inherent features of the nervous system, and partly from learning. Researchers sometimes distinguish between temperament, which refers exclusively to characteristics that are inborn or directly caused by biological factors, and personality, which also includes social and cultural learning. Nervousness might be a factor of temperament, but religious piety is an aspect of personality (overlaid, perhaps, on some temperamental foundation).

The discovery that temperamental differences are real is one of the major findings of contemporary psychology. It could easily have been the case that there were no intrinsic differences between people in temperament. That is to say, it might have been the case that all humans were basically running the same software, which would mean that given the same learning history, the same dilemmas, they would all respond in much the same way. Yet we now know that this is not the case.

Personality measures turn out to be good predictors of your health, your sexual promiscuity, your likelihood of divorce, how happy you typically are – even your taste in paintings. Personality is a much better predictor of these things than social class or age. The origin of these differences is in part innate. That is to say, when people are adopted at birth and brought up by new families, their personalities are more similar to their blood relatives than to the ones they grew up with. The differences begin to emerge early in life and are surprisingly stable across the decades. This is not to say that people cannot change, but major change is the exception rather than the rule.

Personality differences tend to manifest themselves through the quick, gut-feeling, intuitive and emotional systems of the human mind. The slower, rational, deliberate systems show less variation in output from person to person. Deliberate rational strategies can be used to over-ride intuitive patterns of response, and this is how people wishing to change their personalities or feelings have to go about it.

So what are the major ways personalities can differ? The dominant approach is to think of the space of possible personalities as being defined by a number of dimensions. Each person can be given a location in the space by their scores on all the different dimensions. Virtually all theories agree on two of the main dimensions, though they differ on how many additional ones they recognise.

An instantly recognisable dimension is neuroticism or negative emotionality, known as N by psychologists. The mind is equipped with systems for protecting itself from harmful things, like physical danger, disease, humiliation, and loss. These systems are driven by emotions like anxiety, fear and shame. It seems that in some people, the systems are a little more easily set off than others. Such people are high on the N dimension. They are worriers, prone to anxieties and fears. Low N scorers are laid back and un-phased by things. High scorers are vulnerable to depression, anxiety and panic, as well as physical ill health from all that stress. They are at high risk for divorce and likely to report themselves as unhappy at any given moment.

If you think of your friends, you will probably find it easy to rank them in terms of the N dimension. The adjective ‘neurotic’ has even entered everyday speech! This is probably because variation in negative emotion systems is a deep biological characteristic of humans, and even of other mammals. Animal breeders have long known that it is very easy to produce more or less fearful horses or dogs by selective breeding, and they have exploited this for producing military and working animals. We know from laboratory rats that you can produce a fearful and anxious strain in a few generations by breeding from the most fearful individuals.

Another key dimension is extraversion or positive emotionality (E). This is commonly used to mean sociability, but to psychologists it means something broader. The mind also contains systems for identifying rewarding things in the environment – food, comfort, mates, kin – and seeking them out. It is thought that brain circuits using the chemical dopamine function to make these positively rewarding stimuli ‘attention grabbing’ (you know the attention grabbing potential of a piece of chocolate cake or a nice looking person). Now these systems seem to be a little more responsive in some people than others.

So some people (high E scorers) are strongly diverted towards intrinsically rewarding things, and others (low E scorers or introverts) can get on just fine without them. High E scorers, not surprisingly, go out more, talk more, want to be famous more, have more sexual partners, and drink and take more drugs than low E scorers. Low E scorers are often more content with relatively quiet, self-contained jobs or hobbies whose rewards maybe longer coming. Extraverts generally describe themselves as happier than introverts, though there is a significant group of happy introverts who have strong, self-contained interests and vocations.

Extraversion and neuroticism are the two most broadly accepted dimensions of personality. Other influential proposals include Openness (with high scorers interested in art and abstract ideas, low scorers practical and down to earth), Conscientiousness (with high scorers methodical and dutiful, low scorers more distractible), and Agreeableness (with high scorers cooperative and trusting, and low scorers more aggressive and hostile). Together these make up the ‘big five’, or OCEAN (Openness, Conscientiousness, Extraversion, Agreeableness, Neuroticism). The OCEAN system is the subject of an online study that we are currently carrying out here at The Open University.

People often ask what is the optimum personality profile to have. There is no simple answer to this question. It is certainly true that some extremes carry some risks. Low agreeableness is associated with violence, whilst high neuroticism is associated with the risk of depression and anxiety. However, all the systems whose operation is reflected in personality differences are there for a reason. Anxiety and fear are good things to have (at least a little), because the world actually is full of dangerous things. There is evidence that at least some high N scorers do well at college because they strive hard to avoid failure, and people who get convicted of traffic offences may be less neurotic than those who do not (and therefore, presumably, less fearful to the danger of getting caught).

The balance between the different systems is probably all. If you are an extreme introvert, you might want to challenge yourself to experience the rewards of greater spontaneity and exchange; if you are an extreme extravert, you might want to teach yourself to undertake a long and lonely project that will ultimately be very rewarding. As human beings, we have the unique ability to look in at our personality from the outside and decide what we want to do with it.