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.