Suppose you ask people to rate their interest in such things as social activities, travel, competitive success and sex. Perhaps not surprisingly, their separate scores will correlate with each other (0.1 – 0.3). If you now ask them whether they ever feel depressed or ‘blue’, or whether they have sought help for anxiety, their scores for these two items also positively correlate with each other. But the first four sets and the second two sets don't cross correlate at all. This suggests there are deeper traits at work. A technique called factor analysis identifies Extraversion as the common factor in the first set, and Neuroticism as the common factor for the second. These two factors are independent.
When a wide variety of personality-relevant items are rated for large samples of people, factor analysis reliably and repeatedly confirms that there are five underlying, independent personality traits: Extraversion and Neuroticism as already describerelatesscientiousness, Agreeableness and Openness. Each will get a chapter to itself.
The Five Factor Model of personality is often accused of shallowness, and of being atheoretic as the factors simply emerge from statistical processing (in fact just the same factoring procedure generates the g-factor – general intelligence – as measured through IQ tests). The great strength of Nettle’s book is that he can link individual variation within each of the five factors to differences in brain anatomy and metabolism as captured by MRI scanners and then with genetic differences. The five traits seem to be capturing something real about genetically-determined brain variation.
Common observation confirms that we are surrounded by different personalities. If personality is heavily determined by our genes, as it appears, then why haven't we all converged on an ideal personality? You can, of course, ask the same question about any continuously-varying trait which still exhibits variation, such as height or intelligence. The answer seems to be a combination of environmental instability (rewarding different parts of the variability-spectrum in different circumstances) and frequency-dependent selection (as in the way a few rather nasty people can take advantage of the many nice-but-gullible). Nettle discusses this in detail – it will be a recurring point that all positions in personality space help in some circumstances but hinder in others.
The chapter on Extraversion, setting a pattern for those to come on the other traits, links the behavioural attributes of extraverts with brain imaging and genetic studies. Extraversion, it turns out, comes down to a strong reaction to positive emotions – those feelings we find rewarding; introverts just don’t seem to care so much, conserving their energy. There seems to be a link between extraversion and genetic variation in sensitivity to dopamine.
Neuroticism, by contrast, relates to sensitivity to negative emotions: to score highly on this dimension is to be a worrier. The associated brain chemistry seems to involve the neurotransmitter serotonin: inhibitors such as Prozac seem to make us less worried about life’s many sources of anxiety.
Conscientiousness, the third trait to be analysed, seems at first sight a pretty good trait to score highly on. It’s the most reliable predictor of occupational success across the board. Conscientiousness is particularly valuable in structured, rule-based environments such as we find in advanced technological societies. Change the situation to one of unpredictable, fast-changing circumstance however, and the rule-bound are at a disadvantage. The army, for example, has a continual internal conflict as it needs both sorts, but they continually rub each other up the wrong way.
Agreeableness, the fourth dimension, sounds like a trait well-worth having. Who could fault being nice? Perhaps not so strangely, success in business correlates with low scores on this trait. Something about putting other people first and a degree of self-effacement doesn’t sit easily with tough, mission-oriented leadership. This is the one trait where female and male scores are clearly distinct, with women scoring more than half a standard deviation higher in agreeableness. There is a ready evolutionary explanation in the pre-modern sexual division of labour.
The final dimension is Openness to Experience. This is a hard dimension to pin down. Some people equate it with intelligence, but the author is of the opinion that intelligence is a kind of whole-brain efficiency measure implicated across all areas of neural functioning including such non-intellectual tasks as pure reaction times. Nettle believes high-scorers on Openness are artistic, creative people capable of making associations between different – and perhaps surprising – kinds of things. Intellectuals on the science, technology, engineering and maths front don’t look much like famous poets and acclaimed authors. Wherein lies the difference? For once the author doesn’t have good answers, believing the key to excellence in these STEM subjects is more down to general intelligence. But clearly that can't be the whole story.
In the final part of the book the author reviews the evidence for ‘environmental’ influences determining personality and finds they are few and hard to find. Family and parental input (if non-abusive) has been carefully measured to have exactly zero impact: you can't change your child’s personality. Does this give people a deterministic get-out - my genes made me do it? In the final chapter Nettle carefully demolishes this view, showing that dispositions are one thing, but the life choices you make to go with or against the flow of your dispositions are something else.
In summary, this book is a wonderfully accessible and profound exploration of the concept of personality. Everyone will learn something about themselves from reading it and it conclusively takes us beyond the limitations of the Jungian approach as in Myers-Briggs theory. There is a short 12 item questionnaire which you are encouraged to complete before reading (which you can take online here).
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