What a difference a century makes. One hundred years ago, Sigmund Freud’s answer to Daniel Nettle’s question – “What makes you the way you are?” – would have begun with your unconscious mind: the unique pattern of fantasies, defences, and instinctual conflicts that create your neurotic insecurities and self-defeating habits. These unconscious mechanisms would, in turn, have been profoundly influenced by your parents, who overpunished you or underappreciated you, who told you too much about sex or not enough. You can’t do much about your personality, though you can tweak it a bit with years of psychoanalysis.

Today, personality researchers almost uniformly agree that the things that make you the way you are consist of a combination of your genes, your peers and the idiosyncratic, chance experiences that befall you in childhood and adulthood. Your parents influence your relationship with them – loving or contentious, conflicted or close – but not your “personality”, that package of traits we label extroverted or shy, bitter or friendly, hostile or warm, gloomy or optimistic. Your genes, not your parents, are the reason you think that parachuting out of planes is fun, or, conversely, that you feel sick to the stomach at the mere idea of doing such a crazy thing voluntarily. You can’t do much about your personality, though you can tweak it a bit with cognitive therapy.

Freud’s view of personality was passionate, controversial, sexy, unfalsifiable and wrong. But it was a personal theory of personality. Anyone could immediately apply it, party-game style, to his or her own unconscious motivations, hidden fantasies and terrible parents. The behavioural-genetics view of personality is calm, uncontroversial (except to a few diehard Freudians), empirically testable and correct. But it is an impersonal theory of personality. After all, everyone has genes; not everyone has your mother.

Daniel Nettle takes on the task of showing how evolution and genetics have conspired to create “wanderers, worriers, controllers, empathizers, and poets”, along with daredevils and wallflowers. Gone are the old type theories (are you a Thinking or Feeling type?) and single-trait descriptors (do you have a Machiavellian personality? Are you an erotophobe or an erotophile?). Evolutionary theory, the genome project, studies of identical twins reared together and apart, and brain-imaging techniques such as PET scans and MRIs have given scientists the theory and methods of identifying the differences in how people’s nervous systems are wired up and how those differences express themselves in characteristic responses to other people and to events. These characteristic responses statistically cluster into five basic factors, which are pretty much the same in every culture that has been studied, from Britain to Korea, Ethiopia to Japan, China to the Czech Republic. Nettle devotes a chapter apiece to each of the five: extraversion, the extent to which a person is outgoing, talkative, adventurous and sociable, or shy, silent, reclusive and cautious; neuroticism, the extent to which a person suffers from anxiety and other negative emotions such as anger, guilt, worry and resentment; agreeableness, the extent to which a person is good-natured, cooperative and nonjudgmental, or irritable, abrasive and suspicious; conscientiousness, the extent to which a person is responsible, persevering, self-disciplined and tidy, or undependable, quick to give up, fickle, sloppy and careless; and openness to experience, the extent to which a person is curious, imaginative, questioning and creative, or conforming, unimaginative, predictable and uncomfortable with novelty.

You know these people, don’t you? You can see yourself in this list, can’t you? But if you are worrying that you are genetically disposed to worrying, don’t worry about it. Each of these dimensions, Nettle shows, has, in evolutionary terms, benefits and costs. Extroverts may risk their necks to forage for food in strange places, but they are also more likely than their cautious peers to be eaten by strange creatures. Agreeable people may have harmonious relationships, but by putting others first, they may lose status and opportunities to advance. Neurotics are more vulnerable to anxiety and depression, and they see clouds in every silver lining; but they are also hypervigilant to dangers in the environment, some of which, after all, are genuine.

As evolutionary theory would predict, you don’t have to be a person to have a personality. Four of the five factors (apart from conscientiousness, a cognitively complex trait) have been identified in more than sixty species, not only in our fellow primates but also in bears, dogs, pigs, hyenas, goats, cats and even the octopus. For anyone wondering how researchers study octopus personality, the answer is simple. They drop dinner (a crab) into a tank of octopuses and watch what they do. Some octopuses will aggressively grab their dinner at once. Some are more passive and wait for the crab to swim near them. And some are devious; they wait and attack the crab when no one is watching. These “personality dispositions” among octopuses can be reliably identified by independent observers.

Nettle also explains why evolution hasn’t made life easy for itself, so to speak, by simply selecting for one kind of trait among members of a species. Some guppies, for example, are more wary than others. Put them in a tank with one of their natural predators, such as “the splendidly named pumpkinseed”, and in only twenty-four hours, fourteen of twenty highly wary guppies will still be alive, compared to only five of the twenty unwary, extroverted guppies. Shouldn’t evolution have seen to it, then, that wariness would become a universal guppy trait, akin to the long neck of the giraffe? No, because guppies live in different environments. If you are a guppy in a pumpkinseed-free environment, you don’t want to be wasting your time searching for predators when you could be dating and mating (an activity that in humans, if not guppies, requires its own degree of wariness). Most environments provide a constantly changing level of danger from predators, making it maximally beneficial for any group of guppies to have both cautious members and bold ones.

Behavioural-genetic studies have consistently found that the heritability of personality traits, whether the Big Five or one of many others from aggressiveness to happiness, is around 50 per cent. This means that within a group of people, about 50 per cent of the variation in such traits is attributable to genetic differences among the individuals in the group. Most people have assumed that the other 50 per cent comes from the “shared environment” of the home: parental child-rearing methods and the experiences the child shares with siblings and parents. If it did, studies should find a strong correlation between the personality traits of adopted children and those of their adoptive parents. In fact, the correlation is weak to nonexistent. This means that when children resemble their parents and grandparents temperamentally, it is because they share genes with these relatives, not experiences. What, then, is going on in the “unshared environment”, the other half of the influences that “make you the way you are”?

Put simply, Nettle argues, we don’t know. “The area of environmental influences on personality is a morass of unsupported or poorly tested ideas”, Nettle observes. He suggests that the reason there is only one Daniel Nettle, not 200 Daniel Nettles who are “also working on books about the five-factor model of personality”, is that the five factors can be channelled in countless ways, encouraged or impeded given a person’s chance experiences, opportunities, health, peers and immediate circumstances. Moreover, because human beings, unlike the guppy and the octopus, have complex, sense-making minds, they are forever telling stories about themselves to explain why they are the way they are. No one else will experience Nettle’s life as he does or interpret it as he does. Our storytelling brains make each of us unique.

When Judith Rich Harris reported the same information about the genetic origins of personality in her pioneering book The Nurture Assumption (1998) and then developed a richly complex theory about the origins of individual differences in No Two Alike (2007), readers understood that we are in the midst of a revolution in understanding of what makes us the way we are. Daniel Nettle, in contrast, has written an engaging primer on the genetics of personality, but he does not fully examine the implications of this work for child rearing, parent-blaming, literary analysis, memoir, psychotherapy and human hubris. Were he to have done so, readers would have more deeply felt the impact and consequences of coming to the end of the Freudian story, and of being at the exhilarating start of a new one.

Daniel Nettle
PERSONALITY
What makes you the way you are
298pp. Oxford University Press. £12.99 (US $19.95).
978 0 19 921142 5

Carol Tavris is co-author of Mistakes Were Made (But Not by Me): Why we justify foolish beliefs, bad decisions, and hurtful acts, 2007.

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