report good results from encouraging their clients to accept that they cannot change their partners' irritating habits - because they are probably innate - but must find ways to live with them. The parents of a homosexual are generally more accepting when they believe that homosexuality is an immutable part of nature rather than a result of some aspect of their parenting. Far from being a sentence, the realisation of innate personality is often a release.
Suppose you wished to breed a strain of fox or rat that was more tame and less instinctively timid than the average. One way
It parallels another link between neurochemistry and personality in people. Jerome Kagan, a Harvard psychologist, leading a team of researchers studying shyness or confidence in children, found that he could identify unusually 'inhibited' types as early as four months of age — and fourteen years later could predict how shy or confident those same human beings would be as adults. Upbringing mattered a good deal. But intrinsic personality played just as big a role.
Big deal. Nobody, except perhaps the most die-hard social deter¬
minist, would find an innate component of shyness surprising. But it turned out that the same personality traits correlated with some unexpected other features. Shy adolescents were more likely to be blue-eyed (all the subjects were of European descent), susceptible P E R S O N A L I T Y 1 6 7
to allergies, tall and thin, narrow-faced, to have more heat-generating activity under the right forehead and a faster heartbeat, than the less shy individuals. All of these features are under the control of a particular set of cells in the embryo called the neural crest, from which a particular part of the brain, the amygdala, derives. They also all use the same neurotransmitter, called norepinephrine, a substance very like dopamine. All these features are also characteristic of northern Europeans, Nordic types for the most part. Kagan's argument goes that the Ice Age selected those better able to withstand cold in these parts: people with high metabolic rates. But a high metabolic rate is produced by an active norepinephrine system in the amygdala, and brings with it lots of different baggage - a phlegmatic and shy personality being
If Kagan is right, tall, thin adults with blue eyes are slightly more likely to become anxious when challenged than other people. An up-to-date recruitment consultant might find this handy in his head-hunting. After all, employers already seek to discriminate between personalities. Most job advertisements require candidates with 'good interpersonal skills' - something that is probably partly innate. Yet it would plainly be a repellent world in which we were picked for jobs on the basis of our eye colour. Why? Physical discrimination is so much less acceptable than psychological. Yet psychological discrimination is just chemical discrimination. It is just as material as any other discrimination.
Dopamine and norepinephrine are so-called monoamines. Their close cousin, another monoamine found in the brain, is serotonin, which is also a chemical manifestation of personality. But serotonin is more complicated than dopamine and norepinephrine. It is remarkably hard to pin down its characteristics. If you have unusually high levels of serotonin in your brain you will probably be a compulsive person, given to tidiness and caution, even to the point of being neurotic about it. People with the pathological condition known as obsessive—compulsive disorder can usually alleviate their symptoms l 6 8 G E N O M E
by lowering their serotonin levels. At the other end of the spectrum, people with unusually low serotonin levels in their brains tend to be impulsive. Those who commit impulsive violent crimes, or suicide, are often those with less serotonin.
Prozac works by affecting the serotonin system, though there is still controversy about exactly how it does so. The conventional theory put forward by scientists at Eli Lilly, where the drug was invented, is that Prozac inhibits the reabsorption of serotonin into neurons, and thus increases the amount of serotonin in the brain.
Increased serotonin alleviates anxiety and depression and can turn even fairly ordinary people into optimists. But it remains possible that Prozac has exactly the opposite effect: that it interferes with the responses of neurons to serotonin. There is a gene on chromosome 17, called the serotonin-transporter gene, which varies, not in itself, but in the length of an 'activation sequence' just upstream of the gene — a sort of dimmer switch at the beginning of the gene, in other words, designed to slow down the expression of the gene itself. As with so many mutations, the variation in length is caused by a variable number of repetitions of the same sequence, a twenty-two-letter phrase that is repeated either fourteen or sixteen times.
About one in three of us have two copies of the long sequence, which is marginally worse at switching off its gene. As a result such people have more serotonin transporter, which means that more serotonin gets carried about. These people are much less likely to be neurotic, and slightly more likely to be agreeable than the average person, whatever their sex, race, education or income.
From this, Dean Hamer concludes that serotonin is the chemical that abets, rather than alleviates, anxiety and depression. He calls it the brain's punishment chemical. Yet all sorts of evidence points in the other direction: that you feel better with more serotonin, not less. There is, for instance, a curious link between winter, a desire for snacks, and sleepiness. In some people — probably once more a genetic minority, though no gene version has yet been found that correlates with susceptibility to this condition — the dark evenings of winter lead to a craving for carbohydrate snacks in the late P E R S O N A L I T Y 1 6 9
afternoon. Such people often need more sleep in winter, though they find their sleep less refreshing. The explanation seems to be that the brain starts making melatonin, the hormone that induces sleep, in response to the early evening darkness of winter days.
Melatonin is made from serotonin, so serotonin levels drop as it gets used up in melatonin manufacture. The quickest way to raise serotonin levels again is to send more tryptophan into the brain, because serotonin is made from tryptophan. The quickest way to send more tryptophan into the brain is to secrete insulin from the pancreas, because insulin causes the body to absorb other chemicals similar to tryptophan, thus removing competitors for the channels that take tryptophan into the brain. And the quickest way to secrete insulin is to eat a carbohydrate snack.4
Are you still with me? You eat cookies on winter evenings to cheer yourself up by raising your brain serotonin. The take-home message is that you can alter your serotonin levels by altering your eating habits. Indeed, even drugs and diets designed to lower blood cholesterol can influence serotonin. It is a curious fact that nearly all studies of cholesterol-lowering drugs and diets in ordinary people show an increase in violent death compared with control samples that usually matches the decrease in deaths from heart disease. In all studies put together, cholesterol treatment cut heart attacks by fourteen per cent, but raised violent deaths by an even more significant seventy-eight per cent. Because violent deaths are rarer than heart attacks, the numerical effect roughly cancels out, but violent deaths can sometimes involve innocent bystanders. So treating high cholesterol levels has its dangers. It has been known for twenty years that impulsive, antisocial and depressed people - including prisoners, violent offenders and failed suicides - have generally lower cholesterol levels than the population at large. No wonder Julius Caesar distrusted Cassius's lean and hungry look.
