These disturbing facts are usually played down by the medical profession as statistical artefacts, but they are too repeatable for that. In the so-called MrFit trial, in which 351,000 people from seven countries were followed for seven years, people with very low 1 7 0 G E N O M E
cholesterol and people with very high cholesterol proved twice as likely to die at a given age as people with medium cholesterol.
The extra deaths among low-cholesterol people are mainly due to accident, suicide or murder. The twenty-five per cent of men with the lowest cholesterol count are four times as likely to commit suicide as the twenty-five per cent of men with the highest count
- though no such pattern holds with women. This does not mean we should all go back to eating fried eggs. Having low cholesterol, or lowering your cholesterol too far, is highly dangerous for a small minority, just as having high cholesterol and eating high-cholesterol diets is dangerous for a small minority. Low-cholesterol dieting advice should be confined to those who are genetically endowed with too much cholesterol, and not given to everybody.
The link between low cholesterol and violence almost certainly involves serotonin. Monkeys fed on low- cholesterol diets become more aggressive and bad-tempered (even if they are not losing weight), and the cause seems to be a drop in serotonin levels. In Jay Kaplan's laboratory at Bowman Gray Medical School in North Carolina, eight monkeys fed on a low-cholesterol (but high-fat) diet soon had brain serotonin levels that were roughly half as high as those in the brains of nine monkeys fed on a high-cholesterol diet.
They were also forty per cent more likely to take aggressive or antisocial action against a fellow monkey. This was true of both sexes. Indeed, low serotonin is an accurate predictor of aggressive-ness in monkeys, just as it is an accurate predictor of impulsive murder, suicide, fighting or arson in human beings. Does this mean that if every man was forced by law to have his serotonin level displayed on his forehead at all times, we could tell who should be avoided, incarcerated or protected from themselves?5
Fortunately, such a policy is as likely to fail as it is offensive to civil liberties. Serotonin levels are not innate and inflexible. They are themselves the product of social status. The higher your self-esteem and social rank relative to those around you, the higher your serotonin level is. Experiments with monkeys reveals that it is the social behaviour that comes first. Serotonin is richly present in P E R S O N A L I T Y 1 7 1
dominant monkeys and much more dilute in the brains of subordinates. Cause or effect? Almost everybody assumed the chemical was at least partly the cause: it just stands to reason that the dominant behaviour results from the chemical, not vice versa. It turns out to be the reverse: serotonin levels respond to the monkey's perception of its own position in the hierarchy, not vice versa.6
Contrary to what most people think, high rank means low aggressiveness, even in vervet monkeys. The high- ranking individuals are not especially large, fierce or violent. They are good at things like reconciliation and recruiting allies. They are notable for their calm demeanour. They are less impulsive, less likely to misinterpret play-fighting as aggression. Monkeys are not people, of course, but as Michael McGuire of the University of California, Los Angeles, has discovered, any group of people, even children, can immediately spot which of the monkeys in his captive group is the dominant one. Its demeanour and behaviour — what Shelley called the 'sneer of cold command' — are instantly familiar in an anthropomorphic way. There is little doubt that the monkey's mood is set by its high serotonin levels. If you artificially reverse the pecking order so that the monkey is now a subordinate, not only does its serotonin drop, but its behaviour changes, too. Moreover, much the same seems to happen in human beings. In university fraternities, the leading figures are blessed with rich serotonin concentrations which fall if they are deposed. Telling people they have low or high serotonin levels could become a self-fulfilling prophecy.
This is an intriguing reversal of the cartoon picture of biology most people have. The whole serotonin system is about biological determinism. Your chances of becoming a criminal are affected by your brain chemistry. But that does not mean, as it is usually assumed to mean, that your behaviour is socially immutable. Quite the reverse: your brain chemistry is determined by the social signals to which you are exposed. Biology determines behaviour yet is determined by society. I described the same phenomenon in the Cortisol system of the body; here it is again with the serotonin system of the brain. Mood, mind, personality and behaviour are indeed 1 7 2 G E N O M E
socially determined, but that does not mean they are not also biologically determined. Social influences upon behaviour work through the switching on and off of genes.
None the less, it is clear that there are all sorts of innate personality types, and that people vary in the way they respond to social stimuli mediated through neurotransmitters. There are genes that vary the rate of serotonin manufacture, genes that vary the responsiveness of serotonin receptors, genes that make some brain areas respond to serotonin more than others, genes that make some people depressed in winter because of too responsive a melatonin system using up serotonin. And so on and on and on. There is a Dutch family in which the men have been criminals for three generations, and the cause is undoubtedly a gene. The criminal men have an unusual version of a gene on the X chromosome called the monoamine oxidase A gene. Monoamine oxidase is responsible for breaking down serotonin among other chemicals. It is highly probable that their unusual serotonin neurochemistry makes these Dutch men more likely to fall into lives of crime. But this does not make this gene a 'crime gene', except in a very pedestrian sense. For a start, the mutation in question is now considered an 'orphan' mutation, so rare that very few criminals have this version of the gene. The monoamine oxidase gene can explain very little about general criminal behaviour.
But it underscores yet again the fact that what we call personality is to a considerable degree a question of brain chemistry. There are a score of different ways in which this one chemical, serotonin, can be related to innate differences in personality. These are overlaid on the score
C H R O M O S O M E 1 2
S e l f - A s s e m b l y
The egg's ordain'd by nature to that end
And is a chicken in potentia
There are human analogies for almost everything in nature. Bats use sonar; the heart is a pump; the eye is a camera; natural selection is trial and error; genes are recipes; the brain is made from wires (known as axons) and switches (synapses); the hormonal system uses feedback control like an oil refinery; the immune system is a counter-espionage agency; bodily growth is like economic growth.
And so, infinitely, on. Although some of these analogies can mislead, we are at least familiar with the kinds of techniques and technologies that Mother Nature employs to solve her various problems and achieve her ingenious
