In 1994, Stephen Mobley, confessor to a cold-blooded murder, was the first to offer genetic evidence in mitigation in a court of law in Atlanta, Georgia. Despite a privileged homelife, Mobley's family tree showed four generations of violence; his attorneys argued that his crime could be attributed to his genetic make-up. The court rejected the claim. But the introduction of genetic defence into the courtroom was an important step for both law and genetics.
The defence was based upon the discovery of a "criminal gene" in a Dutch family. Although the family was large and living across different regions of the Netherlands, they shared the same problem: for 35 years, many of the men had been aggressive and prone to violent impulsive acts such as rape and arson. Women in the family approached Han Brunner of the Institute of Genetics in Nijmegen, anxious to find out how to stop the trait, which was mysteriously passed on by women and expressed only in men.
The family's violent behaviour followed a pattern that Brunner was able to trace to a single gene. He found that the pattern was being genetically transmitted on the X chromosome, which is not duplicated in men and therefore cannot be "over-ridden" by a partner chromosome as it is in women.
The location of this "criminal gene" caused a great commotion. Suddenly, it seemed that not only physical traits such as eye and hair colour could be passed in the genes from parent to child, but also characteristics so intangible as a tendency for arson, religious behaviour or linguistic aptitude.
The last year has seen the discovery of the "gay gene," "the criminal gene" and even the gene "that stops you from being boring." In April this year, Chris Brand, a psychologist at the University of Edinburgh, caused outrage by suggesting that single mothers should be encouraged to "sire" their children by more intelligent, and hence more genetically desirable men. The American science writer Chandler Burr, who is both gay and a devout believer in a "gay gene," even suggested that once the particular gene is "verified," homosexuality might be "treated" with antibiotics. And in August, a single gene which conferred maternal behaviour upon mother mice was reported in Science. Female mice lacking the gene shunned their babies and left them for dead.
Nature and nurture
Most experts agree that the nature versus nurture debate is pass?. Michael Rutter, professor of child psychiatry at the Institute of Psychiatry in London, has proclaimed it "dead." Steven Rose, professor of biology at the Open University, calls it "fallacious."
No one doubts that behaviour must have some biological basis-where else could it come from? All growth and change in the body is the result of protein synthesis which is under the control of genes. Behaviour is no exception: there are genes predisposing to sexuality, to ageing, to personality.
But the biological root of behaviour is expressed through an individual's environment. Children may have genes favouring high intelligence, but education will affect its development-just as children with the genes to be tall will not grow to their full potential if they do not eat the right foods.
Behaviour is the product of both genes and environment; they play off each other in what Professor Rose terms a "dialectic" of development. The interaction is complex: the wiring of the brain may even be altered by social situations. For example, the dominant male monkey in a hierarchy develops different brain biochemistry to its subordinates.
Diseases, such as Huntington's disease, can be traced to the presence of a single gene. But a single gene can rarely determine any particular form of behaviour, and certainly not one so complex as crime. Genes can merely influence the probability of acting in a particular way in particular circumstances. How could there be genes for criminal behaviour when that behaviour cannot even be described objectively, but varies according to culture and time?
Gene isolation
It is now estimated that all forms of behaviour are between 20 per cent and 70 per cent heritable; but a genetic factor on its own is never strong enough to account entirely for a particular behaviour. Even in heredi-tary diseases, there is no law for the genetic legacy to follow. Illnesses where research seems to point to identifiable genetic roots are turning out to be a lot more complex than was originally thought. For example, although scientists have been able to locate two breast cancer genes, these only account for 5 per cent of breast cancer cases.
Nevertheless, progress in gene isolation, including recent techniques of linkage analysis, where fragments of genetic material can be matched to behaviours with which they consistently occur, has revived the hope of being able to quantify the genetic contribution to a particular behaviour.
Scientists are now able to identify, over a population, what proportion of a behaviour is attributable to genetics, or nature, and what proportion to the environment, or nurture. They are no longer asking whether it is genes or the environment which contribute more to our behaviour, but in what way we can use the information we have.
One school of thought, which includes Professor Rose, asserts that genetic information is not useful when thinking about crime, IQ or sexual orientation. Rose does not deny the existence of genetic links, nor the strength of its input. His objection is that statistics linking genes to behaviour speak only about the prevalence of a particular trait throughout a population; they can reveal nothing about an individual's predispositions. What is more, finding the genes responsible for a trait is not the same as finding a treatment.
Rose's objections are based upon his convictions that the environment is the only variable that we will be able to alter anyway: gene therapy is still too far off to pin our hopes on. Contrasting the huge differences between American and British murder rates, Rose concludes that factors such as access to guns have a much greater impact on crime than any genetic influence.
The other school of thought, which includes Professor Rutter, argues that information about the genetic influences on a particular behaviour can reveal biological clues about the mechanism of that behaviour and, consequently, how to treat it-even if we do not resort to gene therapy. For example, if a child is predisposed to violence which is triggered by certain circumstances such as crowds, then we can put that knowledge to use by making the child avoid crowds.
Some genetic research projects remain more acceptable than others. While the origins of physical diseases are furiously searched for with the approval of the public and funding bodies, it is a different matter with mental illness. Although there is growing evidence for a genetic component to disorders such as autism and schizophrenia, there remains some resistance to the findings. And genetic research into characteristics such as intelligence is even less welcome. The reason for this is simple: it is hard to disentangle research into traits such as intelligence or sexuality from political beliefs and personal prejudices.
IQ and crime
Take IQ for example. The IQ of different racial groups differs consistently, with the mean score for blacks 15 points below that of the mean score for whites. Even though this is a sturdy and well-replicated finding, there is resistance to acknowledging it because the finding is so often misinterpreted: that there might be some genetic contribution is read as total genetic determinism. This is the mistake made by Chris Brand in The g factor, his recently withdrawn book on IQ. Because there may be some genetic contribution to intelligence, he suggests that racial differences in IQ tests may be explained by genetics. But taking in the well-known environmental differences of black and white populations, we can never be so sure. The fact that features such as IQ run in groups and families does not always mean that they are carried by DNA.
Those supporting more research on the genetics of behaviour say that finding the specific genes associated with IQ or homosexuality will help us to understand those behaviours completely, and in the case of undesirable behaviours such as crime, it will help us find a treatment.
But to do this, we first need to find the mechanism, the crucial pathway that begins with biological instructions handed out by genes. Genes cause the synthesis of proteins, which are used in the production of particular chemicals, such as dopamine and serotonin, in the brain. The levels of these chemicals, known as neurotransmitters, are associated with different moods and behaviours.
So in reality the hunt for genes associated with behaviours is more of a search for genetically-based changes in the brain which could lead to these behaviours. Take the case of crime. Crime comes in many forms: arson, rape, petty theft, mugging. But they all have something in common, some inner immunity to the sense of risk or the urge to act on a whim. Finding a gene, or set of genes, which might encourage risk-taking or impulsive behaviour is the real task, rather than looking for a gene which can control car theft.
Some progress has been made in isolating the mechanisms of antisocial behaviour. In January this year, two research groups, one in Israel and one in America, independently reported an association between a mutation of a dopamine D4 receptor with impulsive behaviour. The mutated form of this gene was soon branded the "thrill-seeking gene."
But the urge to take risks need not merely encourage joy-riding. It may also be the same urge which drives the entrepreneur to invest, or leads explorers over new terrain. An extra risk in the forest might have earned a hunter-gatherer an extra bison at dinner time. Any gene which offered such an early advantage would have thrived, not just survived, through natural selection.
In the case of Han Brunner's Dutch family, he found a consistent association between the reported violent behaviour and a mutation on the X chromosome. In men, mutations on the X chromosome show up because it is paired with the smaller Y chromosome. The result of this particular mutation was a lowered production of an enzyme, MAO-A, which regulates levels of the neurotransmitter serotonin.
When research linked both MAO-A and serotonin to aggression it seemed that the idea of a gene for crime might be validated. But things were not so simple. Brunner had found a genetic mutation whose end result was to increase levels of serotonin. The problem was that lowered, rather than raised, levels of serotonin have been associated with aggression in all other research. So Brunner's work implied a very different mechanism for violent behaviour.
Brunner had chanced across a single gene for violent behaviour. But, the gene is so rare that it can tell us very little about the genetic origins of crime in general. It is even possible that no other family has this gene.
The story so far
The evidence for the genetic basis of behaviours is more convincing in some cases than others. For example, research into alcoholism has led to contradictory findings, although there is an emerging pattern suggesting 50 per cent inheritance. East Asians have a low tolerance for alcohol: drinking it leads to hot, unpleasant flushes, which may in turn protect them from alcoholism. The flushing is caused by a reduced amount of a single enzyme, aldehyde dehydrogenase, which enables alcohol to be metabolised-and levels of aldehyde dehydrogenase are genetically determined. Westerners, lacking this protective mechanism, thus have a greater vulnerability to alcoholism.
However, factors which positively increase the risk for alcoholism have been less easy to identify. A single dopamine receptor, DRD2, has been associated with alcoholism, but time and again the evidence slips away as weak, or cannot be replicated.
Similarly, the levels of MAO-A and serotonin in the brain have been implicated in suicide and antisocial behaviour. Lowered levels of serotonin have repeatedly been found in the brains of suicide victims. Moreover, the tendency of suicide to run in some families lends support to the idea of some biological basis to suicide. However, it is easy to question such findings: how do we know whether the lowered serotonin is a cause of depression and then death, or the result of the depression that led to death?
Scientists need to establish causation, not just correlations. Tony Monaco, of the Wellcome Trust Centre for Human Genetics in Oxford, is currently searching for genetic markers for autism, dyslexia and language disorder, which are all known to have a large genetic input. He is looking for stretches of DNA that could be involved in the development of mental abilities, such as language.
There is still much to be learnt in this politically sensitive field. But just as we explore the environmental conditions that contribute to traits such as intelligence or criminality, it surely makes sense to exploit molecular biology to find the genes involved too.
