The DNA made me do it, M'lud

It may not have a snappy Hollywood title like Natural Born Killers, but a book being launched at a press conference in New York tomorrow will add an extra twist to the growing controversy that the source of crime might lie in our genes. Entitled Genetics of Criminal and Antisocial Behaviour, the book is the proceedings of a scientific conference held in London last year by the Ciba Foundation. Although the conference attracted widespread debate, it took place behind closed doors, so tomorrow's publication will be the first chance to see and assess the quality of the arguments put forward.

Crime, and the fear of crime and violence, dominate much of politics in the US and Britain, stimulating a search for simple explanations and simple ways of dealing with the problem. One of the most heated debates is over claims that genetics and biology can help to explain the causes of crime and violence.

In the US the National Institutes of Health has recently renewed its funding and support for a Genetic Factors in Crime conference, despite having to cancel the conference twice since 1992 because of public opposition and accusations of racism. The NIH has also recently increased its budget for violence-related research to $58m (pounds 39m), and these studies continue to grow in number. At the end of last year, Channel 4 presented British viewers with a two-part programme called A Mind To Crime, which speculated that delinquents and seasoned villains may have suffered Attention Deficit Disorder as children.

What is the evidence linking biology to crime? A widely cited piece of research is a Danish study of 14,427 people adopted as children by unrelated families. The theory is that if the biological parents have a heritable predisposition to crime, they might transmit these characteristics to their offspring. The research showed that as the "criminality" of the biological parent increases from having no convictions on court records to three or more convictions, the proportion of adopted sons who are subsequently convicted themselves steadily increases from about 13 per cent to 25 per cent. Since the children are adopted, the correlation ought to be genetic rather than environmental. But the study has thrown up some confounding results: the genetic predisposition is apparently for property crimes rather than violent offences, and it is difficult to see how there could really be "genes for theft". Moreover, the findings are not clearly duplicated by a similar Swedish adoption study.

Other claims stem from new brain-scanning technologies such as positron emission tomography, or PET scanning. The technique enables researchers to observe and map accurately which areas of the brain are active when subjects are presented with different mental tasks, physical sensations or are asked to recall memories.

The apparent connection with crime comes from PET scans of children and adolescents diagnosed as having Attention Deficit Disorder (ADD) in the US and Australia. ADD is not widely recognised by psychologists in Britain, but the disorder is thought to be related to, but distinct from, hyperactivity. Symptoms include an inability to concentrate or focus on a single activity, impulsive behaviour and a tendency to speak and act without thinking. Ann Moir and David Jessel argue in A Mind to Crime that a core of criminals and delinquents may have suffered ADD as children. Hence for them biology might, at least partially, explain why some turn to a life of crime. But although they concede that most ADD children do not grow up to be "delinquent", this does not prevent them from dubbing the children as "the dangerous few" who "lack normal emotions and feelings" and are prone to crime.

The PET scans of such individuals apparently show lower levels of activity in the frontal cortex, the front of the brain involved in higher mental processes as well as regulating the limbic system, which controls the physiological basis to aggression, fear, feeding and sex. The theory is that the under-aroused state of the frontal cortex fails to inhibit the limbic system sufficiently, leading to impulsive behaviour and hyperactivity.

Another report that received attention as evidence of violence and crime being linked to genetics was Hans Brunner's study of an abnormal Dutch family. The research linked a specific genetic defect coding for a neurotransmitter enzyme to a variety of antisocial aggressive acts in this one family. However, Dr Brunner said of his study that it is "highly unlikely that this is anything but an extreme rarity". Such is the hunger for genetic explanations, however, that Dr Brunner's statement has made little difference to the research being greeted as evidence for "a gene for aggression".

The first problem with studies linking biology and crime is that most simply fail to demonstrate an association between the two, let alone a meaningful cause. The second flaw is that where a link is made, it is quite illegitimate to extrapolate these results on to the population at large, even if the claims are hedged with qualifications about biology only "predisposing" individuals to crime.

A cause can be described as such only if it is pertinent and immediate. For example, that step- fathers are 60 times more likely to kill their step- children than natural parents could be attributed to their genes. On the other hand, families with a stepfather might be less financially and emotionally secure than a family that has remained intact.

The gene that determines our gender could be described as a "gene for crime", since men commit the overwhelming majority of crimes. But when one considers the number of intermediary steps that lie between the determination of gender and the social circumstances of men and women in the Nineties, this is not a convincing causal explanation. In his book, The Psychopathology of Crime, Professor Adrian Raine argues that there must be a biological basis to crime; otherwise, why don't all poor people commit crime? The majority of crimes being committed by a minority of criminals is cited as biological evidence; similarly, the 19th-century criminal anthropologist Cesare Lombroso argued that 40 per cent of criminals followed a hereditary compulsion. The remainder committed crime as a result of their circumstances or in the heat of passion, which could explain why some upright men occasionally went astray.

The consequences of arguing that crime has a genetic component can only be authoritarian. The legal and moral implications are likely to be greater calls for authorities to contain and control those deemed to be genetically unfit or dangerous.

Arguing that physiological measures of children can identify those who will grow up to be criminal can result only in their criminalisation, especially in a climate where fear of crime far outstrips the reality. Calls to tackle "one-boy crime waves" such as "Rat boy" or "blip-boy" are increasingly common. Underclass theories on youth now abound.

At the end of the last century, Lombroso searched for atavistic traits in criminals by measuring skull size and comparing criminal face shapes with dogs and flatfish. Today, criminal anthropology simply uses rather more hi-tech equipment.