Is violence written in our genes?
July 10th, 2004
Why do we find Brad Pitt and Eric Bana fighting it out in Wolfgang Peterson's Troy so appealing? Brad certainly does look fetching in his leather skirt and iron helmet but does their portrayal of the ancient enmity between Achilles and Hector touch deeper nerves? US scientists claim to have found a 25 million year old "warrior gene" in the chromosomes of our primate cousins. If the same gene drives us to take up the sword then, as, as the English author Thomas Fuller noted four hundred years ago, anger may indeed be "one of the sinews of the soul".
Scientists became interested in a gene called MAOA when they discovered in 1995 that mice that lacked it suffered serious anger management problems. The enzyme made by the gene mops up the excess neurotransmitters (the brain's chemical messengers) so mice lacking the gene had unusually high levels of neurotransmitters like serotonin. Affected male mice (interestingly, female mice with the defective gene showed no abnormal behaviour) were quick to attack an intruder and failed to establish the dominant-submissive relationships that normally result in fewer scraps overall. The same gene had previously been implicated in human aggression when it was found that members of a Dutch family whose men suffered from excessive bouts of aggression carried a rare MAOA gene mutation. A later study in 2002 by Terrie Moffitt and Avshalom Caspi of Kings College, London found that men who inherited a variant of the gene and who had suffered abuse as children tended to be more prone to violence.
And now Tim Newman, a biological anthropologist, and his colleagues at the US National Institute on Alcohol Abuse and Alchoholism (NIAAA) discovered that forms of the gene linked to aggressive behaviour in macaque monkeys have been around in primates for at least 25 million years. For the aggression gene to survive so long, it must have provided some advantage to its hosts. Newman believes that aggressive behaviour encoded by "warrior" genes is likely to have provided payoffs for our ancestors in competition for mates or resources. Warrior genes might have compelled Achilles and his comrades to launch those thousand ships and fight the Trojans for the return of Helen. But though our ancestors might have cheered when Achilles drove his sword through Hector on the plains of Ileum we don't cheer when those same genes drive their hosts into violent crime today. Should we at least pause for thought before punishing our fellow men for their genetic inheritance?
Claiming biological causes of criminal behaviour is hardly new. The eighteenth century phrenologists saw criminal personalities in the lumps and bumps on a felon's skulls. Only a few decades ago males inheriting the XYY chromosomes were thought to be prone to violence. Both these claims have of course been discredited. The more recent aggression gene claims are built on firmer ground but the findings are still highly controversial because they go to the heart of the debate on the innateness of human nature, the central issue of the "Literature, Science and Human Nature" symposium sponsored by Body&Soul and the University of Surrey. The novelist Ian McEwan opened that day with a reading from Homer and claimed that it is our underlying biologically-determined human nature that allows us to empathise with the Homeric heroes. The biologist Steven Pinker agreed but pointed out that our genes evolved on the African savannah millions of years ago and although they may have found appropriate expression on the battlefields of the Homeric world, they are less likely to be suited to the constraints of modern world. The warrior genes that gave our Palaeolithic ancestors the competitive edge at the local watering hole might similarly have given the Greeks the edge over the Trojans but today they might lead their owners to the inside of a prison cell.
Will judges ever entertain a "It wasn't me it was my genes" defence? At first sight it seems unlikely. Moira MacDaid, lawyer for the Crown Prosecution Service is sceptical. "I cannot see that putting forward one's genetic predisposition to crimes of violence or theft being accepted by the courts … all sorts of genes could be raised as a defence is this was allowed." Courts have known for centuries that the causes of crime include poverty and deprivation yet judges do not look kindly on a "It wasn't me it was my background" defence. But central to the legal concept of guilt is the notion that, whatever the background we suffer, whatever circumstances we experience on the outside, on the inside each of us is capable of exercising our free will to choose between right and wrong. Trainspotting's Renton chose heroin, the Kray brothers chose crime and society feels justified in rewarding both choices with punishment through the penal system.
Courts generally do not convict us of crime when our internal free is compromised. The clerk who hands the bank's money over to the robber isn't convicted of theft because the law recognises that the gun pointed at his head compromised his ability to choose. The law also admits a defence of insanity when the mental state of an individual comprises their ability to make internal rational choices. The problem is that though insanity may mean something in legal terms it has very little medical meaning. There is no diagnosis of "insanity" outside of its legal use. In the real world mental health is never a binary decision and all mental illnesses have degrees of severity that range from relative normality to severely dysfunctional. Genetic research on mental illness and behavioural disturbances from depression to autism suggests that the difference between normal and abnormal is merely a mater of degree with each individual's mental state coloured by both the throw of the genetic dice and the vagaries of upbringing. The type of warrior gene we inherit may be one of those genetic throws of the dice.
Uncovering the gene/environment interactions, the GxE factors, which influence our mental state is the goal of much genetic research today. What made the recent aggression studies so important is that they were able to detect both genetic and environmental causes of human behaviour. On its own, the MAOA gene variant had no effect but if men who carried the MAOA gene were abused as children then they were more than three times more likely to commit violent crime. Similarly, in the macaque study, the influence of the MAOA gene on aggression was only seen in association with increased stress during the monkey's infancy. According to US researcher Tim Newman, the work points more towards a "it wasn't me it was my genes AND my upbringing" defence.
But where in all these GxE interactions is the legal concept of free will? The law that chooses to ignore the external causes of crime still considers that the (sane) individual can rise above deprivation and exercise their free will to choose not to commit a crime. But the genetic data seems to undermine the concept of the internal free agent. Who chooses aggression in those individuals who are unlucky enough to both inherit the MAOA gene and suffer an abusive upbringing? Is it their mind or their genes? Genes for aggression or other antisocial behaviour undermine the legal assumption that the difference between the delinquent and the law-abiding youth who suffered the same abusive upbringing is just a matter of individual choices. We don't choose our genes but they seem to be influencing our choices.
So far only a handful of genes have been associated with any kind of behavioural trends and their influence is not overwhelming. Even in the Caspi and Moffitt study, many children (about 15%) who inherited the MAOA variant and were severely maltreated did not grow up to be criminals. There is still room for a 'free will' influence. But MAOA is just one gene in a genome of about 30,000 genes. It is likely that many more 'behavioural genes' will be uncovered in the coming decades.
What will society's role be if simple tests on a pinprick of blood from a baby's foot could be used to provide the G-side of the GxE equation. If a child is found to be overloaded with aggression genes then will society feel obliged to intervene to ensure that the E side of the equation is conducive to maturing a mellow adult? Or will the onus be laid on the individual to seek appropriate therapy to curb the aggressive influence of their genes? And if the child does suffer an abusive childhood and, as the GxE equation predicted, grows up to possess a violent temper, can we blame him when he is later found brawling in the football stands. The court of Judge Ito in Los Angeles (who tried OJ Simpson) is presently considering whether to admit a "MAOA-plus-child abuse" defence for a death penalty case of serial homicide. Like it or not, judges may soon have to consider the evidence of our genes.
Johnjoe McFadden is Professor of Molecular Genetics at the University of
Surrey and is the author of "Quantum Evolution" (HarperCollins).