There is a fascination with murder. Our interest in murder is seen as prurient and unhealthy. Yet, it is most likely driven by incomprehension at a mind-set that could take life.
The better we can understand how this happens, the more likely we are to reassure ourselves that none of our family or friends possess this tendency.
Even the popularity of crime fiction, that has resulted in the burgeoning of TV murder dramas and made household names of writers like Jo Nesbo and PD James while resurrecting the popularity of Conan Doyle, is driven by a moral desire to see justice done. The detective is the saviour who helps bring order and resolution to a cruel world filled with bloody acts and unimaginable sorrow.
The study of murder and murderers as a scientific exercise began in Italy in 1870 when Cesare Lombroso, a doctor and psychiatrist in Turin, developed his theory of the "born criminal". He believed that those committing very serious crimes were "atavistic" and "primitive" and that they could be identified by certain physical features such as a sloping forehead, ears of unusual size, asymmetry of the face, excessive length of arms, asymmetry of the cranium and others that resembled those of apes and lower primates.
His eugenic theories were challenged by other European Universities where the focus was on the sociological aspects of serious crime.
The question of the relationship between nature and nurture, between genes and environment in those who are murderers, was the subject of a fascinating documentary recently on BBC 4 presented by Michael Mosley with the title The Mystery of Murder.
While the crude analysis of the anatomical make-up of serious criminals carried out by Lombroso has long passed, more sophisticated studies using "imaging" techniques with scanners allows criminologists access to the brain of murderers and other serious offenders.
The work of neuroscientist Professor Adrian Raine, and his book The Anatomy of Violence, has sought to explore this issue. Speaking on the BBC programme he discussed the role of abnormalities in parts of the brain concerned with emotions (the amygdala) and with impulse control and planning (the prefrontal cortex) in creating a "natural born killer".
Another candidate that may play a role in determining who commits murder was also explored in the documentary. The "warrior gene" was first identified in a large Dutch family who were notoriously violent. The gene is involved in making an enzyme called mono-amine oxidase A (MAOA), that metabolises some brain neurochemicals. Low levels of activity in the gene are linked with anti-social behaviour in adulthood, particularly if the person had been brutalised in childhood.
In the US, a man escaped a conviction for first degree murder and the death penalty in 2009 based on a medical argument that he had the "warrior gene" and had been sexually abused in childhood: in others words the lethal combination.
Jim Fallon is a professor of neuroscience and behaviour at the University of California.
Several members of his family have committed violent offences including murder. He himself is not violent and when he and his family were scanned and screened for genetic and brain variants associated with potential violence, he scored positive.
Why does he not behave as his genes might predict? He believes that his happy and loving childhood mitigated the effects of biology. In other words, for him nurture trumped nature.
This is a positive conclusion since it provides hope for families dogged by trans-generational violence that the cycle of bloodshed and death can be broken by nurturing and cherishing within the family. It also overturns the belief that has dominated eugenic thinking which contends that genes trump everything and that these strains can only be extinguished by ceasing to reproduce.
Potentially therefore the finding that a good childhood overcomes the genetic propensity to violence should lead health care and social agencies to develop methods of identifying families at risk of violent crime and developing methods to improve parenting to reduce this likelihood.
Of course other factors also come into play in determining serious violence such as substance misuse, but this too is linked to impulsivity and problems understanding the consequences of actions. The same parts of the brain have been implicated in this as in serious violence and the remedy is probably the same.
The mind of the murderer is complex but studies such as those discussed in the documentary help us understand why not everybody from a violent family turns into a killer and why those whose families are loving sometimes become violent offenders.
Genes and environment work hand in glove in a complex but fascinating manner. No wonder we have an enduring interest in murder, that most terrible of crimes.
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