The defective gene appears to cause a build-up of natural chemical messengers in the brain of male carriers which leads them to overreact in an aggressive manner.
Observers who have seen the research have suggested that although the genetic defect has been identified in only one Dutch family so far, the results could have wider implications in that similar genetic mutations might contribute to aggressive behaviour in the wider population.
However, Han Brunner, who led the research team at the University Hospital in Nijmegen, Holland, emphasised yesterday that any extrapolation of the results to aggression in society remained speculative. He said the research, published in the June issue of the American Journal of Human Genetics, applied only to the family where the inherited disorder had been identified. 'It is highly unlikely that this is anything other than an extreme rarity.'
He said that 14 men in the extended family, stretching back over several generations, had learning difficulties and were prone to bouts of aggressiveness.
An analysis of the family tree showed the behaviour followed a 'sex-linked' pattern of inheritance, with the defective gene being carried by unaffected mothers who passed it on to some of their sons, much like the gene for the haemophilia blood disorder. Dr Brunner and colleagues found that the gene must reside on a stretch of the X chromosome which is known to carry two genes responsible for producing an enzyme that prevents a build-up in the brain of neurotransmitters, the chemicals that trigger nerve impulses.
When the scientists analysed the urine of some of the men, they found higher than normal amounts of chemicals that are normally acted on by one of the enzymes, called monoamine oxidase A.
They concluded that the inherited predisposition to aggressiveness was the result of a mutation in the monoamine oxidase A gene which results in a shortage of the enzyme that in turn causes a build-up of neurotransmitters and over-excitation of the nerves during stressful situations.