Aggression is a naturally occurring behaviour that is present in a diverse range of animals, from insects to mammals. Animals normally fight over resources (food, territory, nesting sites etc), to scare away mating competitors or to defend their territory.
Importantly, the final goal of aggressive behaviour isn’t usually serious injury or death of the individuals. Aggression is a ritualised violent act which often follows precise stereotyped rules, and allows the opponents to gauge dominance and settle competition without causing serious injuries.
In many behavioural traits, including aggression, the relative contribution of nature versus nurture is a matter of continuous debate.
According to Freud, the father of psychoanalysis, aggression as an innate characteristic common to all humans, and operates as a powerful instinct shared by (wo)man with many non-human species; however this theory does not explain why some people are more aggressive than others. On the other hand the social learning theory claims that aggression is learnt in social contexts and encouraged by reward.
The first studies to establish the direct contribution of DNA to this behaviour were performed in the early 1990s. Five male members of a Dutch family were affected by impulsive aggression and had been involved in serious crime. This study revealed that all of them carried a mutated version of the MAOA gene, which regulates the metabolism of serotonin, dopamine, and noradrenaline. High serotonin levels in these males led to abnormally high levels of aggression.
A more recent study followed 500 children from birth to adulthood. If they carried the mutate version of the MAOA gene but had a normal childhood, their aggressiveness levels would be normal. However, if they carried the mutated genes AND were maltreated and abused as young children they would be much more aggressive than maltreated children with a normally functioning MAOA. Environment or genetics alone do not make us aggressive, but their interaction can affect our behaviour. In recent years several other genes have also been discovered which affect aggression levels in a variety of animals.
We have described aggression in the fruit fly Drosophila melanogaster, paving the way to unveil the molecular structure of this complex but interesting behaviour.