A consideration of the evolutionary, physiological and anthropological aspects of aggression suggests that individual differences in such behaviour will have important genetic as well as environmental underpinning. Surveys of the likely pathways controlling the physiological and neuronal processes involved highlight, as obvious targets to investigate, genes implicated in sexual differentiation, anxiety, stress response and the serotonin neurotransmitter pathway. To date, however, association studies on single candidates have provided little evidence for any such loci with a major effect size. This may be because genes do not operate independently, but function against a background in which other genetic and environmental factors are crucial. Indeed, a series of recent studies, particularly concentrating on the serotonin and norepinephrine metabolising enzyme, monoamine oxidase A, has emphasised the necessity of examining gene by environmental interactions if the contributions of individual loci are to be understood. These findings will have major significance for the interpretation and analysis of data from detailed whole genome association studies. Functional imaging studies of genetic variants affecting serotonin pathways have also provided valuable insights into potential links between genes, brain and aggressive behaviour.

Both genes and environment contribute to individual differences in aggression. Surveys of the pathways implicated in the physiological and neuronal processes involved highlight the potential role of genes regulating sexual differentiation, anxiety, stress response and neurotransmission. To date, however, association studies have provided little evidence of a substantially significant role for any single candidate gene in such pathways. This may be because genes function against a background in which other genetic and environmental factors are crucial. A series of recent studies, particularly concentrating on monoamine oxidase A , has emphasised the necessity of examining gene by environmental interactions if the contributions of individual loci are to be understood. These findings have major significance for the interpretation of data, both from individual gene and whole genome association studies. Functional imaging studies of genetic variants affecting serotonin pathways have also provided valuable insights into potential links between genes, brain and aggressive behaviour. Key Concepts: Aggression is an evolutionarily advantageous trait with input from one of the most primitive brain regions, the amygdala. There is significant disparity between aggressive behaviour in males and females. Genes and environment both influence aggressive behaviour and there is evidence that stressful life events can interact with specific genetic variants. DBH , COMT , adrenergic receptors, NET1 and SLC6A2 have been studied as possible candidate genes linking stress and aggression. In the serotonin system, genetic polymorphisms in MAOA , SLC6A4 , TPH1/2 and the serotonin receptor genes have been linked with aggression. Studies have shown a potential link between diet and its effects (e.g. on glucose levels) and aggression. Brain imaging studies are beginning to assist an interpretation of the links between genetic variation and aggression.