Aggression
Genetics

The few laboratory-based analyses have not found significant amounts of individual variation in aggression explicable by genetic variation in the human population. Furthermore, linkage and association studies that seek to identify specific genes, for example that influence neurotransmitter or hormone levels, have generally resulted in contradictory findings characterized by failed attempts at replication. One possible factor is an allele (variant) of the MAO-A gene which, in interaction with certain life events such as childhood maltreatment (which may show a main effect on its own), can influence development of brain regions such as the amygdala and as a result some types of behavioral response may be more likely. The generally unclear picture has been compared to equally difficult findings obtained in regard to other complex behavioral phenotypes. For example, both 7R and 5R, ADHD -linked VNTR alleles of dopamine receptor D4 gene are directly associated with the incidence of proactive aggression in the men with no history of ADHD.
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