Aggression
Some scholars believe that behaviors like aggression may be partially learned by watching and imitating people's behavior, while other researchers have concluded that the media may have some small effects on aggression. There is also research questioning this view. For instance, a long-term outcome study of youth found no long-term relationship between playing violent video games and youth violence or bullying. One study suggested there is a smaller effect of violent video games on aggression than has been found with television violence on aggression. This effect is positively associated with type of game violence and negatively associated to time spent playing the games. The author concluded that insufficient evidence exists to link video game violence with aggression. However, another study suggested links to aggressive behavior.
Aggression
In humans, there is good evidence that the basic human neural architecture underpinning the potential for flexible aggressive responses is influenced by genes as well as environment. In terms of variation between individual people, more than 100 twin and adoption studies have been conducted in recent decades examining the genetic basis of aggressive behavior and related constructs such as conduct disorders. According to a meta-analysis published in 2002, approximately 40% of variation between individuals is explained by differences in genes, and 60% by differences in environment (mainly non-shared environmental influences rather than those that would be shared by being raised together). However, such studies have depended on self-report or observation by others including parents, which complicates interpretation of the results. 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.