Social Learning Theorists, such as Bandura (1965), claim that aggressive behaviour is learned through observing and imitating aggressive models. Aggressive behaviour is strengthened and maintained if it has a desirable outcome (reinforcement). Bandura's series of Bobo doll experiments revealed a number of factors that determine whether observed aggressive behaviour is imitated:

There are several ways in which genetic factors in aggression have been investigated. In the 1960s and 70s a popular theory was that males who have inherited an extra Y chromosome are more 'male' and therefore, more aggressive; however, this hypothesis as been shown to be incorrect. Nevertheless, animal breeding studies consistently show that it is possible to select for aggressiveness and twin and adoption studies in humans suggest that genes influence aggressive behaviour; more recently, psychologists and biologists have developed a better understanding of how genes can influence behaviour by using new technology to investigate the genetic mechanisms that lead to increased aggressiveness.  

There is now a considerable amount of research evidence suggesting that aggression is influenced by biological factors including hormones, the structure of the brain and the neurochemistry of the brain.