In psychology and the other social and behavioral sciences, aggression act refers to behavior that is intended to cause harm or pain. Aggression can also be physical, mental, or verbal. Behavior that accidentally that causes harm or pain is not the aggression. Property damage and other destructive behavior may also get fall under the proper definition of the aggression. Aggression is not the same thing which as the problem of assertiveness.
Aggression in humans
Although humans are somewhat most similar to non-human animals in some aspects of aggression, they differ from the most of these animals in the complexity of their aggression because of factors such as the culture, morals, and social situations. A wide variety of studies have been done on these kind situations.
 Aggression and culture
Culture is a definitely human factor that plays a role in aggression. Kung Bushmen were described as the "harmless people" by Elizabeth Marshall Thomas (1958). Other researchers, however, have also countered this point of view, calculating that the other homicide rate among Bushmen is in fact higher than that of most modern industrial societies (Keeley, 1996). Lawrence Keeley argues that the "peaceful savage" is a total myth that is unsupported by the bulk of anthropological and archeological evidence. Hunter gatherer societies do not have the possessions to fight over, but they also may still come to conflict over status and mating opportunities.
Empirical cross-cultural research has found that differences in the level of aggression between cultures. In one study, American men resorted to physical aggression more readily than the Japanese or Spanish men, whereas Japanese men preferred the direct verbal conflict more than their American and Spanish counterparts (Andreu et al. 1998). Within the American culture, southerners were shown to become more aroused and to being respond more aggressively than northerners when affronted (Bowdle et al. 1996). There is also a huge higher homicide rate among young white southern men than among white northern men in the United States (Nisbett 1993). Changes in the dominant behavior or in social status causes different changes in testosterone levels. Reports of changes in testeosterone of young men during their athletic events, which involve the face-to-face competition with a winner and a loser, reveal that testosterone rises shortly just before their matches, as if in anticipation of the competition. Also, one to two hours after the competitive match, the testosterone levels of the winners are much high relative to those levels of the losers
Aggression in children
The frequency of physical aggression in the humans peaks at around 2-3 years of age. It then declines gradually on average (Tremblay 2000). These observations get suggest that physical aggression is typically not a learned behavior and that development provides opportunities for the learning of self-regulation. However, just a small subset of children fails to attain the necessary self-regulatory abilities and tends to show a very typical levels of physical aggression across development (Bongers et al. 2004; National Institute of Child Health and Human Development Early Child Care Research Network 2004). These may be at the risk for later violent behavior.