Glucocorticoids are released by hypothamic hypothamic nenia (HPA) in response to stress, of which cortisol is most important in humans. Results in adults suggest that reduced cortisol levels, combined with reduced anxiety or stress response, may be associated with increased aggression. However, proactive aggression may be associated with low levels of cortisol, while reactive aggression may be accompanied by high levels. Differences in the assessment of cortisol may also explain a variety of outcomes, especially in children.  Biological approaches see aggressiveness as an internal energy released by external stimuli, a product of evolution by natural selection, in part of genetics, a product of hormonal fluctuations. Psychological approaches conceptualize aggression as a destructive instinct, a reaction to frustration, an affect stimulated by a negative stimulus, the result of observed learning of society and a diversified reinforcement, a result of variables that affect personal and situal environments.   People share aspects of aggression with non-human animals and have specific aspects and complexity regarding factors such as genetics, early development, social learning and flexibility, culture and morality. In 1963, konrad Lorenz found in his classic On Aggressiveness that human behavior is marked by four shoots of animals in search of survival. Together, these impulses – hunger, fear, reproduction and aggression – reach a natural selection.
 E. O. Wilson explained in On Human Nature that aggression is typically a means of taking control of resources. Aggressiveness is therefore exacerbated at a time when high population density is leading to a shortage of resources.  According to Richard Leakey and his colleagues, human assault has also increased by focusing more on property and defending property.  However, unesco adopted the 1989 Seville Declaration on Violence, which refuted the assertions of evolutionary scientists that genetics was in itself the sole cause of aggression.   Pain and discomfort also increase aggression. Even the simple act of putting your hands in hot water can cause an aggressive reaction. Warm temperatures have been implicated as a factor in a series of studies. A study conducted in the midst of the civil rights movement found that riots were more likely on warmer days than on colder days (Carlsmith-Anderson, 1979). After a test in a hot classroom, students were more aggressive and irritable (Anderson et al. 1996, Rule, et al.
1987). Drivers in cars without air conditioning were also more likely to honk their horns (Kenrick-MacFarlane 1986), which is used as a measure of aggressiveness and has shown links with other factors such as generic symbols of aggression or the visibility of other drivers.  The role of chemicals in the brain, especially neurotransmitters, in aggression has also been studied.