This biological explanation proposes that aggression occurs due to physiological causes. It assumes that aggression is located within the biological make-up of the individual through hormonal and neural mechanisms rather than in the environment around them. Neurotransmitters are chemicals that enable impulses within the brain to be transmitted from one area of the brain to another. There is some evidence that at least two of these neurotransmitters, serotonin and dopamine, are linked to aggressive behaviour. Serotonin and dopamine are of particular interest because low levels of serotonin and high levels of dopamine have been associated with aggression in animals and humans.

Serotonin is thought to reduce aggression by inhibiting responses to emotional stimuli that would otherwise lead to an aggressive response (e.g. someone pushing past you rudely). It has been found that low levels of serotonin in the brain have been associated with an increase of impulsive behaviour, aggression, and even violent suicide. Some drugs are thought t alter serotonin levels and thus increase aggressive behaviour. Mann et al (1990) gave 35 health subjects dexfenfluramine, which is known to deplete serotonin. They used a questionnaire to assess hostility and aggression levels, they found that dexfenfluramine treatment in males (but not females) was associated with an increase in hostility and aggression scores.

Although the link between high levels of dopamine and aggressive behaviour is not as well established as with serotonin, there is some evidence to suggest that such a link exists. For example, Lavine (1997) found that increases in dopamine activity via the use of amphetamines have also been associated with increases in aggressive behaviour. Buitelaar (2003) found that antipsychotics which reduce dopamine activity in the brain have been shown to reduce aggressive behaviour in violent delinquents.

Raleigh et al (1991) have added support for the importance of serotonin in aggressive behaviour in a study of vervet monkeys. They found that the monkeys that fed on experimental diets high in tryptophan (which increases serotonin levels in the brain) exhibited decreased levels of aggression. The monkeys that fed on diets that were low in tryptophan exhibited increased aggressive behaviour, suggesting that the difference in aggression could be attributed to their serotonin levels.

Although we cannot establish that dopamine causes aggression, there is evidence to suggest that it might be a consequence of aggression instead. Couppis and Kennedy found that in mice, a reward pathway in the brain becomes engaged in response to an aggressive event and that dopamine is involved as a positive reinforcer in this pathway. This suggests that individuals will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it.

The male sex hormone testosterone is thought to influence aggression from young adulthood onwards due to its action on brain areas involved in controlling aggression. This association comes from a number of sources such as a meta-analysis carried out by Archer (1991) who analysed the results of 230 males over five studies and found a low positive correlation between testosterone and aggression. However, the type of participant and the form and measurement of aggression, differed substantially between studies.

Wingfield et al (1990) proposed a theory that, in monogamous species, testosterone would only need to rise above the normal level when there is a social challenge (e.g. a threat of aggression from another male). As the human species is considered to be monogamous, this would predict that male testosterone levels would rise sharply in response to such challenges. In such situations, a testosterone surge is to be expected, with a consequent increase in aggression, provided the threat is deemed relevant reproductive competition, e.g. a dispute over a female. This theory is known as the challenge hypothesis.

Cortisol appears to have a mediating effect on other aggression-related hormones such as testosterone, possibly because it increases anxiety and the likelihood of social withdrawal (Dabbs et al, 1991). High levels of cortisol inhibit testosterone levels and so inhibit aggression. Studies have reported low levels of cortisol in regular violent offenders and in violent school children. This suggests that although relative high testosterone is the primary biochemical influence on aggression, low cortisol also plays an important role by increasing the likelihood of aggressive behaviour.

Albert et al (1993) claim that despite many studies showing a positive correlation between testosterone and aggression, other studies find no such relationship, particularly those that have compared testosterone levels of aggressive and less aggressive individuals. In addition, most studies showing a positive correlation have involved small samples of men within prisons, using self-report measures of aggression or judgements based solely on the severity of the crime committed. This suggests that the evidence is inconsistent and therefore invalid.

The link between biological mechanisms such as serotonin and testosterone with aggression are well established in non-human animals. However, the position is not quite so clear in the case of humans. This is not to deny that such links exist, but rather that the complexity of human social behaviour means that a biological explanation for human aggression is insufficient on its own to explain all the many different aspects of aggressive and violent behaviour. This shows that the biological explanation of aggression is reductionist.

Most studies concerned with testosterone and aggression have involved male participants and ignore the links with testosterone and aggression in females. Eisenegger el al (2011) found that testosterone could make women act ‘nicer’ rather than more aggressively depending on the situation. This lends supports to the idea that, rather than directly increasing aggression, testosterone promotes status-seeking behaviour of which aggression is one type. The fact that this explanation doesn’t take females into consideration when explaining testosterone and aggression shows that it is gender biased.