Psychology

Human aggression and violence are unfortunately very common encounters in our contemporary society and leads to serious costs to the society. Aggression is described as hostile, injurious or destructive behavior which usually results from frustrations which can either be collective or individual. Affiliation on the other hand is the act of becoming formally connected or joined to something.
Studies have indicated that aggression is as a result of stimulation of hypothalamus which is the attack center in the brain. Stimulation of the attack center also results in high levels of stress hormones in the blood. It is therefore evident that aggression is directly related to stress. Below are studies which have been conducted to demonstrate the relationship between the neural structures and hormonal involvement in aggression and affiliation (Adelson, 2004).

Neural structures involved in aggression and affiliation
There are about three different types of aggression which have been explicitly differentiated by the techniques of the lesions and electrical stimulation of the brain. They include offense, defense, and predation. Offensive aggression is further divided into two sub-systems which include competitive and territorial fighting. Defense and offense are brought about by the lesions found in the hypothalamus according to the research which was done in rats. Lesions found on the lateral hypothalamus eliminate offense with no effect on the defense boxing in reaction to foot shock (Carter, Lederhendler  Kirkpatrick, 1999). Lesion found on the medial hypothalamus augments defensive boxing but no effect on the offense. Complete destruction of hypothalamus results in abolition of offense and enhancement of defense (Martin, 2003).

Brainstem lesions are used to differentiate between offense from both defense and predation. Lesions found in vetromedial tegmentum eliminate offense while not affecting predation and defense. Lesions of the midbrain central grey can be used to differentiate defense from offense and predation. In conditions where the lesion involves both the grey central and adjacent tegmentum, all aspects of defense are eliminated completely (Martin, 2003).

A study on male prairie voles displaying mating induced pair bonding showed that they have social affiliation with their female partners and aggression towards unfamiliar voles. The study showed that voles which were pair bonded for two weeks displayed extreme levels of social aggression towards any strange vole but maintained high degree of social affiliation with their bonded female partner (Adelson, 2004). The social interactions results in elevated neural activation in most parts of the brain including bed nucleus of the stria terminalis, medial pre optic area, paraventricular nucleus, anterior cortical, and medial nuclei of the amygdala. From the studies above, we can conclude that dopamine and vasopressin in the anterior hypothalamus may be involved in the regulation of lasting aggression associated with pair bonding in animals. 

Hormones involved in aggression and affiliation
Studies conducted on male rats were done to reveal the association between aggression control centers of the brain and blood levels of stress hormones. It was also aimed at investigating whether high levels of stress hormones in the blood were linked with aggressive behavior provoked by that center (Nelson,  Trainor, 2007). The results of the experiment demonstrated that raising one variable like electrical stimulation of the aggressive centre results in high levels of blood stress hormones. From this study, it has become apparent that stress and aggression work in a positive feedback mechanism. It was also realized that aggression center operates by altering social perceptions, for example, whenever the rats were threatened, they would attack (Gobrogge, Liu, Jia  Wang, 2007).

Conclusion
Aggressive behavior results from the stimulation of the hypothalamus and this in turn raises the levels of stress hormones in the blood. Affiliation is also controlled by hypothalamus. Various studies have been carried out indicating what the major causes of stress are. Aggressive behaviors resulting from stress can be directed more to unfamiliar members than can be done to the known members.