aggression

aggression has been defined as a behaviour characterized by the intention of inflicting unpleasant stimulation on another individual — but this underestimates its subtlety and omits some important features. It is well known that aggression is not a simple, single category of behaviour. Several classifications of aggression have been proposed, some more elaborate than others. Aggression, unlike some other behaviours, has no biological function or purpose in isolation. Aggressive interactions occur mostly as part of some other pattern of behaviour; for example, as a strategy to achieve sexual goals or access to preferred foods. More generally they form part of the process whereby individuals define their position in the social groups to which they belong, and hence ensure access to restricted resources without the need for constant conflict — a form of social control. However, this does not rule out the possibility that the performance of aggressive acts can itself be rewarding.

Distinctions have been made between different types of aggression, largely on the basis of the context in which it occurs or the stimuli that provoke it: (i) inter-male, or inter-female (territorial, social conflict, etc.); (ii) maternal; (iii) self defence; and (iv) infanticide. Aggression towards members of the same species has been divided more simply into ‘offence’ and ‘defence’. Predation, the hunting of other species, is sometimes included in discussions of aggressive behaviour, but is more properly classed as feeding behaviour.

Human aggression has been separated into ‘emotional’ aggression, carried out by people with the main intention of harming someone, and ‘instrumental’ aggression, with some other objective, such as to obtain something rewarding, rather than specifically to injure a victim. In general, both the form of the aggressive act and the context in which it occurs have to be taken into account.

Social factors

Most species, including human beings, live in social groups whose structure affects access by individuals to items in short supply — such as food, mates, or shelter. Direct aggressive confrontation may be used to determine which individual has priority, but it is more usual that animals come to know, through a process of social learning, who is likely to win in such an encounter. This determines their strategy, and also gives the group its dominance structure. Animals, or people, low in the hierarchy may not challenge those higher in the scale, presumably because of the cost in terms of potential injury. This mechanism of social control, based on previous aggressive interactions, functions to reduce aggression; but it does have potent effects on individuals. If it is to be effective, social control by hierarchy requires extremely sophisticated neural processing; indeed, there are those who claim that the primary function of the human brain is to facilitate social interaction.

Gender influences

There are marked individual and gender-related differences in aggression. In most mammalian species aggression is more common between males than between females. Exposure of the brain to testosterone in the womb may alter infant behaviour: young males show more aggressive-like play than females. Testosterone may also sensitize the individual to the later effects of the same hormones: for example, increasing the likelihood of adult aggressive behaviour, particularly in the context of competition for desirable, but limited, resources. However, giving excess testosterone to men has had rather inconspicuous effects on their aggressive behaviour or tendencies (including thoughts), though levels of testosterone in the saliva have been shown to correlate positively with violent crimes in male prison inmates. But social status also alters testosterone, so the relation between individual differences in aggressivity and testosterone may be indirect. A variety of studies, in both human and non-human primates, has shown that social ‘stress’ (that is, demands made by the social or working environment) lowers testosterone and that ‘dominant’ males have higher levels. However, injecting ‘subordinate’ monkeys with testosterone does not improve their position in the hierarchy, or make them more aggressive.

Aggression is not a male prerogative. It also occurs in females, particularly when they need to defend their young. For example, lactating rats are highly aggressive to intruding males (rather than females). In this context, different hormones may play a role. This aggressive reaction seems to depend upon suckling, and has the obvious biological function of protecting the young. Testosterone given to lactating females actually reduces their aggressive reaction to males.

Are specific parts of the brain involved in aggression?

Since much of aggression in the biological world is part of another behaviour, it is difficult to separate those areas of the brain responsible for the underlying behaviour (getting food, winning mates, etc.) from those associated with the particular behavioural strategy of aggression to achieve these ends. It has been known for many years that damage to a part of the brain called the amygdala results in ‘tameness’ and reduced aggression. The amygdala is part of the limbic system, a set of brain structures particularly concerned with survival, adaptation, and the defence of the body against the metabolic or social demands that constitute stress. The amygdala is closely involved in the ability of the brain to classify stimuli in a motivationally and emotionally meaningful way. Its role is not, therefore, restricted to aggression, but this along with many other behaviours is dependent on proper functioning of this part of the brain. Human cases are known in which disturbances of the amygdala have led to inappropriate or excessive aggression.

Another area of the brain implicated in aggression is the hypothalamus. Lesions or stimulation in several areas of the hypothalamus have altered aggressive interactions. The hypothalamus is implicated in other behaviours. For example, part of it has well-established roles in sexual and maternal behaviour, and it is prominently involved in the regulation of feeding and drinking. Bearing in mind the relation between aggression and other categories of adaptive behaviour, it is clear that there is still uncertainty about the exact role of the hypothalamus in aggression, and whether this can be truly separated from its other adaptive and homeostatic functions. Nevertheless, there are well-documented cases describing humans with tumours in the particular parts of the hypothalamus who became highly aggressive, responding with aggression to stimuli they would have previously considered only annoying. ‘Sedative’ surgical interventions, involving the hypothalamus, have been used in the treatment of aggressive patients.

We have seen that aggression forms an important part of social regulation and social interaction. This is known to involve the cortex of the frontal lobes of the brain. The frontal cortex is also intimately connected with both the amygdala and the hypothalamus and is therefore in a position to influence these other brain centres that control aggression. This behaviour can occur as a feature of frontal lobe damage in man. Patients with damage in one region of the frontal cortex react impulsively, without planning or taking into account the consequences of their behaviour; they are irritable and have short tempers, responding to minor provocation. But the frontal cortex is a complex area of the brain, and it is still not very clear whether particular parts may have distinct roles in aggression.

Are there specific aggression-related chemicals in the brain?

The brain is a chemical machine, and the recognition that different parts can be defined by the chemical transmitters that they use offers a different perspective. In humans, changes in the level and metabolism of serotonin have been correlated with affective behaviour in general and more specifically with aggressive behaviour. Serotonin has become the major focus of biological studies of suicidal behaviour (defined as ‘self-aggression’) and impulsive aggressive behaviour in humans. An association has been reported between low serotonin concentration in the brain and impulsive, destructive behaviours, particularly when aggression and violence are involved. Studies in animals show that a wide range of aggressive behaviours are sensitive to manipulations of the serotonergic system. Depletion of brain serotonin increases aggression. Conversely, serotonergic-enhancing drugs, such as the specific serotonin-uptake inhibitors (SSRIs), reduce aggression. A class of drugs acting on serotonin are known as ‘serenics’; these reduce aggression.

But serotonin is not the only neurochemical implicated in aggression. Animal studies suggest that increasing brain dopamine activity creates a state in which they are more prepared to respond aggressively to stimuli in the environment. Antagonists of dopamine receptors are the most frequently used therapeutic agents in the management of violent patients. However, dopamine has important roles in generalized behavioural categories such as reward or punishment; this may be the real reason why it contributes to the display of aggression.

Hyperactivity of noradrenaline in the brain has been found to correlate with aggressive behaviour in humans, and noradrenergic receptor blockade is clinically useful in its treatment. This is supported by the effects on aggressive behaviours in isolated mice of drugs that modify noradrenaline activity in the brain.

Many peptides are found in the brain, particularly in the limbic system, that act as neurotransmitters. One of these, corticotrophin releasing factor, is present throughout the limbic system. It has an important role in organizing the co-ordinated response to stress; this includes behaviour, hormones, and the emergency systems regulating the cardiovascular and other autonomic responses. It may also increase aggression. Vasopressin (first known as a pituitary hormone) is another peptide found in the limbic system, and microinjections of this into the hypothalamus and amygdala increased offensive aggression in rodents. Although alterations in several peptides, as well as other substances, are known to change aggression, no single one so far has been specifically associated with this behaviour. Clearly, given the current preoccupation with understanding and controlling aggression in man, the existence of such compounds, should they be proved, would be most important.

The complexity of aggression — the behaviour pattern, the contexts in which it occurs, and the uses to which it is put — means that there can never be a single, definable system underlying it. Nevertheless, attempts should continue to define aggression more precisely, since this offers not only greater understanding of the relation between this behaviour and others but also direct help to those who try to control undesirable aggression in either animals or humans.

J. Herbert

Bibliography

Albert, D. J.,, Walsh, M. L.,, and and Jonik, R. H. (1993). Aggression in humans: what is its biological foundation? Neuroscience and Biobehavioural Reviews, 17, 405–25.
Archer, J. (1988). Behavioural biology of aggression. Cambridge University Press.
Valzellli, L. (1981). Psychobiology of aggression and violence. Raven Press Books, New York.

See also hormones; peptides; serotonin; violence.