Analysis of aggression among animals reveals a basic set of conditions under which more or less all species, especially those towards the upper end of the evolutionary scale, resort to inflicting injury and death on other species, as well as on members of their own.

Predatory behaviour for sustenance is the most obvious case of destructive behaviour against another species. Procuring territories that provide the conditions for subsistence is another. These situations translate to aggression for food and shelter, driven by self-preservation. Within-species aggression serves the same purpose in situations where resources are scarce.

Within-species aggression also pursues reproductive ends, as males fight for sexual access to receptive females. Such aggression might be considered to serve the preservation of the species. But it is generally held that within-species fighting does not aim to inflict injury or death. Its object is to induce rivals to yield in the competition over resources.

Is aggression among humans precipitated by the same circumstances? Some scholars suggest that aggression-provoking conditions are in essence the same for all species and that there is nothing unique about human aggression. After all, humans liberally slaughter other species for nourishment, fight wars with each other over territories, inflict injury and death on others to usurp their valuables, and use violence to defend what they hold dear. Often, sexual rivalries escalate to involve brutality.

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Other scholars find these analogies wanting. They point to the evolution of the human neocortex and the fact that its mental capacity far exceeds that of other species. Moral contemplation (the ability to judge what is good and right under given circumstances) and volitional control (the ability to align one's actions with moral assessments) take centre stage in such theorising. Although they may concede the existence of archaic aggressive impulses, these scholars believe that, as a rule, rationality can quash them. They contend that little, if anything, can be learnt from analyses across the human/non-human divide.

I, however, propose a more integrative stance, acknowledging both our ancient evolutionary heritage and the comparatively recent expansion of our cognitive faculties.

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It is generally held that evolution of the brain progressed from a reptilian core, which was encapsulated by palaeomammalian structures, known as the limbic system, which, in turn, were encapsulated by neomammalian structures, the neocortex. Notwithstanding development of an especially large neocortex, our brains have retained the tripartite structure that integrated the earlier evolved, ancient structures. More important, these structures continue to influence all vital human behaviours.

The limbic system controls all human emotions. The amygdala, a part of this system, has emerged as the most significant structure in controlling aggression. It is involved in monitoring the environment for danger cues and responding to them by initiating endocrine processes to help people cope with them in a physical way.

Coping with immediate threats of danger requires an individual to be provided instantly with energy for vigorous action, primarily for warding off a threat by attacking it or for eluding it by retreating swiftly. The energy needed is mediated by the systemic release of mostly adrenal hormones that foster sympathetic excitement, in large measure by providing glucose to the skeletal muscles.

This set of responses defines the fight-or-flight reaction, which is ideal for behavioural emergencies that can be resolved by forceful action. In evolutionary terms, the mechanism for such action has served the species well. It has helped humans to survive unexpected confrontations with predators and hostile humans. Becoming roused and agitated, feeling strong and able to rise to a challenge, and focusing fully on the here and now in coping with danger proved to be highly adaptive.

It is only in modern society that this adaptive value has been compromised. As a rule, threats of danger can no longer be countered with direct assault or spontaneous escape. Instant physical action cannot eliminate the adverse consequences of radon gas in one's home. And the fight-or-flight reaction is no use in the face of problems such as taxation or global warming.

Probably most important is that societal sanctions curtail, by threat of punishment, resolution of common conflict through violent or evasive action. It is inadvisable for someone whose car is dented by a careless driver to beat him up in a fit of rage, and it is usually not feasible for someone who owes child support to flee the country on impulse. Nonetheless, all such cases of provocation and frustration engage the archaic brain structures to initiate potent excitatory reactions, even though these reactions have largely lost their utility. This often fosters unbearable anger and triggers violent action that is incapable of removing the grounds for the emotion.

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In understanding the emotions of fear and anger, it is important to recognise their initial function as well as their more recent dysfunction. The initial function was twofold: to furnish energy for a bout of action and to focus attention to the here and now of the action. These two responses, known as action impulsivity and cognitive deficit, still characterise anger and rage. One urges aggressive action irrespective of that action's ultimate usefulness. The other, because of cognitive preoccupation with the immediate situation, makes individuals negligent of all non-immediate implications of their actions. This deterioration of cognitive control, which makes people oblivious to the consequences of their violent actions, is obtrusive enough to be widely considered a form of temporary insanity, mitigating responsibility.

The propensity to commit acts of destructive violence resides in all of us. Threats of harm and degradation elicit excitatory reactions that at extreme levels are bound to lead to uncontrolled, impulsive, aggressive behaviour. Excitatory residues from unrelated frustrations and the challenges of everyday life often enter the excitatory reaction to specific circumstances. Because anger can be fuelled by excitation from different sources, seemingly minor disagreements often escalate to fury and violent conflict.

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So much for the influence of archaic brain structures. What about the role of the structures that set us apart from other species: the neocortex with its associative anticipatory and inferential powers?

Most scholars of aggression embrace the view that superior rationality, afforded us by the neocortex, is the antidote to violence. Rationality is seen as the panacea to all baser human compulsions. No doubt rationality can, and often does, prevent violent outbursts. But when it comes to preventing violence, reason regularly fails us.

More important, rationality not only fails to provide an effective antidote to violence, it is the direct cause of enormous amounts of violence perpetrated by humans against each other. It is our ability to reason that tells us that taking others' valuables by force in a way that minimises or entirely avoids repercussions is a winning formula. Our anticipatory skills can help construct strategies that make violence pay. They not only place every individual at risk of coercing others by way of aggressive action, they also inspire organised violence and warfare.

Human aggression is not merely helped by superior anticipatory skills and the strategies they yield. It is also driven by what some consider the very highest form of rationality: moral reasoning. The moral concepts of equity and retribution constitute major sources of aggression. Social justice comparisons that place us at the short end of rewards, despite comparable efforts, are infuriating and can instigate aggression.

Violation of our sense of justice demands retaliation. If we have been wronged, we must get even. The desire to retaliate to set things right leads frequently to interpersonal clashes. Wars are often fought because someone persuaded the populace that past humiliations cannot go unpunished. On occasion, even the perpetration of the vilest of atrocities is construed as morally mandated, usually by reference to divine authority.

So the neocortex, while enabling us to recognise the social ills and global dangers of violence, has furnished new and uniquely human reasons and avenues for aggression. The thoroughly reasoned conception of strategies for effective aggression and the moral mandate for aggression are absent in other species. These motives for aggression set us apart from other animals. At the same time, however, we continue to share with other primates, and other species, the motives for aggression that reside in the archaic structures of our triune brains.

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Dolf Zillmann is professor of information sciences and psychology at the University of Alabama.

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