Aggression in Humans

There is no question that a wide range of factors can trigger acts of aggression in people but how much of this is under our control? In recent years great advances have been made in understanding the biochemical processes that can lead to aggressive behaviour. Much is yet to be discovered but at present it looks as if several chemicals play critical roles. Top of the list is serotonin, a neurotransmitter (also known as 5-HT) that has long been linked with many different aspects of human behaviour.

Next there are monoamine oxidases, enzymes which play a critical role in breaking down amine based chemicals in the body, such as adrenaline, melatonin and serotonin. Then there is testosterone, the male androgenic hormone which is synthesized in the testes of males and, to a much lesser extent, in the thecal cells of the ovaries of females. A surprising name on this list of chemicals linked with aggression is none other than the familiar enemy of our coronary arteries, cholesterol. Finally, I will one more compound that is often suspected of wreaking havoc with the processes under discussion; alcohol.

Serotonin plays a crucial role in effecting many different aspects of behaviour, including appetite, sexual behaviour, memory and general mood, as well as a variety of physiological processes, including cardiovascular function and hormone balances. A series of postmortem studies in the 1970s showed that suicide victims often had very low serotonin levels in their cerebrospinal fluid. Later studies showed that similar low levels were often to be found in people suffering from depression or with suicidal tendencies. Anti-depressant drugs appear to act by raising serotonin levels; increasing concentrations of serotonin in the brain appear to inhibit impulsive and aggressive behaviour and combat depression.

A host of studies have produced evidence that people with lowered serotonin levels in their brains are far more likely to commit violent crime, behave impulsively and to commit suicide. At the other end of the scale, higher levels of serotonin are linked with rather more desirable emotions, such as love and a sense of optimism, whilst at higher levels still serotonin may be the cause of various forms of neuroticism, such as obsessive-compulsive behaviour. At really high levels, sometimes experienced by those taking drugs to enhance serotonin levels, it may cause anxiety, confusion and tremors.

Although there is still a lot of debate over exactly how it works, Prozac appears to alleviate depression by preventing reabsorption of serotonin by neurons in the brain and therefore keeping the concentration in the brain at an increased level. This may even add an unexpected danger to the use of Prozac; if taking the pills ceases abruptly, serotonin levels in the brain may plummet to a very low level and it has been suggested that this may bring about outbursts of violent behaviour.

While serotonin clearly plays a role, there are other neurotransmitters, such as dopamine, which are implicated in aggressive behaviour. There is even a dispute about which region of the brain is responsible for controlling aggression. At one time, the amygdalae, a group of neurons deep within the medial temporal lobes, were considered the sites of aggression but this behaviour is now being linked with several other areas of the brain, particularly the hypothalamus (learn about what thalamus does) and parts of the cortex. When the causes of aggression are better understood, it seems certain that serotonin will still figure highly in the chemicals involved but the final explanation may be far more complex than we can know at present.

Monoamine oxidases (MOAO) break down serotonin, so it is easy to see that a high level of MOAO activity could result in low serotonin levels and therefore a tendency towards aggressive behaviour. It also follows that substances which inhibit MOAO activity can act as anti-depressants, by preventing breakdown of serotonin and many drugs have this very effect. This article is about aggression, not depression, but the link between the two is so close that they often seem inseparable. It should be added that serotonin itself is not used as a drug, since it does not cross the blood brain barrier and is therefore of no value in its pure form.

Testosterone has long been regarded as the hormone at the heart of male aggression but the precise nature of its role is still not clear and most of the evidence regarding its role comes from research on other mammals. Countless studies have shown that aggressive behaviour is drastically reduced in castrated animals but the problem here is that removal of the testes has widespread effects on hormone levels and does not just affect testosterone production. Whatever the role of other hormones, testosterone does appear to have a role to play in promoting aggression by lowering serotonin levels and therefore removing its potential inhibitory influence.

One theory suggests that testosterone is correlated with competitiveness and aggressive urges, rather than actual violence, whilst serotonin functions to restrict any acts of aggression to an appropriate moment or setting. Individual animals with high levels of testosterone may exhibit correspondingly high levels of aggression but there are appear to be mechanisms in place for moderating aggressive urges. Violence may occur when these mechanisms are altered by external factors, such as olfactory stimuli or alcohol. Previous experience is also important and studies have shown that individuals that have obtained rewards through aggressive behaviour are more likely to show aggression and use violence in subsequent encounters. Perhaps one of the most fascinating findings in all this is that there is a critical period, shortly after birth, when testosterone acts on the brain to sensitize certain neural circuits in a way needed to elicit aggressive behaviour later in life.

High testosterone levels are often found in animals that are aggressive but high levels can also be found in those that do not show aggression. In humans the evidence for the role of testosterone in aggression is not yet clear and it is evident that many other factors are involved and as humans we should never understate the moderating influence that learning and higher thought processes have on our behaviour.

The link between cholesterol and heart disease is long established but there was widespread consternation in the 1980s when researchers testing drugs designed to lower elevated cholesterol levels found that people taking these drugs seemed to be dying at an unexpectedly high rate in rather violent ways and from causes unrelated to cardiovascular disease. Other researchers have reported similar patterns, finding that the risk of suicide seems to be significantly higher in people with low blood cholesterol levels. Another study showed a strong link between low cholesterol and impulsive, aggressive behaviour, whilst many others reported links between low cholesterol and depression.

No-one is quite sure of what effect low cholesterol has but, once again, it appears that there is a link between blood cholesterol and serotonin. For example, a study on monkeys showed that those kept on low-cholesterol diets had brain serotonin levels that were only about half as high as those kept on high-cholesterol diets. They were forty percent more likely to engage aggressively or antisocially with other monkeys and this observation applied to both sexes. The implication is clear; people on low-cholesterol diets may show aggressive tendencies, especially if they are already disposed to low serotonin levels. Where that leaves people who have been medically advised to go on low cholesterol diets is well beyond the scope of this article but if such a person had a history of impulsive behaviour or depression, he or she would be advised to proceed very carefully.

Alcohol has long been linked to depression, violence and suicide and here, yet again, serotonin figures highly. In many animals, drugs that decrease serotonin activity increase alcohol consumption but when the same animals are given tryptophan, a compound needed to synthesise serotonin in the central nervous system, their serotonin levels rise and their alcohol consumption drops. Many studies have shown that reduced levels of serotonin appear to predispose people to alcoholism and also to make them more likely to behave violently and suicidally. It also appears that such tendencies are more likely to manifest themselves in males whose testosterone levels are high.

What we have seen in this brief account is that a range of chemicals are linked with aggressive behaviour, with serotonin emerging as the key player. The ways in which these chemicals interact in our bodies and influence our personalities are bound to be largely inherited but we can also see that their effects and levels can be influenced by environmental factors. It seems incontrovertible that some members of our society are more predisposed to violence than others but behaviour is not immutable and our brain chemistry is determined by the signals it receives. If we understand this chemistry, we are in a better position to influence it and ensure that our brain receives the right types of signal.