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Approaches to the Study of Aggression
Author Paul Kenyon
Neurotransmitters & Aggression Hormones & Aggression

Neuropsychology of Aggression & Fear
An important theme in this lecture is untangling the relationship between aggression, dominance and impulsiveness. We begin by looking at studies of aggression - behaviours intended to inflict damage on another individual - which have been studied from a variety of different theoretical perspectives.

By the end of the lecture you should be able to evaluate the explanations of aggression provided by ethology, behaviourism and evolutionary psychology.

Hormones, particularly testosterone are linked in the public's mind to aggression. But you will begin to appreciate that, although it is relatively easy to study the effects of hormones in rodents under laboratory conditions, things prove more complicated when we try to apply these insights to human behaviour where attempts to achieve dominance rather than inflict physical harm characterize agonistic interactions.

Another emerging theme from this material is that engaging in agonistic behaviour can influence the chemistry of the body.

The role of serotonin in agonistic behaviour is introduced in order to make the point that there is a link between agonistic and impulsive behaviour, and that aggression is not always directed outwards, but can lead to suicide. You should begin to appreciate that there are often links between the topics you study on this course. For example, work on serotonin and aggression is relevant to studies of the biochemical basis of depression.

Finally the lecture describes the role of the hypothalamus and amygdala in agonistic behaviour. When you study this material you should begin to see how the topics of fear, anxiety and aggression are interrelated. The ability to synthesize material in order to see patterns in evidence is an important intellectual skill that you should try to develop during your studies.

Learning objectives
After studying the material on this page you should be able to:
  • Distinguish between aggressive, dominance and agonistic behaviours
  • Describe the components of the Lorenz model of aggression
  • List two predictions made by the Lorenz model
  • Evaluate the strengths and weaknesses of the Lorenz model
  • Explain the advantages of an operational definition of aggression
  • Recognize the categories of aggressive behaviour proposed by Moyer and Brain
  • List the techniques used to study aggression in laboratory animals
  • Recognize the psychological factors thought to be involved in reproductive success by evolutionary psychologists
  • Evaluate the explanation of aggression provided by ethology, behaviourism and evolutionary psychology
  • Describe the origin and physiological effects of testosterone
  • Describe the effects of castration and hormone-replacement therapy on aggression in male and female mice
  • Critically evaluate the evidence of a role for hormones in human aggression
  • Evaluate the relationship between serotonin, aggression and impulsivity
  • Describe the role of the hypothalamus and amygdala in aggression

'We frown on aggression but thrive on dominance'

Watch Big Brother All Stars 24/7 on SuperPass

Aggression is overt behaviour intended to inflict damage on another individual. Human aggression is relatively rare - when was the last time you inflicted damage on someone, or were the victim of a physical assault? But it is very likely that you have been at the receiving end of a person's dominance behaviour.

Dominance behaviour is designed to achieve, or maintain, high status - to obtain power, influence, or valued assets - over a conspecific. It is displayed on our TV screens in programmes such as "Big Brother".


Tail-gating' is a threatening form of driving in which a car is driven close to the rear of the car in front with the intention of getting the slower driver to move over. Atzwanger (1995) videotaped drivers when they drove up close to another car. He found:

  • Men drove close up faster than women
  • Individuals, who drove alone, were more likely to tailgate than those with passengers.
  • Drivers of cars with higher status drove closer up than others.

In this example we see several themes that will be developed in this lecture: sex differences in agonistic behaviour, competition for resources, and impulsiveness in the face of danger.

Humans use threats and violence to dominate, but we also resort to cheating, deceit or negotiation to obtain status and valued resources. In fact a lot of our behaviour involves strategies for existing with each other without causing or risking actual physical damage. The TV program 'Big Brother' is an example of how people compete, co-operate and negotiate in pursuit of financial reward and status.

The term agonistic behaviour is a convenient way of referring to the constellation of 'aggressive' behaviours.

An important theme in this lecture is untangling the relationship between aggression, dominance and impulsiveness. We begin by looking at studies of aggression - behaviours intended to inflict damage on another individual

Perspectives on aggression

Aggression has been studied from a variety of different theoretical perspectives. Here is a flavour of three different approaches:

which have all offered something important to our understanding of this topic.

Lorenz - is aggression inevitable?

In a previous lecture you learnt about a simple biological theory of aggression based on Lorenz' theory of motivation.

Lorenz argued that aggression in animals and humans is an inherited, spontaneous tendency much the same as the motivation to eat, drink and make love.

Action specific energy accumulates in a reservoir until released by the appropriate external stimulus, represented by weights on a scale pan, or until the pressure on the valve causes an action pattern to occur spontaneously (vacuum activity). The consummatory response or fixed action pattern(s) released vary depending upon how much action specific energy is released from the valve.

This theory predicts predicts that:

  • Aggression is inevitable - the accumulating energy must find an outlet
  • Humans & animals will actively 'look for fights'.

Limitations of hydraulic models

Perhaps the strength of Lorenz' model is that it highlighted the importance of internal state and external stimuli in aggressioin.

But the fundamental weakness is that it fails to acknowledge that the consequences of behaviour on the animal's external environment can feed back to affect subsequent behaviours. This is the central point addressed by more sophisticated models such as that put forward by Deutsch.

It is now realized that aggression is not simply due to an accumulation of internal action specific energy.

We now appreciate that aggression is influenced by a variety of internal and external factors.

For example, in North America and Europe the number of assaults peaks during the hottest months of the year (Anderson, 1989).

But how do we explain this finding? Does increasing temperature make people more irritable. Does irritability lead to aggression? Are all forms of aggression due to irritability?

Clearly we need to begin to define more precisely what we mean by aggressive behaviour, and what pre-conditions lead to aggression.

aggression peaks in hottest months

Laboratory studies of aggression

Aggression is often due to a conflict between the interests of two or more individuals. Conflicts arise over limited resources such as territories, food and mates. According to behaviourists, one of the first steps necessary before any type of behaviour can be studied is to develop some method of operationally defining the behaviour so that everyone is agreed what constitutes a particular behaviour.

There have been several attempts to distinguish between types of aggression based upon antecedent conditions i.e. conditions that precede or lead up to the aggressive behaviour.

Types of aggression
Moyer (1968) distinguished between:
  • predatory
  • intermale
  • fear-induced
  • irritable
  • territorial
  • maternal
  • instrumental
Brain (1981), divided aggression into:
  • predatory attack
  • self-defensive behaviours
  • parental-defensive behaviours
  • social conflict

An important first step was to distinguish between affective (highly emotional) and non-affective -'cold-blooded' -attack. Predatory attack is clearly non-affective and may be a component of feeding behaviour.

Animal models of aggression

A number of animal models have been used to study the effects of internal states which can be manipulated by drugs, hormones and brain lesions on aggressive behaviours.

Ethological studies of aggression

Although the theoretical model of aggression developed by Lorenz have been abandoned, ethologists have provided important insights into the role of external stimuli in controlling aggressive behaviours.

"The overwhelming impression one gets from watching animal disputes is of remarkable restraint and self-control. The spilling of blood is not the norm - it is a rare event." Morris (1990).

The reason for this is because aggressive encounters involve threat and submissive/appeasement displays.

Human threat behaviours- raised arm threat.

Human attack behaviour - rapid, overarm blow at opponents head, humans don't have body parts that are specialized to damage opponents.

Submissive behaviours are very effective in stopping attacks. For example a wolf whose competitor rolls over and exposes his jugular vein will stop his attack.

In humans: person makes their body appear small, limp, shoulders hunched, face winces, hands spread, voice high and whining. "bowing and scraping" (Morris, 1977)

Submissive behaviours appear to be less effective in inhibiting human aggression e.g. CCTV pictures of attacks continuing even when victim is on the ground in the foetus position.

The term agonistic behaviour was introduced to cover threat, attack and submissive behaviours

The main problem for humans is the use of weapons as extensions of our bodies. They are impersonal - we can't see submissive behaviours of our opponents. Humans co-operate in their aggressiveness.

Humans are very sensitive to threat signals. For example, Hansen & Hansen (1988) found that subjects were much quicker and more accurate in picking out the one angry face in a crowd of happy faces, than the happy face in a crowd of angry people.

Human violence: An Evolutionary Psychology Perspective

Evolutionary psychology is an area of psychology that has been influenced by ethology, particularly the idea that behaviour has evolved to meet challenges posed by the environment in which a species has evolved.

Evolutionary psychologists are particularly interested in psychological mechanisms that:

  • are universal i.e. do not vary greatly between individuals
  • are closely related to reproductive success : e.g.
    • attracting a mate
    • choosing a mate
    • raising offspring
    • kin recognition
    • maintaining relationships
    • acquiring status
    • cheater-detection
    • maintaining group cohesion

Evolutionary psychologists are interested in why behaviours evolved in particular ways. They argue that males compete for mates whilst women choose dominant males. Males become dominant by being violent.

According to evolutionary psychologists, in species where one sex makes a higher parental investment than the other, the high investing sex is a resource for which the opposite sex competes. In humans females make a higher parental investment than do males. Males compete with each other for access to females. Males use their dominance and resources to attract females. Evolutionary psychologists argue that the higher rate of aggression in men shows the crucial importance of status to male reproductive success.

Females compete for access to males who will provide them with resources that will enable them to successfully raise their children.

Sex differences in human aggression

Over 80% of homicides are committed by men . Most of the victims are also men. The most common cause of homicide is due to the escalation of a relatively trivial disagreement over status that starts with words and escalates into lethal violence. It seems that men resort to violence to protect or gain status and honour.

This sex difference is found across all cultures. Criminal violence is most likely between the ages of 14 and 24.

Traditional psychologists argue that boys are trained to be aggressive and girls learn to be passive. However, Dyson-Hudson (1995) found that 'low-conflict societies' with affectionate socialization and aversion to inter-personal confrontation (e.g. Inuit, !Kung Bushmen, Gebusi of lowland New Guinea) have high rates of violent death. In contrast, Turkana pastoralists (East Africa) are taught to fight as children; and most men reported having participated in inter- personal fights intended to cause injury, having engaged in recreational within-group fighting mimicking warfare, and having taken part in raids on the neighboring Pokot. Yet demographic data indicate that within- group homicide rates among the 'violent' Turkana are lower than those reported for the 'low-conflict' societies.

It may be that Turkana rules which require bystander intervention and adjudication by elders, are effective in preventing within-group aggression and violence from escalating into lethal fights.

Male aggression

bullitt.jpg (11758 bytes)Richard Wrangham (Wright and Wrangham, 1998) presents an interesting analysis of male violence in terms of evolutionary psychology. He argues that:

weweresoldiers.jpg (11343 bytes)However warfare is a uniquely human behaviour.   In a battle both sides will suffer casualties regardless of who finally wins. Consequently battles involve a failure to assess the true costs of combat by both sides. Wrangham suggests that this failure is due to 'positive illusions' by each set of combatants that they will emerge victorious

Female aggression

Until recently, relatively little attention was focussed on female aggression. Campbell (1999) argues that

".. lower rates of aggression by women reflect not just the absence of masculine risk-taking but are part of a positive female adaptation driven by the critical importance of the mother's survival for her own reproductive success."

phoolan_devi_bandit_queen.gif (20452 bytes)
Campbell reviews evidence that:
  • women show  greater fear of physical harm compared to men.
    For example:
    • women show more fear of open spaces, dogs, snakes, insects, and rodents than men
    • women  are less likely to engage in hazardous sports, dangerous driving, military combat, and drug abuse, than men
    • women are more afraid of being victims of crime involving aggression, and are more likely to visit a doctor to seek advice on preventative care, than men
  • women commit fewer violent crimes than men (see Campbell et al, 2001)
  • women show less concern for status compared to men
  • greater adoption of dispute resolution strategies that involve a low risk of physical harm by women compared to men
  • female 'maternal aggression' to defend their offspring; paternal aggression is rarer
  • female menopause - an infertile period after the birth of the last child will ensure its survival

This is a picture of Phoolan Devi, (Seema Biswas) so-called "Bandit Queen of India", she led a gang  who roamed north central India during the late 1970s and early 1980s; she became a folk hero after taking bloody revenge against men who raped her.

These stills are taken from the French film 'Baise Moi" which was banned in several countries. The film deals with a young woman who has been raped, and an accomplice, who embark on a spree of violence and promiscuous sex. It is interesting to reflect on this film's treatment by censors in the light of Campbell's argument that

"...Women's aggression has been viewed as a gender-incongruent aberration or dismissed as evidence of irrationality. These cultural interpretations have "enhanced" evolutionarily based sex differences by a process of imposition which stigmatises the expression of aggression by females and causes women to offer exculpatory (rather than justificatory) accounts of their own aggression."

baise-moi-film.jpg (20738 bytes) baise-moi-film3.jpg (8546 bytes)

Hormones & Aggression

Testosterone and aggression in rodents

Testosterone is an androgen . It is made by Leydig cells in the male testis, as well as the adrenal cortex and ovary of both sexes .Testosterone in men is secreted into the bloodstream in spurts, so levels can change dramatically within minutes. The hormone is released in a circadian rhythm in both sexes, highest and most variable in the morning, lower and more stable during the afternoon. Synthetic testosterone such as testosterone propionate is a synthetic drug which is absorbed more easily, and has longer lasting effects than the naturally occurring hormone.

Testosterone has

There is evidence that androgens affect aggression:

Experiments in rodents suggest that androgens increase aggression in male and female mice

Male aggression & testosterone

Redrawn from Wagner et al (1980)

In male mice

  • castration reduces aggression
  • testosterone restores aggression in castrated mice
  • androgens have an organizational effect on aggression in infancy
  • exposure to androgens in infancy increases the sensitivity of the adult brain to androgen

There is evidence that high levels of testosterone are necessary but not sufficient to trigger aggression. There are individual differences in the aggressive behaviour of mice. In one experiment male mice were rated as aggressive or non-aggressive. They were then castrated which reduced their aggression. When they were given testosterone replacement therapy, only those mice who were initially rated as aggressive showed a restoration of aggressive behaviour. In other words testosterone is necessary for aggressive mice to exhibit aggression, but injecting testosterone is not sufficient to turn a previously non-aggressive mouse into an aggressive mouse.

Can you think of an explanation for this finding based on the idea that early exposure to testosterone organizes the brain?

Female aggression & testosterone

Redrawn from Carlson (1998)

In female mice

  • testosterone - but not oestrogen - increases aggressiveness in ovariectomised females (van de Poll et al, 1988)
  • exposure to androgens prenatally - due to intrauterine position - increases aggressiveness in adulthood

Hormones and human aggression

Nelson (1995) reviews the relationship between hormones and aggression in humans. Unlike the animal studies we have reviewed, these studies rely on interviews, questionnaires or criminal records to assess aggressive behaviour.

One problem in this area is the time that elapses between the aggressive act(s) and the analysis of hormone levels. Nevertheless it does appear that there is a positive correlation between violence in male and female prisoners and circulating levels of androgen. But it is unclear whether androgens:

Mazur & Booth ( available online ) argue that "high levels of endogenous testosterone (T) seem to encourage behavior apparently intended to dominate -- to enhance one's status over -- other people. Sometimes dominant behavior is aggressive, its apparent intent being to inflict harm on another person, but often dominance is expressed nonaggressively. Sometimes dominant behavior takes the form of antisocial behavior, including rebellion against authority and law breaking... T not only affects behavior but also responds to it. The act of competing for dominant status affects male T levels in two ways. First, T rises in the face of a challenge, as if it were an anticipatory response to impending competition. Second, after the competition, T rises in winners and declines in losers. Thus, there is a reciprocity between T and dominance behavior, each affecting the other. "

There may be important individual differences in our response to testosterone. In a recent report Harrison et al (2000) administered testosterone to 56 men aged 20 to 50 years. Testosterone treatment significantly increased aggressive responses on a frustration-inducing computer game involving a fictitious subject. This effect, however, was not uniform across individuals; most showed little psychological change, whereas a few developed prominent effects.

Androgens and sport

It is possible that these effects are due to changes in status because:

Consequently these effects may be related to changes in dominance. Jeffcoate et al (1986) found that testosterone level changed in men as a function of their changing dominance-ranking within a group aboard a holiday yacht.

'Basal' and 'Reciprocal' Models of Testosterone Effects on Behaviour

It is difficult to know whether testosterone causes changes in a person's dominance - the basal model , or if testosterone levels vary as a function of a person's position within a dominance ranking - the reciprocal model.

According to the basal model testosterone level is a stable trait, and consequently it is possible to predict behavior from testosterone measured at a single point in time. It suggests that men with high basal testosterone level engage in dominating or antisocial behavior.

Mazur & Booth ( available online ) review studies showing that men with higher levels of testosterone are more likely to:

According to the reciprocal model testosterone levels vary as a function of a person's dominance.

In a study of marital status among 2,100 male Air Force veterans who received four medical examinations over a ten year period, testosterone levels fell and remained low with marriage, and rose with divorce. These results are consistent with the reciprocal model (Mazur & Booth available online )

Neurotransmitters & Aggression

Serotonin and aggression- animal studies

Increased serotonergic activity tends to reduce aggressive behaviour in rodents.

Adapted from Fig 11.18 Carlson (1998)

Higley et al (1996) studied free-ranging rhesus monkeys living on an island. Used behavioural observations and sampled CSF (cerebro spinal fluid) to measure 5-HIAA levels (5-HIAA is a breakdown product of 5-HT - the more 5-HIAA the greater 5-HT release).
  • found negative correlation between 5-HIAA and aggression. No relationship between aggression and NA or DA metabolites.
  • Low 5-HIAA associated with high risk-taking behaviour -aggression towards older larger animals, took long leaps from tree to tree. Many died as a result of attacks from mature males.
  • low 5-HT turnover may reflect low impulse control rather than increased aggression per se

"Dominance and aggression are not synonymous." (Carlson,1998)
landseer22.jpg (6099 bytes)

(Sir Edwin Landseer.
The Monarch of the Glen. 1851)

boxing_free.jpg (11185 bytes)

Serotonin levels are affected by dominance rank. Raleigh et al, (1984)

Redrawn from Fig 9.25 Feldman (1997)

Raleigh et al (1991) investigated effects of serotonergic drugs on dominance and aggression. Used 12 groups of vervet monkeys. Temporarily removed dominant male from each group. The two remaining subordinate monkeys were treated with
    • serotonergic drug
    • control (placebo)

The serotonergic drugs used were

    • 5-HT agonists (tryptophan or fluoxetine) which increase 5-HT activity
    • 5-HT antagonists (cyproheptadine or fenfluramine) which decrease 5-HT activity (chronic treatment with fenfluramine depletes 5-HT levels )

Used a crossover design so that each monkey received agonist and antagonist treatments.

    • monkeys given agonist drugs became dominant
    • monkeys given antagonist drugs became subordinate
    • monkeys given agonist drugs initiated fewer aggressive events
    • monkeys given antagonist drugs initiated more aggressive events

Note that because a cross over design was used the same animal could be dominant or subordinate depending on what type of serotinergic drug they received.

Serotonin and human aggression

  • Reduced concentrations of 5-HT and 5-HIAA in brains of suicide victims.
  • Maybe suicide and violence towards other people represent the same underlying aggressive tendency
  • Low 5-HIAA levels in brains of suicides who used violent means to end their own lives (using guns or jumping from heights rather than by ingesting pills or taking a poison)
  • in normal adults there is a negative correlation between 5-HIAA level and 'urge to act out hostility' subscale of the Hostility and Direction of Hostility Questionnaire
  • in psychiatric patients there is a negative correlation between 5-HIAA level and psychological measures of aggression
  • low 5-HIAA linked to impulsive, antisocial aggressiveness
  • low 5-HIAA reported in children with disruptive behaviour

Fenfluramine challenge

The release of the hormone prolactin from the pituitary gland is controlled by serotonin.

The drug fenfluramine:

  • releases 5-HT from presynaptic terminals
  • which in turn increases the release of prolactin from the pituitary into the blood stream

The amount of prolactin released by a fixed dose of fenfluramine varies between individuals. Measuring the amount of prolactin released by fenfluramine (fenfluramine challenge) is used an an index of the status of the 5-HT system. Coccaro et al (1989) studied male patients with personality disorders. Patients were given a fixed 'challenge' dose of fenfluramine. Blood samples were taken and analysed for prolactin levels.

There was a significant negative correlation between prolactin concentration scores on BDHI (Buss-Durkee Hostility Inventory) subscales which measure irritable, impulsive aggression . The more aggressive patients showed an attenuated response to fenfluramine suggesting that they had reduced serotonergic activity.

This result is consistent with a picture of low serotonin activity being associated with increased aggressive and impulsive behaviours.

Pharmacological treatment of aggression

It may be possible to treat aggression by increasing serotonergic activity by administering the antidepressant drug Prozac (fluoxetine) which is a 5-HT reuptake inhibitor (Coccaro & Kavoussi, 1997)

Serotonin and mood: Conclusions  

Neuropsychology of Aggression & Fear

Brain and aggression



Fear and the amygdala

amygdala-hippocampus.jpg (13570 bytes)The central nucleus of the amygdala may be a control centre for fear, receiving fear-related sensory information and transmitting fear-related motor instructions.

LeDoux (1994) suggests that the amygdala receives input from three areas:the thalamus, cortex and the hippocampus

The thalamus and cortex convey information from the environment The thalamus to amygdala pathway carries information rapidly to the amygdala

The thalamus to cortex to amygdala pathway is slower but allows the external stimuli to be cognitively appraised.

The hippocampus is involved in memory and may store information about the emotional significance of external stimuli . Memories can elicit fear presumably through a hippocampus to amygdala pathway .

Source: LeDoux (Scientific American, 270(6), 50-57, 1994)

See also, Laura Arendal The Emotional Brain (2000). Discusses the idea that fear been hardwired into our brains over the course of evolution, and  LeDoux's theory of emotion.

Online resources


Copyright Dr. C.A.P. Kenyon 1994-2006