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Two opposing views of aggression
Author Paul Kenyon
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Konrad Lorenz
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Fred Skinner

'A Harvest of Death', photograph taken after Battle of Gettysburg, July, 1863

Two opposing views of aggression

  1. Aggression in animals and humans is an inherited,
    spontaneous tendency much the same as
    the motivation to eat, drink and make love.

    "Aggression is in our nature"

  2. There is nothing inevitable about the expression of aggression, it depends as much upon experience and external factors as the internal state of the animal.

    "Aggression is in our nurture"



Complexity of Human Aggression

'More confused than most, less informed than many'

Nature Genetic Inheritance of behavioral capacity for aggression. Behavioural genetics james-watson2.jpg (3820 bytes)

James Watson

Physiological Control of aggression by brain, blood and hormones. Physiological psychology beach.gif (11970 bytes)

Frank Beach

Organism Interaction between genetic and physiological factors. Ethology - animal behaviour lorenz.gif (11496 bytes)

Konrad Lorenz

Nurture Organism Psychology of learning.
  • Pavlovian conditioning
  • Operant conditioning
  • Social modeling


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Fred Skinner

Society Cultural and political determinants of aggression
  • sociology
  • anthropology
  • political science
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Steven Rose

Lorenz' Hydraulic Model of motivation

Action specific energy accumulates in a reservoir until released by the appropriate 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.

Here is a VRML version of Lorenz' model. This requires a VRML plug-in



A photo taken during the Viet Nam war titled 'You-VC'

Hydraulic Theory predicts:

  1. Aggression is inevitable - the accumulating energy must find an outlet
  2. Humans & animals will actively 'look for fights'.
  3. After an attack an animal / human will become less aggressive.
  4. Animals reared in isolation will show aggressive behaviour.

Guppy courtship

The picture below illustrates the main concept behind the Lorenz model and the relationship between external sign stimuli, accumulating action specific energy and behaviour. In the diagram:

The person in this picture would eat ice-cream even if it was presented shortly after a meal, but wouldn't eat any of the other foods. But if 10 hours had passed since their last meal they would eat a beefburger if it was the only food available!

According to Lorenz's theory the degree of consummatory response is a function of the amount of accumulated action specific energy and the sign stimuli to which the animal is exposed. Baerends and his colleagues have provided an elegant demonstration of this principle. Male guppies exhibit several patterns of courtship behaviour. The external markings of a male guppy vary with its readiness to show courtship. In terms of Lorenz's model, the external markings are an indication of the level of action specific energy for courtship. The sign stimulus value of the female increases with her size. Baerends conducted experiments in which males with different external markings were exposed to females of various sizes. The results of these tests are shown below and indicate that for each pattern of male courtship behaviour, the size of the female needed to elicit the pattern was less the greater the readiness of the male to court.

guppy experiment

The picture above shows the influence of the strength of external stimulation (measured by the size of the female) and the internal state (measured by the colour pattern of the male) in determining the courtship behaviour of male guppies.
Each curve represents the combination of external stimulus and internal state that produces the sigmoid courtship patterns of increasing intensity P, Si, S (After Baerends et al, 1955).
Note how internal state and external stimulation have been operationally defined in this experiment

Raza's experiment on aggression in male cichlids

Can Hydraulic Theory explain these results?

Deutsch's Model of motivation

deutsch's model of motivation

The recommended textbook (Manning, An Introduction to Animal Behaviour) gives a very good account of the strengths and weaknesses of these two models of motivation. Deutsch's model addresses a fundamental weakness of Lorenz' Hydraulic mode. It includes a feed-back link from the environment to the Analyser component which signals to the animal that its goal has been achieved. In turn the receptor system inhibits the Central link (shown in blue) and thereby the Motor link which is responsible for triggering behaviour is switched off.


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 & 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?


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

Online resources

The mind of a killer. A neurologist who studies murderers' brains talks about factors that make someone kill, the difficulty of predicting violence and why most murderers can never be rehabilitated.

Crime Times is an online resource which contains research reviews and information on biological causes of criminal, violent and psychopathic behaviour

Follow this  link to the Index of Crime Times and search for articles on research into the links between aggression and the hormone testosterone, and aggression and the neurotransmitter serotonin. Here are some useful articles I found useful in  'Crime Times' :

In a provocative article (Science, Biology, and Conflict ) Keith Webb from the University of Kent argues for a total rejection of biological explanations of human aggression. Be warned this is a very long and scholarly article. We do not require you to read it as part of this module, but you may find it interesting if you are interested in the tensions within the social sciences arising from different philosophical approaches to the study of human behaviour
Here is his concluding section which gives a flavour of the intensity of his argument:
The conclusion of this paper is a wholehearted rejection of any consideration of biological, genetic, or evolutionary factors in the explanation or analysis of conflict. While a number of different arguments are presented, and it is admitted in some cases that the outcome is not conclusive, the net consequence of these arguments is in favour of the rejection of biology and the acceptance of social explanations based on human openness and what have been described as the essential features of man. But this has been said before: the proper study of man is man.
But we can, perhaps, go further than this. The biological interpretation of the world is not just a description of the world, but like all social theory has prescriptions embedded within it. These prescriptions are profoundly reactionary and operate to justify the existing social order. Thus Shaw and Wong (1989: 195-209) attack 'top-down' and 'bottom-up' approaches to peace, as well as functional or world society approaches (e.g., Banks, 1984) as approaches which do not recognise the essential nature of man. Their own scheme for peace is no more convincing than that of Lorenz before them - educational efforts, a redefinition of inclusive fitness, and a voluntaristic change in attitudes and beliefs. But, in terms of their own theory, such hopes are utopian; rather, they should accept the dismal consequences of their own ideas and recognise that peace is unattainable. The real message of biological arguments is that the human race will forever be embroiled in violent confrontations and thus arms races, nuclear deterrence, balances of power, and military alliances, are the best means of attaining our security. Given the weakness of biological arguments, however, there is no need to accept these conclusions, and hence no good reason to succumb to a mindless fatalism.

Here is a much more accessible article Violence in Sports by Ismat Abdal-Haqq which may be of interest to some of you sports fans!

Here are several articles provided by an Internat magazine called Psychiatric Times that are concerned with the practical aspects of dealing with aggression:


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