Biological Explanations of Aggression


There are three types of approach for biological theories of aggression. These are:







The 47 XYY Karyotype, first identified by Sandberg, 1961.

Numerous media reports identify a large number of criminals, imprisoned for violent behaviour, as carrying the XYY chromosone. Such reports have not been subjected to proper critical evaluation and much of the research also accepts the proposition uncritically. For example Court-Brown (1967) sampled 314 patients over a two year period (1965-1967)  and concluded those with XYY would be

best hospitalised due to an increased likelihood of aggressive behaviour

One classic study in the 1960s (Jacobs et al., 1965) found that a surprising number of men in prison had XYY sex chromosomes instead of the normal XY. The researchers hypothesised that the extra Y chromosome might make the men more aggressive. Later studies have found that such genetic abnormalities are in fact widespread throughout the general population and therefore can’t explain aggression.

More recently, studies have identified genetic trends in twins and families. For example, Brunner et al. (1993) identified a common gene in male members of a Danish family who all exhibited abnormal aggressive behaviour.



One way to explain why men are generally more aggressive than women is in terms of the male hormone testosterone.

Kalat (1998) reports that men aged 15 to 25, who have the highest levels of testosterone, also show the highest levels of violence as measured by crime statistics. Further evidence can be gleaned from the fact that in non-human animals, those males who have been castrated (and thus produce no male hormones) fight least.

Female aggression has also been linked to hormones. For example, Floody (1968) reviewed research on pre-menstrual syndrome and found evidence to support the view that during this time of hormonal fluctuation women increase in irritability and hostility, and also are more likely to commit a crime.

The neurotransmitter serotonin has also been linked with increased aggression. For example, people with a history of criminal behaviour have been found to have low levels of serotonin (Virkkunen et al., 1987).

A third way of explaining aggression in physiological terms is with reference to brain anatomy Raine, Buchsbaum, and LaCasse (1997) found significant differences in the brain structure of murderers and normal individuals, such as reduced activity in both sides of the prefrontal cortex and in the amygdala. One major difficulty with physiological explanations of aggression is that it is difficult to know whether physiological correlates are causes or effects of aggressiveness.


Ethologists and evolutionary psychologists argue that aggression must be understood in terms of its natural function. Animals, especially males, are thought to be biologically programmed to fight over resources. One of the classic ethological accounts was from Lorenz (1966). His conclusions were based on observations of non-human animals in their natural environment. He felt that his view was equally applicable to humans because they are governed by the same laws of natural selection. He argued that aggression is a highly adaptive response because an individual who is aggressive controls food, territory, and mating, and thus is the one most likely to survive to reproduce.

Evaluation of Biological Theories



+ Biological Theories have some practical usefulness, especially in the use of drug treatments to combat excessive aggression.


+ The ethological approach provides some evidence that aggression is an innate characteristic of all species


+ Biological theories lend strong support for the notion of instrumental aggression




– However they are highly deterministic and reductionist


– There is no conclusive evidence that aggressive behaviour can be carried through the genes.


– Biological explanations ignore the importance of environment. Further they cannot explain why cross cultural variations in aggressive behaviour exist.


– Important social psychological contributing factors are ignored

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