To read up on biological explanations of aggression, refer to pages 170–177 of
Eysenck’s A2 Level Psychology.
Ask yourself


Are men more aggressive than women because they are “built” that way?
Indeed, are men actually more aggressive than women?
 Are children in aggressive families aggressive themselves because of seeing role models in their families, as a response to being treated badly, or is it a straightforward case of inheritance?
What you need to know
NEURAL AND HORMONAL
MECHANISMS
GENETIC FACTORS
 The neural and hormonal mechanisms involved in
 aggression
How maladaptive brain structures may cause aggression
 The genetic factors involved in aggression, including inherited maladaptive genes
In this section, we will consider research and evaluate it as we go along.
NEURAL AND HORMONAL MECHANISMS
RESEARCH EVIDENCE
 Animal studies indicate that aggressive behaviours come from the prefrontal cortex, hypothalamus, and amygdala (Gregg, 2003, see A2 Level Psychology page 171). However, it cannot necessarily be concluded that this applies to humans, who are controlled to a greater extent by learning and social convention than are non-human animals.

 Haller and Kruk (2003, see A2 Level Psychology page 171) describe how adrenaline, noradrenaline, and other stress hormones are influenced by the limbic system and may result in dysfunctional behaviour such as aggression.
They suggest a positive correlation between noradrenaline and aggression.
This and other evidence indicates that long-lasting stressors can lead to aggressive behaviour. However, correlation does not mean causation. It is

 possible that, rather than hormones triggering behaviour, certain behaviour causes the release of hormones so cause and effect goes in the other direction. Alternatively, a third factor, such an environmental trigger, could cause both aggressive behaviour and an increase in the level of hormones such as testosterone.
 Virkkunen (1985, see A2 Level Psychology page 171) found that habitually violent offenders show lower levels of glucocorticoids than non-violent people. This has been conformed in men and women (Pajer et al., 2001).
Haller and Kruk (2003) suggest that low levels of glucocorticoids lead to abnormal fears in social settings, which in turn lead to an aggressive response. This has been supported by research on rats.
 Virkkunen et al. (1987, see A2 Level Psychology page 172) suggest that low levels of serotonin result in aggressive behaviour. They found low levels of serotonin in people who are habitual criminals.
Brunner et al. (1993, see A2 Level Psychology page 172) found low levels of monoamine oxidase A (MAOA) in a family in which there was a history of violent behaviour in the men. Since MAOA breaks down serotonin and noradrenaline, it appears that these individuals had higher levels of these substances. With respect to serotonin, this contradicts the previous finding.
 Raine, Buchsbaum, and LaCasse (1997, see A2 Level Psychology page 172) used PET scans to compare the brains of normal individuals and murderers who had pleaded guilty on grounds of diminished responsibility, and found significant differences in structure of the prefrontal cortex and the amygdala between the groups. The murderers also showed reduced activity in these areas compared with non-murderers.
 Testosterone has also been implicated in aggression and may account for why males are more aggressive than females since they produce far more of it. Kalat (1998, see A2 Level Psychology page 172) reported that in 15- to 25year-old men those with the highest level of circulating testosterone also showed the highest levels of aggression as shown by crime statistics.
EVALUATION
 The research studies provide useful indicators of the biological roots of

aggression. They also provide material for further research.
Supportive evidence. There is supportive evidence for the role of biology in
 aggression, especially in non-human animals.
Reductionist approach. The approach is reductionist and cannot explain the wide range of aggressive behaviours.
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
Deterministic. It is deterministic and does not allow for freewill; for people to make the decision not to be aggressive or, indeed, to be aggressive.
Issues of generalisability. Sample sizes of brain studies are small and therefore it is difficult to generalise from them.
 It does not account for individual differences. For example, it does not account for why stress leads some people to be aggressive but does not affect everyone that way.

 Cultural differences indicate that aggression is to some extent learnt. In the last section we considered cultural differences in aggression. There are also considerable differences in male/female aggression which indicates that aggression is not entirely caused by biology (for example by testosterone) but is partly a consequence of social norms.
GENETIC FACTORS
The genetic influence on aggression has been shown by numerous studies including the following.
RESEARCH EVIDENCE
 Cairns, Gariepy, and Hood (1990, see A2 Level Psychology page 173) showed that in non-human animals genetic factors are important in aggression. There are many studies that demonstrate that non-human animals can be bred for aggression or non-aggression.
 Hudziak et al. (2003, see A2 Level Psychology page 173) and several other researchers have shown that certain personality characteristics and behaviours that may be linked to aggression have greater concordance in identical (MZ) twins than in non-identical (DZ) twins. Note, however, that the link is not direct in humans in the way that it is in non-human animals—it simply shows that characteristics that might lead to aggression are likely to be genetically transmitted. Certain temperaments (e.g. those with a disposition to anti-social behaviour or impulsiveness) have been shown to have a genetic component and this could result in aggressive behaviour.
 Rhee and Waldman (2002, see A2 Level Psychology pages 173–174) conducted a meta-analysis of twin and adoption studies and estimated that genetic influences are the cause of 41% of the variability in anti-social
 behaviour. Note, however, that this means that 59% of variability is not genetic.
Brunner et al. (1993, see A2 Level Psychology page 174) found that MAOA deficiency is genetic. Recall from earlier comments that this deficiency is likely to lead to raised levels of noradrenaline, which in turn can lead to aggression. This has been shown in one large family and in a study of 110 men who had both abnormalities in the MAOA gene and showed aggression and poor impulse control (Manuck et al., 2000).
EVALUATION
 Research support. The evidence from the studies just discussed indicates that genetic factors do contribute to aggressive behaviour, or at least to personality traits that may lead to aggressive behaviour. The meta-analysis by Rhee and Waldman (2002) showing a 41% genetic contribution to variation is important in this respect.
 Small biased samples. Many studies of humans consist of a small number of families, or just extended family, and are therefore not likely to be representative of the general population. This limits the extent to which the findings can be generalised.

 Low ecological validity and low mundane realism. These are an inevitable consequence of small biased samples.
 Aggression could be the result of learning, not genetics. It is quite possible that the extent to which aggressive behaviour is genetic is exaggerated by learning. If, for example, members of a family are very aggressive (so there is a genetic element), this may be considered as inevitable by other people and accepted as unavoidable. This, in turn, exaggerates the differences between these individuals and others who are disciplined to discourage aggression.
 Much research is correlational and does not show cause and effect. This applies to much research, including the link between raised noradrenaline levels and aggression.
 Genes do not inevitably determine behaviour. Just because a person inherits certain tendencies, their behaviour is influenced by nurture as well as nature. Caspi et al. (2002, see A2 Level Psychology page 176) point out that in a family with a variant of the abnormal MAOA gene only those who suffered abuse as children were prone to aggressive behaviour and 15% of those with the abnormal gene were within the normal range for aggression.
SO WHAT DOES THIS MEAN?
The biological approach to aggression provides very good pointers to the causes of aggression. There is no doubt that biological factors make some contribution to a variety of personality traits and behaviour that may result in violent behaviour.
Nevertheless, it is a reductionist and deterministic approach. It is reductionist in that it does not take sufficient account of the role of learning in shaping an individual’s behaviour. It is deterministic because it gives little scope for the freewill of the individual to decide whether or not they choose to make an aggressive response to any particular situation. The interactionist approach allows for the fact that some individuals are innately predisposed to be more aggressive than others but their experiences shape whether that aggression is given full rein or is controlled. Likewise, non-aggressive individuals may be provoked to such an extent that they respond with aggression or they may be surrounded by violent successful role models whose example encourages them to be aggressive. Both nature and nurture inevitably contribute to the behaviour of such complex beings as humans.
OVER TO YOU
1. Describe the role of neural and hormonal mechanisms in aggression. (10 marks)
2. Evaluate research into neural and hormonal mechanisms in aggression. (15
marks)