Biological Explanations of Aggression

1. Testosterone-aggression link

   Annotations:

   • Two meta-analyses have established a weak but positive relationship between testosterone and aggression.
   1. Archer, 1991

      Annotations:

      • Archer analysed the results of 230 males over five studies and found a low positive correlation between testosterone and aggression.
      • However, the type of participant, and the form and measurement of aggression, differed substantially between studies.
      1. Book et al, 2001

         Annotations:

         • A larger meta-analysis of 45 studies established a mean correlation of 0.14 between testosterone and aggression.
         1. Archer et al, 2005

            Annotations:

            • Although, Archer claims that methodological problems with this study meant that a correlation of 0.08 was more appropriate.
2. Testosterone

   Annotations:

   • The male sex hormone testosterone is thought to influence aggression from young adulthood onwards due to its action on brain areas involved in controlling aggression.
   • Evidence for this association comes from a number of sources.
   1. Dabbs et al, 1987

      Annotations:

      • Dabbs measured salivary testosterone in violent and non-violent criminals. 
      • Those with the highest testosterone levels had a history of primarily violent crimes whereas those with the lowest levels had committed only non-violent crimes. 
      • Studies of non-prison populations have found similar trends.
      1. Lindman et al, 1987

         Annotations:

         • Lindman found that young males who behaved aggressively when drunk had higher testosterone levels than those who did not act aggressively.
   2. The challange hypothesis

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      • Wingfield proposes that, in monogamous species, testosterone levels should only rise above the baseline breeding level in response to social challenges such as male-male aggression or threats to status.
      • As the human species is considered to be monogamous, this would predict that male testosterone levels would rise sharply in response to such challenges.
      • In such situations, a testosterone surge is to be expected, with a consequent increase in aggression, provided the threat is deemed relevant to reproductive competition, e.g. a dispute over a female.
      1. Wingfield et al, 1990
   3. Inconsistent evidence

      Annotations:

      • Albert claims that despite many studies showing a positive correlation between testosterone and aggression, other studies find no such relationship, particularly those that have compared testosterone levels of aggressive versus less aggressive individuals.
      • In addition, most studies showing a positive correlation have involved small samples of men within prisons, using either self-report measures of aggression or judgements based solely on the severity of the crime committed.
      1. Albert et al, 1993
   4. Aggression/Dominance?

      Annotations:

      • Mazur suggests we should distinguish aggression from dominance.
      • Individuals act aggressively when their intent is to inflict injury, whereas they act dominantly if their wish is to achieve or maintain status over another.
      • Mazur claims that aggression is just one form of dominance behaviour. In non-human animals the influence of testosterone on dominance behaviour might be shown in aggressive behaviour.
      • In humans, however, the influence of testosterone on dominance is likely to be expressed in more varied and subtle ways (e.g. through status-driving behaviour.)
      1. Mazur, 1985
3. Cortisol
   1. Dabbs et al, 1991

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      • Cortisol appears to have a mediating effect on other aggression-related hormones such as testosterone, possibly because it increases anxiety and the likelihood of social withdrawal.
   2. Vikkunen, 1985

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      • High levels of cortisol inhibit testosterone levels and so inhibit aggression.
      • Studies have reported low levels of cortisol in habitual violent offenders...
      1. Tennes and Kreye, 1985

         Annotations:

         • ...and in violent schoolchildren.
         • This suggests that although relatively high testosterone is the primary biochemical influence on aggression, low cortisol plays an important role by increasing the likelihood of aggressive behaviour.
   3. Commentary

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      • The moderating effect of cortisol on aggressive behaviour is supported in a four-year study of boys with behavioural problems.
      • Those boys with consistently low cortisol levels began antisocial acts at a younger age and exhibited three time the number of aggressive symptoms compared to boys with higher or fluctuating cortisol levels.
      • Researchers concluded that cortisol levels were "strongly and inversely related to aggressive conduct disorder."
      1. McBurnett et al, 2000
4. Reductionism

   Annotations:

   • The links between biological mechanisms such as serotonin and aggression, and testosterone and aggression are well established in non-human animals.
   • However, the position is not quite so clear in the case of humans. This is not to deny that such links exist, but rather that the complexity of human social behaviour means that a biological explanation for human aggression is insufficient on its own to explain all the many different aspects of aggressive and violent behaviour.
5. Gender Bias

   Annotations:

   • Most studies concerned with testosterone and aggression have involved male participants, but does testosterone also increase aggression in females?
   1. Archer et al, 2005

      Annotations:

      • If anything, research suggests that the association between testosterone and aggression is higher for female than male samples.
   2. Baucom et al, 1985

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      • A further study showed that women with higher testosterone levels had higher occupational status, possibly as a result of being more assertive.
      • These studies indicated that women may also respond to challenging situations with increased testosterone, displaying characteristics such as aggressiveness and dominance.
   3. Archer and Coyne, 2005

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      • However, in some circumstances this may be a disadvantage, as high testosterone levels are associated with an assertive style that hinders the formation of alliances as well as the more subtle forms of competition that occur in female groups.
6. Gun related crime vs testosterone

   Annotations:

   • Statistics suggest a sharp increase in gun-related crime in the UK, but why does the presence of guns in the environment lead to increased aggression?
   • Perhaps the presence of a stimulus, such as a gun or knife, triggers increases in testosterone levels (the gun is seen as a threat), which in turn increases aggressive behaviour, a chain of events that would be predicted by the challenge hypothesis.
   • To test this, Klinesmith had male college students provide a saliva sample (to measure testosterone), interact either with a gun or a child's toy for 15 minutes, and then provide another saliva sample.
   • Males who interacted with the gun showed significantly greater increases in testosterone and behaved more aggressively toward another participant compared to those who played with the child's toy.
   1. Klinesmith et al, 2006

Annotations:

• Biological explanations of aggression offer a completely different perspective to the social psychological explanations we have looked at so far.
• Biological models assume that aggression is located within the biological make-up of the individual rather than in the environment around them.
• Many research studies in the last 30 years have shown that violent criminals were high in the hormone testosterone, encouraging some people to conclude that castration of highly aggressive males would stop them from killing or injuring innocent people.
• However, the relationship between biological mechanisms and aggressive behaviour is not that simple.

1. Neurotransmitters-aggression link

   Annotations:

   • A meta-analysis of 29 studies published before 1992 examined neurotransmitter levels in antisocial children and adults.
   • These studies consistently found lower levels of serotonin in individuals described as being aggressive, but found no significant rise or fall in dopamine levels.
   • Indications of reduced levels of serotonin were found in all antisocial groups, but were particularly marked in those individuals who had attempted suicide.
   • This suggests that serotonin depletion leads to impulsive behaviour, which in turn may lead to aggressive behaviour in various forms.
   1. Scerbo and Raine, 1993
2. Neurotransmitters

   Annotations:

   • Neurotransmitters are chemicals that enable impulses within the brain to be transmitted from one area of the brain to another.
   • There is some evidence that at least two of these neurotransmitters, serotonin and dopamine, are linked to aggressive behaviour.
   • Serotonin and dopamine are of particular interest because low levels of serotonin and high levels of dopamine have been associated with aggression in animals and humans.
   1. Serotonin

      Annotations:

      • Serotonin is thought to reduce aggression by inhibiting responses to emotional stimuli that might otherwise lead to an aggressive response.
      • Low levels of serotonin in the brain have been associated with an increased susceptibility to impulsive behaviour, aggression, and even violent suicide.
      • Some drugs are thought to alter serotonin levels and thus increase aggressive behaviour.
      1. Mann et al, 1990

         Annotations:

         • Mann gave 35 healthy subjects dexfenfluramine, which is known to deplete serotonin.
         • Using a questionnaire to assess hostility and aggression levels, they found that dexfenfluramine treatment in males (but not females) was associated with an increase in hostility and aggression scores.
      2. Evidence from non-human studies
         1. Raleigh et al, 1991

            Annotations:

            • Raleigh has added support for the importance of serotonin in aggressive behaviour in a study of vervet monkeys.
            • They found that individuals fed on experimental diets high in tryptophan (which increases serotonin levels in the brain) exhibited decreased levels of aggression.
            • Individuals fed on diets that were low in tryptophan exhibited increased aggressive behaviour, suggesting that the difference in aggression could be attributed to their serotonin levels.
         2. Popova et al, 1991

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            • Other evidence for the importance of serotonin in aggression has shown that in animals that are selectively bred for domestication and for increasingly docile temperaments, there is a corresponding increase, over generations, in brain concentrations of serotonin.
      3. Evidence from antidepressants

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         • If low levels of serotonin are associated with low impulse control and aggressive behaviour, drugs that clinically raise serotonin levels should produce a concurrent lowering in aggression.
         1. Bond, 2005

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            • Bond has established that this is exactly what happens in clinical studies of antidepressant drugs that elevate serotonin levels.
            • She established that such drugs do tend to reduce irritability and impulsive aggression.
   2. Dopamine

      Annotations:

      • Although the link between high levels of dopamine and aggressive behaviour is not as well established as with serotonin, there is some evidence to suggest that such a link exists.
      1. Lavine, 1997

         Annotations:

         • For example, increases in dopamine activity via the use of amphetamines have also been associated with increases in aggressive behaviour.
      2. Buitelaar, 2003

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         • Antipsychotics, which reduce dopamine activity in the brain, have been shown to reduce aggressive behaviour in violent delinquents.
      3. Cause or consequence?

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         • Although research is fairly inconclusive about the causal role of dopamine in aggression, recent research suggests that its influence might be as a consequence instead.
         1. Couppis and Kennedy, 2008

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            • Couppis and Kennedy found that in mice, a reward pathway in the brain becomes engaged in response to an aggressive event and that dopamine is involved as a positive reinforcer in this pathway.
            • This suggests that individuals will intentionally seek out an aggressive encounter solely because they experience a rewarding sensation from it.

Annotations:

• The biological approach to aggression includes the belief that the propensity for aggressive behaviour lies in an individual's genetic make-up.
• Researchers must try to establish whether genetically related individuals are more similar in their aggressive tendencies than non-related individuals.
• This also has important implications for understanding the origins for understanding the origins of violent crime.
• Although the question of genetic influences for aggression and violent crime has perhaps not interested researchers quite as much as the general public, research suggests that aggressive tendencies may, at least in part, be inherited.

1. Is aggression inherited?

   Annotations:

   • Trying to determine the role of genetic factors in aggression is essentially a question of nature and nurture.
   • To disentangle the relative contributions of nature (genetic inheritance) and nurture (environmental influences), researchers have employed a variety of methodological techniques, including twin and adoption studies of individual genes and studies of violent populations.
2. Twin studies

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   • MZ twins share all of their genes, while DZ twins share only 50%.
   • In twin studies, researchers compare the degree of similarity for a particular trait (such as aggression) between sets of MZ twins to the similarity between sets of DZ twins.
   • If the MZ twins are more alike in terms of their aggressive behaviour, then this should be due to genes rather than environment (both types of twin share same environment as each other but MZ twins are more genetically alike).
   • Most twin studies have focused on criminal behaviour generally, but one of the few studies to specifically study aggressive behaviour using adult twin pairs found that nearly 50% of the variance in direct aggressive behaviour (i.e. aggression toward others) could be attributed to genetic factors.
   1. Coccaro et al, 1997
3. Adoption studies

   Annotations:

   • Adoption studies can help to untangle the relative contributions of environment and heredity in aggression. 
   • If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents, a genetic effect is implied.
   • If a positive correlation is found between the adoptee's aggressive behaviour and the rearing family, then an environmental is implied.
   1. Hutchings and Mednick, 1975

      Annotations:

      • A study of over 14,000 adoptions in Denmark found that a significant number of adopted boys with criminal convictions had biological parents (particularly fathers) with criminal convictions, providing evidence for a genetic effect.
4. Aggression gene?
   1. Role of MAOA

      Annotations:

      • Although no individual gene for aggression has been identified in humans, a gene responsible for producing a protein called monoamine oxidase A (MAOA) has been associated with aggressive behaviour.
      • MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with impulsive and aggressive behaviour.
      1. Brunner et al, 1993

         Annotations:

         • In 1980s, a study of a Dutch family found that many of its male members behaved in a particularly violent and aggressive manner, and a large proportion had been involved in serious crimes of violence including rape and arson.
         • These men were found to have abnormally low levels of MAOA in their bodies, and a defect in this gene was later identified.
   2. Gene-environment interaction
      1. Caspi et al, 2002

         Annotations:

         • A second study linking MAOA to aggressive behaviour, involved 500 male children.
         • Researchers discovered a variant of the gene associated with high levels of MAOA and a variant associated with low levels.
         • Those with low levels of MAOA were significantly more likely to grow up to exhibit antisocial behaviour but only if they had been maltreated as children. 
         • Children with high levels of MAOA who were maltreated, and those with low levels who were not maltreated, did not display antisocial behaviour.
         • This shows that it is the interaction between genes and environment that determines behaviours such as aggression.
5. Genetics and violent crime

   Annotations:

   • Researchers do not suggest that there is a gene for violent crime per se.
   • Rather it is claimed that inherited temperamental or personality characteristics place some individuals more at risk of committing violent crime.
   • Adoption studies have shown that the highest rates of criminal violence in adopted children occur when both biological and adoptive parents have a history of violent crime - clear evidence of a gene-environment interaction.
   • However, a series of adoption studies in which the criminal history of an adopted male was compared with the criminal history of both his biological and his adoptive fathers, found that genetic influences were significant in cases of property crime but not in cases of violent crime.
   1. Brennan, 1993
6. Meta-analyses
   1. Miles and Carey, 1997

      Annotations:

      • Miles and Carey carried out a meta-analysis of 24 twin and adoption studies that had investigated the genetic basis of aggression.
      • Most studies had relied on parental or participants' self-reports of aggressive tendencies, although some involved observation of aggressive behaviour.
      • The results suggested a strong genetic influence that could account for as much as 50% of the variance in aggression.
      • Age differences were notably important, with both genes and family environment being influential in determining aggression in youth, but at later ages the influence of genes increased.
      1. Rhee and Waldman, 2002

         Annotations:

         • A later meta-analysis by Rhee and Waldman combined the results of 51 twin and adoption studies and concluded that aggressive antisocial behaviour was largely a product of genetic contributions.
         • However, in this study, as with the Miles and Carey study, several variables, including age of participant and assessment method for aggression moderated the genetic influence on aggression.
         • This suggests that although genetic factors play a significant part in the development of aggressive behaviours, the influence of other factors affects their expression.
7. Problems of sampling

   Annotations:

   • Many of the studies in this area have focused on individuals convicted of violent crime. Two particular difficulties arise when trying to draw meaningful conclusions from these studies.
   • The first problem lies with the participants themselves.
   • Convictions for violent crime are relatively few compared to the vast number of violent attacks by individuals that never result in a conviction, therefore they represent just a small minority of those regularly involved in aggressive behaviour.
   • Second, contrary to popular belief, offenders designated as "violent" on the basis of a court conviction are not necessarily the most serious, persistent offenders.
   • For example, a convicted murderer would be designated as violent for one offense despite, perhaps, having otherwise had a lifetime free from crime.
   • This might explain why so many studies have found little or no evidence of heritability for violence.
8. Applicability?

   Annotations:

   • Although studies of the biological basis of aggression interest nearly everyone, research findings are far too uncertain to be valuable in understanding the causal factors affecting those who engage in violent activities.
   • Nevertheless, there have been suggestions that public policy should be informed by the results of this research. 
   • If people are predisposed towards aggressive behaviour or violent crime, then questions about the treatment of such behaviours inevitably arise.
   • Some commentators advocate genetic engineering but others go much further. As long as violence remains at the forefront of public concern, ways of dealing with it that address the problem more directly (e.g. through chemical castration) remain an attractive option to many.
   • Given the extremely tentative nature of conclusions that can be reached from this research and the far-reaching ethical consequences of labelling an individual as a threat to society on the basis of their genetic inheritance, an awareness of the limitations of these studies is extremely important.
9. Value of animal research

   Annotations:

   • Studies of aggressive behaviour in non-human animals have an important role in helping us understand aggressive behaviour in humans.
   • For example, rodents offer the advantage of experimental manipulation to test the effects of specific genes on aggressive behaviour.
   • Manipulations may involve selective breeding programmes and "knockout" techniques (where a single gene is eliminated from a group of experimental animals in order to study its effect).
   1. Young et al, 2002

      Annotations:

      • An example of such a study that has potential for an understanding of human aggression was by Young et al. 
      • These researchers claim to have indentified a genetic mutation that causes violent behaviour in mice.
      • This mutation, nicknamed "fierce," has a range of effects on mice, including extremely violent behaviour towards other mice.
      • A counterpart for this gene does exist in humans, although its precise function is not known.
10. Can we determine a genetic role?

    Annotations:

    • What does the role of genetic factors in aggression really mean?
    • The connection between genetic factors and aggression is far from straightforward because of problems determining what is, and what is not, a product of genetic inheritance.
    • It is difficult to establish genetic contributions to aggressive behaviour for three reasons.
    • 1) More than one gene usually contributes to a given behaviour. 2) As well as genetic factors there are many non-genetic (i.e. environmental) influences of the manifestation of aggressive behaviour 
    • 3) These influences may interact with each other. Genetic factors may affect which environmental factors have an influence, and vice versa (gene-environment interaction). See Caspi's study on gene-environment interaction for support.
11. Problems of assessing aggression

    Annotations:

    • Many of the reported studies of aggression have relied on either parental or self-reports of aggressive behaviour, whereas other studies have made use of observational techniques.
    • In the Miles and Carey meta-analysis reported on the left, mode of assessment was found to be a significant moderator of aggressive behaviour in the 24 studies that made up their analysis.
    • They found that genetic factors explained a large proportion of the variance in aggressive behaviour in studies that had used parental or self-reports.
    • However those that had made use of observational ratings showed significantly less genetic contribution and a greater influence of environmental factors.
    1. Plomin et al, 1981

       Annotations:

       • For example, in a replication of Bandura's bobo doll study, twin pairs were encouraged to act aggressively toward the doll by being exposed to an adult model who also acted aggressively towards it.
       • Researchers found no difference in correlations between MZ and DZ twin pairs, suggesting that individual differences in aggression were more a product of environmental influences (e.g. family upbringing) than genetic influences.
12. Inheritance of criminal violence

    Annotations:

    • Studies that have investigated the role of genetic factors in criminal behaviour often fail to distinguish between violent and non-violent crime, making it more difficult to untangle the role of genetic factors in specifically aggressive violence.
    • These studies also often fail to distinguish between criminals who are habitually violent and those for whom their violent crime is a one-off. 
    • The evidence for violent crime being inherited is also far from conclusive.
    1. Walters, 1992

       Annotations:

       • A meta-analysis of studies in this area found only a low to moderate correlation between heredity and crime, with better designed and more recently published studies providing less support for the gene-crime hypothesis than more poorly designed and earlier published studies.
       1. Surgeon General's report on youth violence, 2001

          Annotations:

          • A more recent review of published studies of youth crime concluded: "the data do not suggest a strong role for heredity in violence."