One evolutionary explanation of human aggression is sexual jealousy. Infidelity triggers sexual jealousy which is an evolved adaptive response to infidelity. Men have developed many strategies from vigilance to violence to stop their partner committing adultery. Men never know for certain that the baby is theirs and could be investing in another man’s baby. In a questionnaire, if women reported that their partner did not like them talking to other men, they were twice as likely to report violence from their male partner. 72% needed hospital treatment. Studies on battered women have shown that extreme jealousy of their partner as the main cause. Under this explanation Uroxide is an unintended consequence of the evolutionary adaptation designed to control, not kill. Support for the explanations comes from a study in the USA involving 461 men and 560 women that were all in committed heterosexual relationships. Men were asked about their use of mate retention techniques and how often they used 26 different types of violent acts, women were asked similar questions. From the male responses negative mate retention techniques were positively correlated with violence scores. Results for women were similar.
The idea that uroxide was unintentional was challenged by Shakelford. He analysed half a million homicides and picked 13670 where men killed women and discovered that younger women were more at risk, which undermines Daly and Wilson’s idea that uroxide was unintentional. A strength of the research is this area is its practical applications. Particular use of mate retention techniques can be an early indicator of violence towards partner, alerting family, friends and partner to possible danger. You can sort the problem out before violence occurs. A second evolutionary explanation of human aggression is the evolution of homicide.
One cause of aggression could be a lack of resources. Daly and Wilson carried out a study in Brazil and found that approximately 40% of victims and killers were men that were unemployed and around 70% of male victims and killers were not married. So, the inability to attract a long term partner and a lack of resources appears to lead to social competition and therefore aggression. A second cause of aggression could be status. In the EEA, we used to live in small groups and status was very important. A loss of status could have had catastrophic results on reproductive potential and resources. Although this is not the case now, the gene may still exist, so there has been a genome lag which is causing aggression in humans. A third cause could be sexual jealousy. A summary of 8 studies that involved love triangles showed that 92% of homicides were male – male and only 8% were female – female.
A weakness of this explanation is that it cannot explain why people respond differently when face with the same situation. In Shakleford’s study, 3 men were faced by their wives infidelity and one resulted in beating their wife up, one husband got drunk and one husband killed their wife. For any evolutionary explanation, a limitation is that it cannot be falsified because you cannot go back to the EEA and see if the behaviour has really evolved, so you cannot prove or disprove an evolutionary explanation of human aggression. A limitation of those explanations are that they are both reductionist and deterministic. Assuming the aggression is simply caused by your genetic make up or because its adaptive to be aggressive is too simple. It ignores other factors such as culture which has shown to cause aggression as a result of socialisation e.g. the Simba tribal people.
Discuss the role of neural and hormonal mechanisms in aggression. (24 marks) Testosterone is a hormone produced in the male testes and in female ovaries however males produce 10 times more than females. Testosterone has been linked to aggression and as it plays a part in increased muscle and bone mass, this suggests that it plays a structural role in influencing aggression. Testosterone has been shown to correlate with the level of aggression displayed by humans and animals. For example when levels of testosterone peak around the start of puberty there is also a corresponding level of aggression displayed in young males during this time. Testosterone modulates levels of various neurotransmitters that mediate effects on aggression. There seems to be a critical period early in life, where exposure to testosterone is essential to elicit aggression in adulthood. It is though testosterone helps sensitise androgen responsive system. There have been many experiments done to see the effects of adding and removing testosterone.
All of which have been performed on animals due to methodological and ethical issues with testing on humans. It was found that male mice that are castrated at birth showed a decreased level of aggression. These findings were also supported by similar studies involving different species fo animals and so it is not solely specific to mice. Although castration research can be useful, castration disrupts other hormone systems as well as testosterone and os these may be playing a part in the reduced aggression. However as these findings come from animal studies, we cannot apply it to humans and so we cannot learn much about human behaviour from this specific experiment. This is because humans have a more complex physiology, for example, and so may respond quite differently. Instead, we have to do a cost-benefit analysis, in that is the potential harm done to the animals worth the benefit to humans of increased knowledge. We cannot rely solely on findings from these studies or other similar research, but at the very least we need to verify these animals’ findings with findings from human studies. Also, another problem with using animals to research testosterone affects is that certain brain structures are involved with different types of aggression in different species.
For example, cingulated gyrus is linked to fear-induced aggression in monkeys, but to irritability in cats and dogs, creating problems in trying to generalise in humans and to identify aggression. Research on humans has however, provided support for this theory. Dabbs measured salivary testosterone in violent and non-violent criminals and found that those with the highest levels of testosterone tended to have committed the most violent crimes. Likewise, Lindman found young males who behaved aggressively whilst drunk had higher testosterone levels than those who did not act aggressively. A disadvantage of the theory of hormonal mechanisms as an explanation of aggression is that as all the research is Correlational, we cannot prove causality. Also, studies have failed to identify whether aggression was caused by testosterone or whether testosterone was secreted due to increased aggression. Also there are inconsistent findings which is a disadvantage of this explanation. Book carried out a meta-analysis of 45 studies and established a mean correlation of 0.14 between testosterone and aggression. It could be argued that although a positive correlation was found, it was extremely weak and so we cannot draw any conclusions from it. Likewise, a meta-analysis by Archer also found low positive correlation between the two variables.
However, the type of participant and the form and measurement of aggression differed substantially between studies and so may not be completely valid. Archer also criticised Book’s meta-analysis, claiming that methodological problems in the study meant that a correlation of only 0.08 was more appropriate which is barely a correlation. Mazur also suggested distinguishing aggression from dominance. Individuals act aggressively when their intent is to inflict injury, whereas they act dominantly if their wish is to achieve. Mazur claims aggression is just one form of dominance behaviour. In animals particularly, the influence of testosterone on dominance behaviour might be shown through aggressiveness. Research investigating the role of hormonal and in particular testosterone in aggression is also very gender bias. Most studies involve all male participants and so we cannot generalise these findings to females and cannot draw any conclusions about the affect of testosterone on female behaviour. Archer actually found that the association between testosterone and aggression is higher for female than male samples. Neurotransmitters are chemical that allow impulses in one area of the brain to be transmitted to another area.
Two neurotransmitters in particular are thought to be important in the control of aggressive behaviour, these are serotonin and dopamine. Low levels of serotonin have been associated with increased aggression levels. In normal levels, it is thought that serotonin has a calming, inhibitory effect on neuronal firing in the brain however low levels remove this inhibitory effect and so individuals are less able to control their impulsive and aggressive responses. Some drugs are thought to alter serotonin levels and thus increase aggressive behaviour. Mann gave 35 participants a drug which depleted serotonin levels and by using a questionnaire, found an increase in hostility and aggression scores. However, there are methodological issues involved with this study, in particular with the measurement of aggression. Using a questionnaire could lead to social-desirability bias and in particular is more likely with sensitive topics such as aggression. Individuals are more likely to under-report undesirable behaviour such as aggression and over-report desirable behaviour leading to invalid findings. Dopamine is the other neurotransmitter associated with aggression however, unlike serotonin, high levels of dopamine are associated with increased aggression.
Although the link between dopamine and aggression is not as well established, there is evidence to suggest such a link exists. The use of amphetamines increases dopamine activity and it has also been found to increases in aggressive behaviour. Antipsychotics which reduce dopamine however, have been shown to reduce aggression, thus supporting this explanation. A meta-analysis of 29 studies examined neurotransmitter levels in antisocial children and adults. These studies consistently found lower levels of serotonin but no significant difference in dopamine levels. This conflicts with the theory that dopamine has an impact on aggressive behaviour however, supports the role of serotonin. Individuals with the lowest levels of serotonin were found tp have attempted suicide, suggesting a serotonin depletion leads to impulsive behaviour, which may lead to aggressive behaviour.
Evidence from non-human studies also supports this theory. Releigh found that monkeys fed on a diet which decreased their serotonin levels increased their aggressive behaviour however, on a diet that increased serotonin levels, the monkeys displayed reduced aggression. It is hard to generalise these findings to humans however, as many animals do not have the same capacity for cognitive processing and self-awareness and animal behaviour tends to rely more on biological factors. Both neural and hormonal mechanisms in aggression are the biological approach and so both are reductionist. Due to the complexity of human social behaviour, the biological explanation alone is insufficient in explaining all the many different aspects of aggressive and violent behaviour. It ignores important social explanations of aggression such as social learning theory- It also fails to explain why there are cross cultural differences. If neural and hormonal mechanisms were the sole cause of aggression, we would see a worldwide distribution of aggression.