Specification: Evolutionary explanations of human aggression. 

Evolutionary explanations propose that human aggression exists today because it helped our ancestors to survive and reproduce. Behaviours that increased an individual's reproductive success were more likely to be passed on through their genes, so over many generations they became part of human nature. These behaviours evolved in the environment of evolutionary adaptedness (EEA), the environment in which modern humans evolved. Our ancestors lived in small groups, competed for scarce resources and had to find and keep mates. Evolutionary psychologists argue that aggression was a useful solution to several of these problems.

This page explains how sexual jealousy and mate-retention strategies can lead to aggression and violence towards partners, and how bullying can be understood as an adaptive behaviour. It then evaluates the explanation.

Key terms

• Environment of evolutionary adaptedness (EEA): the environment in which a species evolved, to which its behaviours are adapted.

• Reproductive success: the number of offspring an individual has who survive to pass on their genes.

• Paternity uncertainty: a man can never be completely certain that a child is his, whereas a woman always knows that a child is hers.

• Cuckoldry: a man unknowingly investing his resources in raising another man's child.

• Sexual jealousy: an emotional response to the threat of a partner's infidelity, which evolutionary psychologists see as adaptive.

• Mate-retention strategies: behaviours used to keep a partner and prevent infidelity, including direct guarding and negative inducements.

• Bullying: repeated aggression by a more powerful individual towards a less powerful person.

Description: aggression as an adaptation

Evolution works through natural selection. Random genetic mutations produce variation between individuals. Any inherited trait that helps an individual to survive and reproduce is passed on to more offspring, so over generations it becomes common. Aggressive individuals were better able to compete for scarce resources such as food, territory and mates, and to gain status in their group, so they left more descendants, and the genes underlying aggression spread.

This competition is mainly between males, because a man's reproductive success depends on how many mates he can attract and keep. Women are thought to prefer men who can provide resources and protection. Sadalla et al. (1987) found that dominant behaviour made men, but not women, more sexually attractive, although being merely aggressive or domineering did not. Aggression is therefore adaptive when it is a route to dominance and resources. This competition is thought to be most intense among young men, who are seeking status and partners, which may explain why young males commit the most violence, often in disputes over respect, territory or rivals for a partner.

Description: sexual jealousy and paternity uncertainty

Sexual jealousy is a major motivator of aggressive behaviour in males, and the evolutionary explanation accounts for it in terms of reproduction. Fertilisation happens inside a woman's body, so a woman always knows that a child is hers. A man, however, can never be completely certain that he is the biological father of a child. This is called paternity uncertainty.

If a man's partner is unfaithful, he risks cuckoldry: investing years of resources in raising another man's child. In evolutionary terms, this reduces his reproductive success, because he gains no genetic benefit while his rival's genes are passed on using his resources. Evolutionary psychologists argue that sexual jealousy evolved in men to increase anti-cuckoldry behaviours. Men who felt jealous at signs of possible infidelity were more likely to act to prevent it, so they were less likely to be cuckolded and more likely to pass on their genes. Aggression is one way of acting on this jealousy: aggression towards rivals deters other men, and aggression or threats towards a partner are intended to deter her from being unfaithful or leaving.

This also explains a sex difference. Because women do not face paternity uncertainty, evolutionary psychologists argue that sexual jealousy, and the aggression linked to it, is stronger in males than in females.

Description: mate-retention strategies

Behaviours used to prevent a partner from straying are known as mate-retention strategies. They are considered adaptive because they increased reproductive success in our evolutionary past. According to Wilson and Daly (1996), mate-retention strategies involving aggression include:

• Direct guarding: vigilance over a partner, such as checking who they see, keeping tabs on where they go or, today, installing a tracking app on their phone. This restricts the partner's opportunities to meet rivals.

• Negative inducements: threats of consequences for infidelity, such as "I'll kill myself if you leave me". These make infidelity or leaving seem too costly.

Description: physical violence against partners

Wilson and Daly argued that mate-retention strategies can escalate into physical violence, and in extreme cases homicide, when a man perceives a serious threat of infidelity or of his partner leaving. Wilson et al. (1995) found that women who reported that their partners used these mate-retention strategies were twice as likely to suffer physical violence from them. Of these women, 73% required medical attention and 53% feared for their lives. This shows that mate-retention behaviours are not just controlling but can be closely linked to serious violence.

Key study: Shackelford et al. (2005)

Aim: To test whether men's use of mate-retention strategies predicts their violence towards their female partners.

Procedure: In one part of the study, 107 married couples completed questionnaires. Husbands reported their own use of mate-retention behaviours, such as direct guarding and negative inducements. Wives separately reported any violence they had experienced from their husbands.

Findings: Men's use of mate-retention strategies correlated positively with their violence towards their partners. Vigilance, a form of direct guarding, was a particularly consistent predictor of violence.

Conclusion: Mate-retention strategies and partner violence are linked, as the evolutionary explanation predicts. Violence towards a partner may be one part of a wider set of behaviours that evolved to prevent infidelity.

Description: bullying

Bullying occurs when a more powerful individual behaves aggressively towards a weaker person. It is usually seen as a problem caused by poor social skills or a difficult upbringing. Evolutionary psychologists offer a different view: bullying may have been an adaptive strategy that increased an individual's chances of survival and reproduction. Bullying can help an individual gain access to valuable resources and increase their social status and dominance.

Male bullying

Volk et al. (2012) suggested that bullying helps adolescent bullies to achieve dominance and sexual goals. This may give bullies access to more sexual partners, which increases their reproductive success. Bullying is therefore a way for males to display dominance and strength, signalling to potential partners that they can provide resources and protection, and deterring rivals.

Female bullying

Female bullying is more often verbal or relational, such as spreading rumours or excluding others, and it often occurs within a relationship. Evolutionary psychologists argue that this helps a woman to secure her partner's fidelity, for example by keeping other women away from him. Securing fidelity increases access to the partner's resources for raising future offspring. From this perspective, bullying behaviours continue today because they were once adaptive in promoting survival and reproduction.

Evaluation

A strength of the evolutionary explanation is that there is research support for the link between mate retention and aggression. Shackelford et al. (2005) found that men's use of mate-retention strategies correlated positively with their violence towards their wives, with vigilance a particularly consistent predictor. Because husbands' own reports of mate retention were linked to their wives' separate reports of violence, the finding does not depend on a single source. This suggests that sexual jealousy, mate retention and aggression are linked, and that partner violence may function to reduce the risk of infidelity and cuckoldry, as the adaptive explanation predicts. However, the data are correlational and based on self-report, so they cannot show that mate retention causes violence, or that the link evolved rather than being learned. The study therefore supports the explanation without confirming it.

A further strength is that the evolutionary explanation can account for gender differences in aggression. Across cultures, men are more physically aggressive than women. According to Campbell (1999), it is not adaptive for a female with offspring to be physically aggressive, because such behaviour would put her own survival at risk, and with it the survival of her child. A more adaptive strategy is to use verbal rather than physical aggression. This means the explanation does not simply say that women are less aggressive; it explains why the type of aggression differs between the sexes, and it can predict these differences in research. This increases the validity of the explanation, although critics argue that focusing on sex differences risks alpha bias, exaggerating differences between men and women.

A limitation is that some cultures show little aggression, which suggests aggression may not be a universal adaptation. The evolutionary explanation predicts that aggression should be present in all cultures, because all humans share the same evolutionary history and aggression supposedly increases survival through greater reproductive success. However, the !Kung San people of the Kalahari actively discourage aggression, and it is not valued or rewarded in their culture. In contrast, the Yanomamo of the Amazon rainforest in Venezuela and Brazil use aggression to gain status in their structured society. This suggests that there are substantial cultural differences in aggression, and that whether aggression is used depends heavily on cultural norms, which fits social learning theory's view that aggression is learned by observing which behaviours are rewarded. This challenges the claim that aggression is a universal evolved behaviour, and supports an interactionist view in which an evolved potential for aggression is encouraged or suppressed by culture.

A strength is that the evolutionary explanation of bullying has real-world application. Rigby (2010) reviewed several established anti-bullying interventions and found that, despite these programmes being available, bullying remains prevalent. Most interventions assume that bullying is maladaptive. However, Ellis et al. (2016) argue that bullying is adaptive, meaning bullies stand to gain advantages for themselves, such as status and resources. This would explain why traditional interventions often fail. Ellis et al.'s "meaningful roles" intervention aims to increase the costs of bullying and reward prosocial alternatives, for example by giving bullies roles and responsibilities within the school, so that they can gain status without aggression. This suggests that an evolutionary understanding of bullying may help to design more effective interventions, although the success of such programmes still needs to be established through further research.

A limitation is that there are methodological issues with the research evidence. It is extremely difficult to test hypotheses about how behaviours evolved to solve problems in our evolutionary past, because the evolution of behaviour over millions of years cannot be observed directly or tested experimentally. As a result, most research, such as Shackelford et al. (2005), is correlational. Even strong correlational research does not allow us to conclude that evolutionary factors such as sexual jealousy cause aggression. Evolutionary explanations are also post hoc, meaning they are developed after the behaviour has been observed, and it is easy to create an evolutionary story for almost any behaviour. This makes it very difficult to establish precisely how valid an evolutionary explanation of aggression is, and it weakens the explanation's scientific status.

Overall, the evolutionary explanation offers a powerful account of why sexual jealousy is so often linked to partner violence, why men and women differ in the ways they are aggressive, and why bullying persists despite efforts to stop it. However, it relies largely on correlational evidence, it cannot easily explain cultural variation such as the low aggression of the !Kung San, and it is difficult to test directly. It is also deterministic and socially sensitive: if male violence is presented as evolved and natural, it could be used to excuse abusers from personal responsibility. The most defensible conclusion is that evolution may have given humans a predisposition to aggression in competition for status and mates, but culture, learning and individual choice determine whether and how that predisposition is expressed.

Exam tips

• Use human examples. Animal research only earns credit as an evaluation point.

• Make every point about aggression: say what the aggressive behaviour is and how it increased reproductive success.

• Do not say evolutionary explanations "ignore biology" or "ignore genetic explanations". Examiners treat this as a basic misunderstanding.