Evolutionary Psychology Lecture 9: Aggression.

Learning Outcomes.

At the end of this lecture you should be able to:

1. Outline evolutionary explanations for the male propensity for increased aggression.

2. Discuss experimental and survey evidence for sex differences in aggression.

Nature of Aggression.

Wrangham & Peterson (1996) point out that only humans and chimpanzees carry out the following aggressive actions:

Male-initiated territorial aggression. Groups of males raiding neighbouring territories. Lethal male/male group aggression. Traditional psychological theories explain aggression as

being caused by cultural factors such as observation and imitation via the influence of TV, increased alienation, and social crowding. However such explanations do not explain:

Long-recorded history of male aggression and warfare. Existence of higher levels of male aggression in all

cultures. Existence of similar patterns of aggression in the

chimpanzee.

The Evolutionary Perspective.

Evolutionary psychologists instead see aggression as a solution to particular adaptive problems and according to Buss & Shackelford (1997) such problems are as follows:

1). Coopting the resources of others: Humans stockpile resources which are important for survival and reproduction.

2) Defence against attack: The presence of individuals who may attack you prompts groups or individuals to use aggression as a means of preventing ones resources from being taken.

3) Inflict costs on intrasexual rivals: Same-sex rivals will compete for access to valuable resources that will attract females, or for females themselves.

4) Negotiate status hierarchies: Aggression can enhance ones personal status within a group, in Western cultures aggression has been ritualised within sporting contexts and top performers can achieve very high status.

5. Deter Mates From Infidelity Aggression, or the threat of aggression may deter

current partners from sexual infidelity. There is much evidence to suggest that male sexual

jealousy is a key factor in spouse abuse. Daly & Wilson (1988) argued that males will use

violence and threats as strategies to limit their partners autonomy and so decrease the chance of infidelity.

Spousal homicide is common, especially for women who:

Have left their partners. Have threatened to do so. Have been suspected of planning or actually

committing adultery. Detroit (1972): 19% of homicide victims were related to

the killer by marriage, compared to 6% who were blood-relatives.

Miami (1980): 10% of murder victims were marital relatives, compared to 2% of blood relatives.

Relationship Status and Homicide Risk.

From Daly & Wilson 1988, in Buss 1999 p58

Homicide Rates in Marriage, Canada 1974-1983

Wives killed by husbands

Husbands killed by wives

From Daly & Wilson 1988, p521.

Contextual Factors.

Wallace, (1986) found that recently estranged wives were at a very high risk of being murdered by their former husbands particularly if they were young or very attractive.

Another key factor is where the male lacks the necessary resources (ie unemployment).

Women whose partners lose their jobs or fail to provide resources are more likely to have affairs.

Almost half of the 1156 women murdered in New York between 1990-1994 were killed by husbands or boyfriends, and 67% of the crimes were committed in poor neighbourhoods with high rates of male unemployment (Belluck, 1997).

Why are Males more Aggressive?

One key aspect of aggression is the fact that males are much more likely than females to act as aggressors, e.g in Chicago between 1965-1980, 86% of murders were committed by men, with 80% of the victims being other males (Daly & Wilson, 1988).

In all known cultures males commit more murders and are more likely to be the victim of assault than are females. If extreme violence is ignored, males still show the following:

They take more risks. They are more likely to choose immediate rewards. Males show aggressive behaviours from age 2 onwards. Males are much more likely to escalate an altercation. Large sex differences favouring males are seen for

aggressive fantasies, physical aggression, imitative aggression, and willingness to shock others (Hyde, 1986).

Same-sex bullying involving direct physical aggression is more common in males (Ahmad & Smith, 1994).

Evolutionary Explanations.

Evolutionary psychologists see aggression as an adaptive solution to sexual selection.

Daly & Wilson (1999) argue that due to inequalities in parental investment males have to compete with one another for access to the higher investing females.

E.g male elephant seals are much larger than females and much aggressive as they defend a harem of females. A small number (around 5%) of successful dominant males will sire 85% of all offspring in a breeding season (Le Boeuf & Reiter, 1988).

Human males are the product of ancestral males who had to engage in risky strategies of intrasexual competition for access to the higher investing females, males die on average around 7 years younger than females (Trivers, 1985).

1. Youth.

Competition amongst males is highest in those entering the breeding market as they have to gain status to enable them to compete against other males (older and higher status).

Wilson & Daly (1985) found that young males are more likely to engage in dangerous confrontations when the reward is a rise in social status.

Young males are also more likely to escalate trivial altercations when there is potential 'loss of face' in front of other competing males or potential female partners.

This is referred to as the ’Young Male Syndrome'. In adolescence the killing of males drastically increases,

reaching a peak in the early 20's, by this age males are 6 times more likely to be murdered by other young males.

Homicide Victims By Age.

From Buss, 1999 p 292

2. Social Status. It is important for males to be able to initially achieve

a certain social standing and then defend or improve it.

Winners gain social status while their opponents lose social status.

This is sensitive to social context -eg a man who beat up a child would lose more status than he gained.

Males at the bottom of a social hierarchy face increased pressures to compete.

We would predict that males who lack resources or social status would engage in correspondingly riskier behaviour to get what they want.

Wilson & Daly (1985) showed that males who were poor or unmarried were more likely to commit murder than were wealthier or married males.

Step-Parents and Aggression.

We would predict that substitute parents would care less for their adopted children than genetic parents.

Children of homes involving a step-parent (especially a father) are 40 times more likely to appear in abuse statistics, juvenile crime statistics, or to run away from home (Daly & Wilson, 1985).

Hill & Kaplan (1988): found that in the Ache Indians, out of 67 children raised by mother and stepfather, 43% had died before age 15 compared with 19% of children raised by their genetic parents.

A survey in the USA (1976) revealed that a step-child was 100 times more likely to be fatally abused than a same-age child living with genetic parents. The children most at risk are those aged 0-5.

Child Homicide Rate in Canada 1974-1983.

Natural parents

Step parents

From Daly & Wilson, 1988 p 520

Parent-Type and Abuse Rate

From Daly & Wilson, 1985 p 202

However..

Temrin et al., (2000) analysed data from children aged 0-15 killed by their carers in Sweden between 1975-1995 (a total of 39 cases).

They found that the percentage of children killed by their carers was as follows:

Two genetic parents = 56.2% One genetic parent = 38.7% One genetic and one non-genetic parent = 5.1% Two non-genetic parents = 0.0%

Female Aggression.

We would perhaps expect that females would be much less violent than men as they face the brunt of childrearing and so the survival of the mother is of major importance to the well-being of the child (particularly in infancy).

For example in the Ache Indians of Paraguay if the mother dies in the first year of the infant’s life, the subsequent infant mortality rate is 100%.

Females therefore have a greater tendency than males to protect their own lives and this will have enhanced their reproductive success.

Female Behaviours Which Enhance Their Reproductive Success.

Females display more 'anxious' behaviour particularly with regards health and personal welfare issues.

Certain phobias (animals, dangerous places) are more common in women.

Women are less likely to engage in sensation-seeking behaviours.

Women have lower rates of accidental injury. Women are less likely to take drugs. Women report higher levels of fear of crime. Women rate the importance of health higher than

men, know more about health issues and are more likely to adopt preventative care.

Women overestimate the dangers of a potential aggressive encounter.

Female Aggression is an Adaptive Behaviour.

Female aggression has traditionally been viewed as a gender-incongruent aberration.

Campbell (1999) has however argued that certain aspects of female aggression are just as adaptive as certain kinds of male aggression.

While males compete with one another for dominance and its rewards, females compete with one another for resources (i.e. other males) which can directly enhance their reproductive success.

We would thus expect the severity of competition to be related to the availability of resource-rich males, where males are few or are of poor quality then female competition and aggression should be higher (Campbell, 2001).

Female Aggression in Context.

Women are significantly more likely to be attacked by another woman (generally an acquaintance) than a man.

In the USA, Campbell et al., (1998) found that out of 297 female-female fights, 121 were concerned with men and 67 were about subsistence concerns (food, money, domestic goods etc).

Normally though, the fear of direct physical assault means that females are less likely to form dominance hierarchies which would entail direct physical aggression to develop and maintain.

They are thus much more likely to form small co-operative groups (often with other female relatives). Evidence:

Group Behaviours.

When placed into group’s girls cooperate whilst boys compete.

Girls who show strong competitive or dominance behaviours are rejected by their peer group.

Boys use direct commands while girls use polite persuasion.

Girls are very concerned to develop cohesion and shared norms within the group.

Collaborative interchanges are more common in female groups while domineering exchanges are more common in male groups.

Males are more likely to adopt an autocratic leadership role and accentuate differences between individuals and groups.

Female Aggression is Indirect.

Males are more likely to favour direct physical or verbal aggression.

Such aggression would not be adaptive for females as they may get injured.

Female aggression is therefore more likely to be 'indirect', i.e. it takes the form of social manipulation where:

The 'attacker' may hide their identity by spreading nasty gossip.

The individual may shun other members of the group or using their influence in the group to get other members ostracised.

Girls are more likely to destroy an adversary's property or tell tales on them, use social ostracism and manipulation of others opinions.

Female bullies are more likely to use indirect aggression rather than direct aggression.

Female Criminal Behaviour is also Indirect

Female criminal behaviour comes close to that of males only in larceny/theft, particularly where direct confrontations are absent (i.e. credit card fraud as opposed to mugging).

Where female-female physical violence does occur, it is most often triggered by competition over scarce resources (usually men) and is most common between current wife/girlfriend and ex wife/girlfriend.

Female-female homicide is very rare and women are much less likely to use weapons when aggressing.

A Study of Female Aggression.

According to Campbell et al., (1998), female-female aggression occurs most often in lower-class females aged 15-24 who generally know one another.

The most frequent trigger for female-female aggression is competition for the attention of men and triggered by insults that slight the others sexual reputation.

They analysed female-female assaults in Massachusetts during 1994 (482 in total) and found the following:

The majority of these cases were committed by females <24 years old.

The number of female-female assaults rose with increased dependency on welfare.

Male unemployment was unrelated to female-female aggression.

Women committed more property crime (fraud, shoplifting) and were more likely to engage in prostitution.