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Sexual Attraction and Human Reproduction
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Seminar readings:
  • Buss, (1995). "Psychological sex differences: Origins through sexual selection".
  • The Evolutionist "In conversation with David Buss"
  • Hewett (2002) "Theory of Sexual Selection: The Human Mind and the Peacock's Tail"
  • Miller (1998) "Sexual Selection and the Mind" - interview with Miller in Edge magazine on May 26th 1998
  • Kanazawa (2000). "Scientific discoveries as cultural displays: a further test of Miller's courtship model". Evolution and Human Behavior, 21, 317-321, 2000.
  • Miller (2000) "The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature
  • Chick (1998) "What is Play For? Sexual Selection and the Evolution of Play"
  • Wilson and Daly (1998) "Lethal and nonlethal violence against wives and the evolutionary psychology of male sexual proprietariness"
  • Daly and Wilson (1996) Evolutionary psychology and marital conflict: the relevance of stepchildren"
  • Daly and Wilson (2001). "Risk-taking, Intrasexual Competition, and Homicide".

The status of sexual selection in evolutionary psychology

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View the PBS (2001) Evolution video "Sex" to get an overview of the factors involved in sexual selection

The peacock fascinated Darwin: How could natural selection alone have led to such an elaborate plumage? Surely such an encumbrance would have jeopardized the bird's survival?

He proposed that secondary sexual characteristics of male animals evolved because females preferred to mate with individuals that had those features.

Darwin wrote: "[Sexual Selection] depends, not on a struggle for existence (i.e. natural selection), but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring." Charles Darwin, Origin of Species (1859). Italics mine

View the PBS (2001) "Tale of the Peacock" video featuring Petrie's work on peacock tail feathers.
"Peahens often choose males for the quality of their trains -- the quantity, size, and distribution of the colorful eyespots. Experiments show that offspring of males with more eyespots are bigger at birth and better at surviving in the wild than offspring of birds with fewer eyespots."

In this  short video by Richard Dawkins discusses how phenotypes and extended phenotypes play an important role in sexual selection

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In essence, sexual selection can operate through two mechanisms:


In these examples of intersexual selection -selective mate choice
  • a female Satin Bower Bird inspects an avenue of twigs a foot or so apart constructed by the male which has bright shiny blue feathers. The male arranges blue objects in front of this avenue in order to attract a mate. Notice the sexual dimorphism in this species - a brightly coloured male bird courts a relatively dowdy female
  • a male suitor presents a woman with a token of his affection


Familiar examples of males competing with each other for access to females -intrasexual selection- include:
  • stags fighting during the rutting season
  • the creation and maintenance of 'dominance hierarchies'
  • and possibly, the tendency of young men to engage in risk taking and inter-male aggression
primate-aggression.jpg (6867 bytes)

Frequently inter- and intrasexual selection behaviours are exquisitely intertwined.

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This picture shows a modern re-enactment of medieval joust in which two knights charge at each other on either side of a central barrier with the aim of unhorsing their adversary with a lance in order to win their lady's favour
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Inter- and intrasexual selection?   In medieval times knights swore to uphold the values of courage and courtesy. Henry VII built a 'tiltyard' at Hampton Court where he could joust with his courtiers as they recreated the chivalrous exploits of medieval knights such as the legendary Sir Galahad - a member of King Arthur's court. The tiltyard had  a special grandstand built in the middle for the queen and the ladies of the court to get a better view. The winner of the jousts was awarded a prize by the 'Queen of Beauty', elected for the occasion from amongst the women present.

Parental investment and sexual selection

Darwin's (1871) theory of sexual selection was developed further  by Trivers (1972) who argued that because of parental investment, the sex that invests greater resources in offspring will evolve to be the choosier sex in selecting a mate. In contrast, the sex that invests fewer resources in offspring will evolve to be more competitive with its own sex for access to the high-investing sex.

(In some species the roles of the sexes may be reversed. See Goodenough et al, 2001 Chapter 14 for a very good description of sexual selection in animals).

Buss (1999 p 41) provides a clear description of why evolutionary psychologists have applied Trivers' theory of parental investment to human mating:

"The differences between men and women in terms of the fitness costs of making a poor mate choice are profound. An ancestral man who made a poor choice when selecting a mate could have walked away without incurring much loss. An ancestral woman who made a poor choice when choosing a mate might risk becoming pregnant and perhaps having to raise the child alone, without help."

The BBC website "More Science About Lonely Hearts" provides a useful introductory overview of some topics researched by evolutionary psychologists interested in sexual selection.

You can listen to Lynn Segal, Professor of Psychology and Gender Studies at Birkbeck College, London and Ruth Mace, Lecturer in the Department of Anthropology at University College London debate the contribution of evolutionary psychology to contemporary attitudes to marriage and parenting. (BBC radio broadcast as part of "Woman's Hour" on Wednesday 8 August 2001)


Human mating systems: monogamous females and polygamous males - a myth?

There are several technical terms used to describe mating systems that can cause confusion. This table may help to clear things up or create more bewilderment - let me know!

Mating system Male female-icon.gif (190 bytes) Femalefemale-icon.gif (190 bytes)
Monogamy female-icon.gif (190 bytes)mates with one female female-icon.gif (190 bytes)mates with one male
Polygamy:  
Polygyny female-icon.gif (190 bytes)female-icon.gif (190 bytes)mates with more than one female  
Polyandry   female-icon.gif (190 bytes)female-icon.gif (190 bytes)mates with more than one male
Promiscuity female-icon.gif (190 bytes)female-icon.gif (190 bytes)mates with more than one female female-icon.gif (190 bytes)female-icon.gif (190 bytes)mates with more than one male


Answer these questions after you have studied the table:

 


Male and female attitudes to multiple sexual partners

Evolutionary psychology suggests that males have evolved an approach to mating that leads them to seek multiple copulatory partners. This prediction - based on Trivers' theory of parental investment - is consistent with the following observations:
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  • When they are asked how many sexual partners they would like over a certain period of time, men report that they would prefer more partners than women,
  • When they are asked if they would agree to have sexual intercourse with an attractive member of the opposite sex that they have known for varying lengths of time, men and women express different likelihood's of consent. Men reported that they would be slightly disinclined to have intercourse with a woman they had known for just an hour. In contrast, it is very unlikely that a woman would agree to intercourse after knowing a man for this length of time. Source: Buss & Schmidt, Psychological Review , 100, 204-232, 1993
sex differences in number of partners sex difference in agrrement to intercourse
A note of caution; These results are based on what men and women say about their desires and preferences. People's replies to questions about what they think they would do should be treated with caution. Researchers need to be wary of demand characteristics influencing participants' behaviour. Demand characteristics refer to participants awareness of experimenters' goals or cultural expectations influencing participants responses. See Mace (2000) for a discussion of this confounding factor in evolutionary psychology research.

Another problem with this type of prospective questionnaire in which people say what they might do is that it does not show that the behaviour has a measurable outcome on reproductive fitness. Contrast this research technique with that adopted by Pawlowski, Dunbar and Lipowicz (2000) who examined  the medical records of nearly 4,500 Polish men aged between 25 and 60. They found that men who had fathered at least one child were on average three centimetres (1.25 inches) taller than men who had not fathered children. You can listen to a radio interview in which Dunbar discusses various explanations for this result.

Short-term mating: "A dance between the sexes?"

Although the data (Buss & Schmidt, 1993) suggest that - compared to females - males would like to mate with more partners over time, it does not support the hypothesis that females are exclusively monogamous. If you look carefully at the graph you will see that females would like more than one partner in the next three years. This opens up the possibility that a male may be cuckolded and consequently waste his parental investment. Bear this in mind as you progress through the readings on this page. These factors may be relatedness to 'male sexual proprietariness' which has been used to explain domestic violence which is discussed below.

Also examination of these results suggests that whereas:

  • men reported that they would be slightly disinclined to have intercourse with a woman they had known for just an hour
  • women reported that they would be slightly disinclined to have intercourse with a man they had known for three months

It is often claimed by evolutionary psychologists that the cost of mating for men are relatively slight. For example: "A man in human evolutionary history could walk away from a casual coupling having lost only a few hours or even a few minutes." (Buss, 1999, p102). But the interview-data suggests that men may have to invest between three and six months in courtship behaviours before they get the opportunity to mate. Whilst this is much less than the nine months a woman devotes to pregnancy plus the years of postnatal care, there is nevertheless a greater cost to the male than is sometimes implied. The pre-mating costs for men seem to have been discounted by evolutionary psychology.

A small, but significant, proportion of women in long-term relationships engage in short-term matings (see figure). There must have been some selective advantage for women to engage in short-term mating otherwise the inclination to engage in this behaviour would never have had a selection advantage for men. Buss (1999) distinguishes between different types of explanations for female short-term mating that have some experimental support in the human and animal literature:

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cinderella2.jpg (6366 bytes)To sum up, I would suggest that the notion of monogamous females and polygamous males is a myth. I would suggest that there is more symmetry in the costs and benefits of short-term mating for both sexes than hitherto acknowledged. It would advantage the inclusive fitness of both males and females to engage in short-term mating where the partner offers 'good biology', and any offspring would be sufficiently resourced to ensure their survival to reproductive age. Thus:

This analysis suggests the following hypotheses about the attitudes of genetic relatives to short-term mating:

  1. a man's relatives can increase their inclusive fitness by permitting / encouraging him to engage in short-term mating particularly if the woman has access to sufficient resources to promote the survival and reproductive success of his child.
  2. a woman's relatives can increase their inclusive fitness by permitting / encouraging her to engage in short-term mating IF her mating partner has 'good biology', and she has access to sufficient resources to promote the survival and reproductive success of her child(ren).
  3. a woman's relatives can jeopardize their inclusive fitness by permitting / encouraging her to engage in short-term mating IF she does not have access to sufficient resources to promote the survival and reproductive success of her child(ren).

Long-term mating:"A battle between the sexes"?

In a groundbreaking study of long-term mating strategies involving 10,047 participants from 33 countries Buss (1989) showed that:

It is interesting that the commentary elicited by this paper focussed on alternative explanations involving economic powerlessness for the female preferences. The findings on male desires did not provoke a spate of alternative explanations based on cultural interpretations to challenge the evolutionary explanation. Buss (1989) and Buss' responses to peer commentaries are both available online

According to conventional evolutionary psychology (e.g. Buss, 1996) these psychological differences between the sexes evolved because the adaptive problems posed by reproduction are different for men and women.   However, if we examine the adaptive problems posed by long-term mating there is remarkable symmetry between the problems faced by the sexes:

  • Both men and women need to select mating partners with 'good biology', i.e. to maximize fitness both sexes need to mate with fertile, healthy  partners who are likely to produce fertile, healthy children.
  • Both men and women need to mate with partners who will remain 'faithful' i.e.
    • Women need to select men able and willing to provide resources (physical and behavioural) to promote the survival and reproductive success of her child(ren),
      AND
    • Men need to select women able and willing to provide resources (physical and behavioural) to promote the survival and reproductive success of his child(ren)
      - this particular point is implied, but not stressed, by the evolutionary psychologists I have read.
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'Good biology' or 'good genes'

Today's paper carries a story headed "Technological advances may lead to genetic apartheid, says scientist" (Akbar 2003).

"Sir Paul Nurse, a Nobel prize-winner, who is the chief executive of cancer research UK, predicts that in 20 years' time, it will be technically possible to sequence the genome of each new baby." I read this and wondered if a 'genetic identity card' might be an essential piece of equipment for future generations steeped in evolutionary psychology setting off to secure a 'one-night-stand'. But I am not convinced that being armed with a person's genome would provide better clues to their 'good genes' than relying on the adaptations that have evolved to spot fit short- and long-term partners.

I have used the phrase 'good biology' in previous sections rather than the more common expression 'good genes' after reading Keller (2000). Keller's book is well worth reading if you are seeking answers to questions such as "What does a gene do?" (see chapter 2) or "What are genes for?" (see Conclusion). Genes are important for evolutionary psychology because they permit the transmission of evolved adaptations from parents and offspring (Buss 1999). However the term 'gene' is borrowed from biology, and recent advances in molecular biology suggest that the concept 'gene' is either very difficult to tie down, or has now outlived its usefulness (Keller pp 66-72). Way back in 1953, Watson and Crick's description of DNA's double helical structure lent support to the idea that one gene controlled the production of one enzyme. Abnormalities in a single gene are known to give rise to severe disorders such as Huntington's disease or phenylketonuria (PKU). This simple picture was captured by Francis Crick who stated in 1957

"DNA makes RNA, RNA makes protein, protein makes us." (See Keller., p 54).

But this simple theory left open the question of what regulates the genes to control the time, place and amount of protein production. All cells have a nucleus with a full set of genes, but all cells do not produce all the potential proteins that they are capable of manufacturing all of the time. In 1959 Jacob and Monod suggested a distinction between 'regulator' and 'structural' genes to deal with this problem. It has been suggested that 97% of the human genome is involved in regulating the 3% of genetic material that actually builds protein. The discovery in 1977 that genes are fragmented along a chromosome and interspersed with lengths of 'junk DNA' opened up the possibility that the variety of protein constructed from DNA can vary during an organism's life (see section on 'alternative splicing', Keller p 60). Consequently, the notion of 'one gene - one protein' has been abandoned to be replaced by 'one gene - many proteins'. Perhaps the most unsettling message for evolutionary psychology from molecular biology comes from recent 'knockout' studies in which specific genes are disrupted in an intact animal. "In many cases, knocking out a normal gene and replacing it with an abnormal copy had no effect at all, even when the gene was thought to be essential..." (Keller 2000, p 112). These results have added to the belief that there is extensive redundancy built into an animal's DNA.

Redundancy is a recurring familiar feature of biological systems. For example, our own work (Kenyon et al.1983) on retrieving behaviour in rats revealed that although cutting the infraorbital branch of the trigeminal nerve - which transmits tactile sensation from vibrissae to the brain - severely disrupts retrieving behaviour for about 12 hours, this complex behaviour returned to normal 24 hours post-operatively even though tactile sensation was abolished. Thus the mother rat was able to utilize some other (redundant?) system to enable her to carry out this vital maternal behaviour.

 


Seminar readings:

The paper by Buss, D. M. (1995). "Psychological sex differences: Origins through sexual selection". American Psychologist, 50, 164-168, available online provides a short and clear explanation of sexual selection seen through the eyes of an evolutionary psychologist. As you read the paper consider the following points:


Read The Evolutionist "In conversation with David Buss" which is available online and consider the following points:


Read Hewett (2002) "Theory of Sexual Selection: The Human Mind and the Peacock's Tail" (online article) and consider the following points:

Read the interview (Sexual Selection and the Mind) with Miller in Edge magazine on May 26th 1998 (available online). This article provides an accessible introduction to Miller's view of the role of sexual selection in the evolution of human behaviour. As you read the article consider the following points:

female-scientist.jpg (6427 bytes)Read Kanazawa "Scientific discoveries as cultural displays: a further test of Miller's courtship model". Evolution and Human Behavior, 21, 317-321, 2000. (Available from library offprints collection) which describes an empirical study based on Miller's theory. As you read this paper consider the following points:


Here are rough graphs showing some of the results presented by Kanazawa. You can use this 'slide projector' to display the age distributions of peak scientific achievement for:
  • married or
  • unmarried male scientists

Kanazawa reported a significant difference in the mean age between the married and unmarried scientists. What statistical test would you carry out to determine if the distributions of the age of peak scientific performance are significantly different between married and unmarried scientists?

 

 


courtship.jpg (15171 bytes)Read Miller (2000) "The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature", Doubleday/Heinemann. A précis of the book is available online. Bear the following points in mind as you read:


Read Chick (1998) "What is Play For? Sexual Selection and the Evolution of Play", which is available online.

Bear in mind Burghardt's comment (cited in Chick):

"In most areas of behavior, the functional approach has yielded great rewards rather quickly once adaptive explanations have been carefully stated and explored"

as you consider the following points:


Domestic violence

Cuckold:  a man whose wife has sex or becomes impregnated by a man other than himself . Here is a 19th century French print entitled "The celebration of the Order of Cuckoldry before the throne of her majesty, Infidelity" Notice the repentant husband whose wife is pointing to the antlers on her own head!

rodin-kiss.jpg (4279 bytes)Wilson and Day (1998a) have postulated a 'male sexual proprietariness' module that is adapted to protect human male reproductive fitness against undetected cuckoldry   i.e. a set of behaviours to prevent a man's partner being impregnated by another man. They suggest that because of paternal uncertainty, and the cost of paternal investment, male fitness will be reduced if a male is cuckolded. Wilson and Daly postulate that male sexual proprietariness is an adaptation that has evolved through natural selection to overcome this risk.

devil.jpg (2114 bytes)It is very important to stress that this is a classic example of the type of proposal that can generate considerable controversy if the suggestion that something "is" is mistakenly taken to mean that therefore it is "natural" or "ought" to be. No evolutionary psychologist is suggesting that male sexual proprietariness is a "good thing". (Here is a discussion of common misconceptions about evolutionary psychology).

At the risk of stating the blindingly obvious it is worth pointing out that women are not monogamous. The phrase "till death do us part" is all-to-often a romantic illusion. When they are asked how many sexual partners they would like over a certain period of time,  women responded that they would like more than one partner in the next year (see above). Of course this is an average figure for the group of women interviewed; some will prefer to remain monogamous, others may anticipate more than two partners. Nevertheless, cuckoldry is a real potential risk to male fitness. Both men and women can experience strong emotions if they see or believe their partner is, or has, formed a romantic relationship within another. Although Wilson and Daly's work focusses on male sexual proprietariness evolutionary psychology predicts a comparable module in females.

Read Wilson and Daly (1998a) "Sexual rivalry and sexual conflict: recurring themes in fatal conflicts". Theoretical Criminology.vol. 2(3):291-310, available online and consider the following points:

This was a 'forced-choice' dilemma. Participants had to choose one option over the other. Why do you think 40% of men chose option# 1.

tortoiseandhare.jpg (3998 bytes)In the light of the results reported by Buss & Schmidt (1993):


mother_children2.gif (34037 bytes)burqa2.jpg (5650 bytes)Male sexual proprietariness consists of a set of controlling behaviours of increasing severity that can culminate in 'uxoricide' i.e. murder of a female by her male partner. Wilson and Daly have focussed on uxoricide because it is a clear behavioural endpoint. Nevertheless there is a spectrum of preceding behaviours that they interpret as being male attempts to control female reproductive effort.

The Independent newspaper reported recently that "According to the Home Office, there are 635,000 incidents of domestic violence a year. One in four women will be abused by their husbands or boyfriends during their lifetime and, on average, two women a week are killed by a current or former partner." (Goodchild, 2003).

As you read the evidence in the next section, it may help to organize the material into a table along the lines shown here to identify 'risk factors' that may increase a woman's chance of becoming the victim of domestic violence, and a corresponding list of factors that may reduce her risk. Here is a copy of the table that you can print out

Risk factor for domestic violence Conditions that increase risk Conditions that decrease risk Reference and caveats
Age risk higher in women 15-34 risk declines after 35 see Wilson & Daly (1998) Figure 8.2. Caveat: may be confounded with man's age
Type of union unmarried married see Wilson & Daly (1998)
Add factors you identify in this column and ... fill in these cells as you progress through the readings

Read Wilson and Daly (1998) "Lethal and nonlethal violence against wives and the evolutionary psychology of male sexual proprietariness" available online. and consider the following points:

Examine these data from Wilson and Daly (1998) table 8.1 and 8.2

Percentage of women who have experienced increasing levels of violence agreeing to statements about the behaviour of their male partner. (Redrawn from Wilson and Daly 1998, table 8.1)
  Violence
None
(n=7060)
"Nonserious"
(n=1039)
"Serious"
(n=286)
"He is jealous and does not’t want you to talk to other men" 3.5 13 39.3
"He tries to limit your contact with friends or family" 2 11.1 35
"He insists on knowing who you are with and where you are at all times" 7.4 23.5 40.4
"He calls you names to put you down or make you feel bad" 2.9 22.3 48
"He prevents you from knowing about or having access to the family income, even if you ask" 1.2 4.6 15.3
Autonomy-limiting Index (Average number of items affirmed by women) 0.17 0.74 1.78
To what extent do you think the 'Autonomy-limiting Index' might provide the basis for creating a risk index to assess the dangers faced by women in abusive relationships?


This figure is taken from Daly and Wilson (1996) "Evolutionary psychology and marital conflict: the relevance of stepchildren", available online. Are a woman's age, and the the presence of stepchildren in the home risk factors for domestic violence?



Evolutionary psychology and artificial insemination by donor

According to evolutionary psychology some aspects of men's behaviour are directed towards controlling female reproductive behaviour. For example, 'male sexual proprietariness' is seen as part of an adaptation to reduce the risk to male reproductive fitness posed by cuckoldry which reduces the returns from paternal investment.

Artificial insemination is one of the oldest and simplest treatments for human infertility. There are two fundamentally different types of artificial insemination:

A man who agrees that his partner be artificially inseminated with donor sperm is agreeing to provide resources for a child that does not share his genes. In contrast, a man who agrees that his partner be artificially inseminated with his sperm is agreeing to provide resources for a child that does share his genes. Does this decision affect his behaviour towards his partner and child? Evolutionary psychology would predict that couples who have children through artificial insemination with husband sperm would be less likely to experience family problems than couples who have children through artificial insemination with donor sperm.


Intrasexual selection

Read Daly and Wilson (2001). "Risk-taking, Intrasexual Competition, and Homicide". Nebraska Symposium on Motivation, 47:1-36. available online  and consider the following points:
  • Explain the meaning of the terms 'risk proneness / seeking', 'risk aversion' and 'loss' in decision making research.
  • Explain, with examples, the conditions under which men might choose higher-risk options. You should add to your answer as you proceed through the paper.
  • Why might a normally risk-averse animal switch to high risk behaviour?
  • Explain in your own words Daly and Wilson's comment that "...sex differences in the variance of reproductive success are widely considered indicative of sex differences in intrasexual competition." my underlining (Daly and Wilson, 2001, p. 7-8)
  • Make brief notes to support the claim that men are polygynous.
  • To what extent are men less sensitive to risk than women ?
  • What is the relationship between gender, risk taking behaviour and status?
  • If risk taking is an adaptation, under what conditions would you expect a man's attitude to risk to change as he gets older?
  • Is it legitimate to use homicide as a marker for male risk taking behaviour?
  • Is male-on-male homicide simply the unfolding of a 'biological predisposition' to violence in young men? Or is it an adaptation sensitive to unconsciously perceived environmental conditions? If so, what are these environmental conditions? You should add to your answer as you proceed through the paper.
  • Are people's perceptions about future prospects 'rational' when viewed from an evolutionary perspective?
  • What are the implications of Daly and Wilson's paper for social policy?
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Building links between evolutionary psychology and psychobiology

Psychobiology is a broad term and somewhat loose term that groups together a wide range of disciplines studied by psychologists and biologists. Psychobiologists are interested in studying the biological bases of behaviour. The term psychobiology covers a wide range of sub-disciplines within psychology and biology including: physiological psychology, psychopharmacology, developmental psychobiology, ethology, neuroethology, animal behaviour, psychoneuroendocrinology, behavioural and cognitive neuroscience, and psychoneuroimmunology.

Evolutionary psychology focusses on why behaviours evolved in particular ways. For example, according to evolutionary psychologists, in species where one sex makes a higher parental investment than the other - the high investing sex - is a resource for which the opposite sex competes.

In the next section we will examine some material that suggests that psychobiology may be able to shed some light on sex differences in risk-taking behaviours.

Aggression and sexual selection

In humans, females make a higher parental investment than do males. Males compete with each other for access to females. Males use their dominance and resources to deter rivals and attract females. Evolutionary psychologists argue that the higher rate of aggression in men shows the crucial importance of status to male reproductive success.

However women can, and do show, aggressive behavious (see Campbell, 1999). Females compete with each other, and men, for resources that will enable them to successfully raise their children.

Sex differences in human aggression

Over 80% of homicides are committed by men. Most of the victims are also men. The most common cause of homicide is due to the escalation of a relatively trivial disagreement over status that starts with words and escalates into lethal violence. It seems that men resort to violence to protect or gain status and honour.

This sex difference is found across all cultures. Criminal violence is most likely between the ages of 14 and 24. (see Campbell, 1999).

Some psychologists have argued that boys are trained to be aggressive and girls learn to be passive. However, Dyson-Hudson (1995) found that: " 'low-conflict societies' with affectionate socialization and aversion to inter-personal confrontation (e.g. Inuit, !Kung Bushmen, Gebusi of lowland New Guinea) have high rates of violent death. In contrast, Turkana pastoralists (East Africa) are taught to fight as children; and most men reported having participated in inter- personal fights intended to cause injury, having engaged in recreational within-group fighting mimicking warfare, and having taken part in raids on the neighboring Pokot. Yet demographic data indicate that within- group homicide rates among the 'violent' Turkana are lower than those reported for the 'low-conflict' societies.

It may be that Turkana rules which require bystander intervention and adjudication by elders, are effective in preventing within-group aggression and violence from escalating into lethal fights. "

Male aggression

bullitt.jpg (11758 bytes)Richard Wrangham (Wright and Wrangham, 1998) presents an interesting analysis of male violence in terms of evolutionary psychology. He argues that:

weweresoldiers.jpg (11343 bytes)However warfare is a uniquely human behaviour.   In a battle both sides will suffer casualties regardless of who finally wins. Consequently battles involve a failure to assess the true costs of combat by both sides. Wrangham suggests that this failure is due to 'positive illusions' by each set of combatants that they will emerge victorious



Female aggression

Until recently, relatively little attention was focussed on female aggression. Campbell (1999) argues that

".. lower rates of aggression by women reflect not just the absence of masculine risk-taking but are part of a positive female adaptation driven by the critical importance of the mother's survival for her own reproductive success."

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Campbell reviews evidence that:
  • women show  greater fear of physical harm compared to men.
    For example:
    • women show more fear of open spaces, dogs, snakes, insects, and rodents than men
    • women  are less likely to engage in hazardous sports, dangerous driving, military combat, and drug abuse, than men
    • women are more afraid of being victims of crime involving aggression, and are more likely to visit a doctor to seek advice on preventative care, than men
  • women commit fewer violent crimes than men (see Campbell et al, 2001)
  • women show less concern for status compared to men
  • greater adoption of dispute resolution strategies that involve a low risk of physical harm by women compared to men
  • female 'maternal aggression' to defend their offspring; paternal aggression is rarer
  • female menopause - an infertile period after the birth of the last child will ensure its survival

This is a picture of Phoolan Devi, (Seema Biswas) so-called "Bandit Queen of India", she led a gang  who roamed north central India during the late 1970s and early 1980s; she became a folk hero after taking bloody revenge against men who raped her.

These stills are taken from the French film 'Baise Moi" which was banned in several countries. The film deals with a young woman who has been raped, and an accomplice, who embark on a spree of violence and promiscuous sex. It is interesting to reflect on this film's treatment by censors in the light of Campbell's argument that

"...Women's aggression has been viewed as a gender-incongruent aberration or dismissed as evidence of irrationality. These cultural interpretations have "enhanced" evolutionarily based sex differences by a process of imposition which stigmatises the expression of aggression by females and causes women to offer exculpatory (rather than justificatory) accounts of their own aggression."

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Neurotransmitters & Aggression

Serotonin and aggression- animal studies

Increased serotonergic activity tends to reduce aggressive behaviour in rodents.

Adapted from Fig 11.18 Carlson (1998)

Higley et al (1996) studied free-ranging rhesus monkeys living on an island. Used behavioural observations and sampled CSF (cerebro spinal fluid) to measure 5-HIAA levels (5-HIAA is a breakdown product of 5-HT - the more 5-HIAA the greater 5-HT release).
  • found negative correlation between 5-HIAA and aggression. No relationship between aggression and NA or DA metabolites.
  • Low 5-HIAA associated with high risk-taking behaviour -aggression towards older larger animals, took long leaps from tree to tree. Many died as a result of attacks from mature males.
  • low 5-HT turnover may reflect low impulse control rather than increased aggression per se

"Dominance and aggression are not synonymous." (Carlson,1998)

. Serotonin levels are effected by dominance rank. Raleigh et al, (1984)

Redrawn from Fig 9.25 Feldman (1997)


Raleigh et al (1991) investigated effects of serotonergic drugs on dominance and aggression. Used 12 groups of vervet monkeys. Temporarily removed dominant male from each group. The two remaining subordinate monkeys were treated with
    • serotonergic drug
    • control (placebo)

The serotonergic drugs used were

    • 5-HT agonists (tryptophan or fluoxetine) which increase 5-HT activity
    • 5-HT antagonists (cyproheptadine or fenfluramine) which decrease 5-HT activity (chronic treatment with fenfluramine depletes 5-HT levels )

Used a crossover design so that each monkey received agonist and antagonist treatments.
Results:

    • monkeys given agonist drugs became dominant
    • monkeys given antagonist drugs became subordinate
    • monkeys given agonist drugs initiated fewer aggressive events
    • monkeys given antagonist drugs initiated more aggressive events

Note that because a cross over design was used the same animal could be dominant or subordinate depending on what type of serotinergic drug they received.


Serotonin and human aggression

  • Reduced concentrations of 5-HT and 5-HIAA in brains of suicide victims.
  • Maybe suicide and violence towards other people represent the same underlying aggressive tendency
  • Low 5-HIAA levels in brains of suicides who used violent means to end their own lives (using guns or jumping from heights rather than by ingesting pills or taking a poison)
  • in normal adults there is a negative correlation between 5-HIAA level and 'urge to act out hostility' subscale of the Hostility and Direction of Hostility Questionnaire
  • in psychiatric patients there is a negative correlation between 5-HIAA level and psychological measures of aggression
  • low 5-HIAA linked to impulsive, antisocial aggressiveness
  • low 5-HIAA reported in children with disruptive behaviour

 
Author Paul Kenyon


References and online resources