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Evolutionary Psychology

The Rise of Evolutionary Psychology


DarwinEvolutionary psychology is a new way of thinking about - human behaviour - my student's will be relieved to hear that rats don't get much of a look-in during this lecture!

Evolutionary psychology can be applied to any topic that is studied by psychologists. It is built upon the rock of Darwin's theory of evolution published in 1859. We usually associate Darwin with the evolution of structure and function, but Darwin suggested that evolution acted on patterns of behaviour as well as the anatomy and physiology of the body. He believed that human behaviour would eventually be explained in terms of evolutionary principles.

Biology is built upon the foundations provided by Darwin's theory of evolution. In contrast, psychology rests on the Standard Social Science Model . This model contains the barely concealed idea that human behaviour is guided by reason , whereas non-human animal behaviour which is influenced by instinct . According to the Standard Social Science Model the human mind is blank at birth (a tabula rasa - blank slate) and is filled as the result of experiences during the individual's lifetime. Behaviourism is a classic example of this approach to understanding human behaviour.

The idea that human behaviour is based on instincts was popular with psychologists about 100 years ago. [Incidentally, the best place to read about the decline of this idea, and the rise of behaviourism is Boakes ' From Darwin to behaviourism ', Cambridge University Press, Cambridge, 1984) see also Daly & Wilson (1999) ]

In 1975 E.O. Wilson wrote a groundbreaking and challenging book ' Sociobiology the New Synthesis ' in which he argued that human social behaviour could be explained in evolutionary terms. This caused an outcry in many quarters. Alcock (1998) provides a useful summary of the debate. I think it is always worth bearing in mind that Sociobiology (or any other approach ) is merely a theory . A theory has a purpose - to explain a body of knowledge and suggest directions for further investigation.

A theory is a bit like a Swiss army knife. It contains a number of useful tools. But eventually it wears out, or you need a tool that is not provided. But of course - in the wrong hands - you can be killed by a Swiss army knife ....

Nicky Hayes provides a very clear critique of the concepts underlying Sociobiology. One of the reasons why Sociobiology generated so much intellectual heat is the memory of what occurred in the past when evolutionary theories were presented in 'watered-down' or distorted versions to the general public.

Social Darwinism is a prime example. Social Darwinism emphasized evolution based upon "survival of the fittest" and painted a picture of "Nature red in tooth and claw". This view of evolution could be used to support oppressive economic policies and racial discrimination. This way of thinking about people can end up in mass extermination camps where genocide, compulsory sterilization, and breeding programmes, are used to ensure racial purity.

The term Sociobiology has now been replaced by the term evolutionary psychology . A good place to begin reading about this new area is Cosmides and Tooby Evolutionary Psychology: A Primerwhich is available online.

Cosmides and Tooby argue that psychology is a branch of biology. To some of you this may appear to be 'a self-evident-truth'. Others may not have thought about this, or reject it immediately. But the relationship between biology and psychology is an important issue, and one that you should form an opinion about.

Evolutionary psychologists believe that behaviour is strongly influenced by inherited factors, and that every human being acts (consciously, but mostly unconsciously) to enhance their inclusive fitness - i.e. to increase the frequency and distribution of their genes in future generations. As Steven Pinker puts it, 'the ultimate goal that the mind was designed to attain is maximizing the number of copies of the genes that created it'.

Principles of Evolutionary Psychology

Cosmides and Tooby describe the principles that guide an evolutionary psychology approach to any topic within psychology:

  1. "Modern skulls house a stone age mind." This is a catchy way of conveying the idea that human evolution occurred in a very different environment to the one in which we now live. Humans evolution is thought to have started 68-million years when we diverged from our primate ancestors. Evolutionary psychologists believe that natural selection designed our minds for life in an environment resembling the African savannah , in which our hunter-gatherer ancestors lived for thousands of years. For 99% of our evolutionary history we probably lived in hunter-gatherer societies. It is only about 10,000 years since humans first started growing their own food. The technical term used to refer to the environment in which we evolved is the Environment of Evolutionary Adaptedness EEA) . The EEA does not refer to some short period of time in our past. It refers to an array of factors that have influenced inclusive fitness during our evolution over the last 200,000 years. See Daly & Wilson (1999) for a very good discussion of this important distinction.
  2. The human brain consists of neural circuits designed by natural selection to solve problems that our ancestors faced during our evolutionary history. Our minds are an adaptation . Adaptions evolve to meet challenges in the environment, challenges faced in our EEA.
  3. Most of what goes on in the mind is unconscious . Most problems that we think are easy to solve are in fact very difficult to solve and require complicated neural circuitry. Consider vision, vision appears easy - open your eyes and you see the world - but this apparent simplicity hides a complex evolved system that we have not been able to reproduce artificially.
  4. Different neural circuits are specialized for solving different adaptive problems. Evolutionary psychology views the mind as consisting of specialized modules that have evolved to cope with adaptive problems. In contrast, psychologists have tended to view the mind as consisting of general purpose circuits involved in many different behaviours e.g. learning, intelligence, memory, reasoning, decision-making. You can see this approach reflected in the way your degree programme is broken down into modules covering these topics.

Boaz and Almquist (1999) provide a detailed account of human evolution. You may find it useful to view the video"The First People" which shows how rock paintings may give us an insight into the minds of our ancestors .

The overarching concern of evolutionary psychology is to identify factors that maximize reproductive success .

Contemporary psychology is very concerned with measuring and understanding the causes of differences between people. For example, most of you will be carrying out experiments as part of your third year projects specifically designed to manipulate experimental conditions to produce differences between groups of people.

In sharp contrast evolutionary psychologists are particularly interested in psychological mechanisms that:

  • are universal i.e. do not vary greatly between individuals
  • are closely related to reproductive success : e.g.
    • attracting a mate
    • choosing a mate
    • raising offspring
    • kin recognition
    • maintaining relationships
    • acquiring status
    • cheater-detection
    • maintaining group cohesion

(See Definition of Evolutionary Psychology )

The debate between The Standard Social Science Model and evolutionary psychology boils down to whether the mind is constructed during development of the individual, or has evolved during development of the species.

Evolution of Behaviour

In our daily lives we are surrounded by examples of animals that have evolved. One of the reasons for believing in evolution is our own ability to modify animals through selective breeding . For example, Man has selectively bred dogs, cats and farm animals for thousands of years. Clearly there have been changes in the appearance of these animals as a result of domestication . But can behaviour be altered by selective breeding?

In a classic experiment Thompson (1954) trained rats to run through a maze for a food reward. Some rats learnt this task faster than others. Thompson selectively bred fast and slow learners, so that over several generations rats were descended from a line of fast or slow maze learners. Eventually two strains of rats developed - 'maze dull' and 'maze bright'. After only six generations, the maze dull rats made twice as many errors before learning their way through the maze than the maze bright strain of rats.

Parental Investment

A baby's brain is very large at birth compared to the size of the rest of the body, and barely fits through its mother's pelvis. The brain continues to grow rapidly during childhood. The adult human brain is four times larger than would be expected in a primate with our size of body.

Human babies have restricted motor capabilities when they are born. For example, it takes a human twice as long as a chimpanzee or gorilla to develop the ability to walk and hold on to its mother.

Babies were part of our ancestors Environment of Evolutionary Adaptedness (EEA) .

Babies represent one of the challenges that our behaviour has adapted to during evolution. It takes a long time for a child to reach a state where it could survive without its parents. Parents input into caring for the child is called parental investment . Parental investment represents a cost for both mother and father. But the production of babies that mature and reproduce are the expression of parents' inclusive fitness.

According to evolutionary psychology sex differences in human behaviour evolved because of differences in the parental investment needed to ensure the reproductive success of males and females.

  • Males produce millions of sperm and could inseminate thousands of women during their lifetime.
  • Females produce relatively few eggs, pregnancy and lactation extends overall several years which restricts the number of children they can bear.


  • Men can increase their inclusive fitness by mating with many healthy females who are likely to give birth to a healthy baby
  • Women can increase their inclusive fitness by mating with men who are likely to donate sperm that will contribute to the birth of a healthy baby

While the mother nurses one child she cannot be pregnant with the next. It would benefit the mother's inclusive fitness if the father made a parental investment in her child. Likewise it would benefit the fathers inclusive fitness if he made a parental investment in his child. But, if the father invests resources in a child he has less resource to invest in another child.

Thus children are rate limiting factors in their parents quest for increased inclusive fitness. Children were part of our ancestors EEA, and our behaviour has adapted to meet the challenge.

Genetic Tests Reveal True Fathers!

Fathers can never be sure that they actually are the father of the child in which they are investing resources.
  • Soraya Khashoggi was married to the arms dealer Adnan Khashoggi when she had a child by Jonathan Aitken, the disgraced former Conservative minister.
  • Paula Yates , the television personality, discovered at 37 that her real father was Hughie Green, the 'Opportunity Knocks' star.

Here is a link to an article from The Times (23rd January 2000) with the headline One in seven fathers 'not the real parent' which suggests that a large number of children are not fathered by the man who believes he is their father.

This startling conclusion is based on results of DNA genetic testing of blood samples from supposed parent and offspring.

Quotation from The Times article:

"There are now seven government-approved laboratories doing paternity testing. Cellmark Diagnostics in Abingdon is the largest and receives more than 10,000 requests a year. One in five of them is "private" and has not been ordered as a result of a court or Child Support Agency dispute. David Hartshorne, spokesman for Cellmark, said that in about one case in seven, the presumed father turns out to be the wrong man. "

Do you think these tests were carried out on a random selection of the population? What is the implication of your answer for the conclusion that a large number of children are not genetically related to their 'fathers'?

Point to ponder: What is childhood for? Seems like a silly question. What's the answer then? Mace (2000) points out that children could grow faster than they actually do grow. What do children learn during childhood that increases their inclusive fitness? Apparently there is no evidence on what this might actually be. Not such a silly question then!

Selecting a Partner

Short Term Relationships

Evolutionary psychology suggests that males have evolved an approach to mating that leads them to seek multiple copulatory partners. This prediction is used to explain the the following observations:

  • 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

Source: Buss & Schmidt, Psychological Review , 100, 204-232, 1993

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.

Long Term Relationships

Although there is a sex difference in the attitude of males and females to casual sex, this difference disappears when men and women are asked about the qualities they seek in a partner for a long term relationship . College students were asked about the minimum intelligence they would require in a partner for casual sex. Men, but not women were prepared to have casual sex with a partner of much lower intelligence than women. However, men and women have similar standards for the minimum intelligence that they would accept in a marriage partner.

In this diagram the results are expressed in terms of the percentile intelligence of an acceptable partner. A value of 40 means that the prospective partners intelligence would lie at the 40th percentile - 40% of the population would have lower intelligence. A value of 70 means that 70% of the population would have lower intelligence. Thus a person with a percentile score of 70 is more intelligent than someone with a percentile score of 40 ( Kenrick et al , 1990).

Males and females seem to look for the same psychological characteristics in long-term partners such as : kindness, understanding, intelligence, personality, adaptability, and creativity (Buss and Barnes, 1986).

Sex Differences in Desirable Partner Characteristics

On the 28th September 2000, a judge in the United States awarded a former Playboy model, 26 year old Anna Nicole Smith, $449m after she was left out of the will of her 90-year-old husband.

Ms Smith was 26 when she married J Howard Marshall a Texan oil tycoon who died 14 months after they married.

A lawyer in the case commented: "He's given her cars, a company. He had given incredible things that were intended to help her get ahead and survive and make money" .

Females and males have a particular interest in selecting partners that will enhance their reproductive success and inclusive fitness. Evolutionary psychology calls these sexual strategies . One hypothesis is that:

  • Females have evolved mechanisms that enable them to detect men that will transfer resources to their offspring (i.e. health and paternal investment ). These are sometimes referred to as 'good provider' and 'good genes' attributes in the male
  • Males have evolved mechanisms that enable them to detect females that promise rapid production of offspring, and a disinclination to mate with other men (i.e. health, fertility and faithfulness )

    Male paternal investment and female faithfulness have costs and benefits to both sexes that must be reconciled to maximize individual inclusive fitness.

    Fertility in the human female is 'silent' - there are no clear-cut external cues that a woman is ovulating. In terms of inclusive fitness it may be beneficial for a woman to marry a 'good-provider' , but mate with a man with 'good-genes'. Thus a woman's husband and mate may be different males. It is very difficult to obtain data on the rate at which this occurs in humans - for obvious reasons! But Mace (2000) suggests that about 10% of children are the result of extramarital mating. It was difficult to find a documented example of this having happened. For that reason she is represented by a negative image at the top of this page.

    From a man's point of view, time invested in paternal care cannot be spent pursuing additional mates, and may be wasted if he is not the father of the child he is investing in. Consequently evolutionary psychology suggests that male have developed adaptions that overcome female unfaithfulness which will be discussed shortly.

    Evolutionary psychologists study jealousy because it may have evolved as a way of coping with a lack of commitment which has fitness costs for both parties in a sexual relationship. They are also interested in power and status behaviours in men because these may act as signals to females that the male has 'good-provider' attributes.

    [ A note about the pictures - I have chosen pictures of people who are well known in the UK to illustrate male 'good provider' and 'good gene' features, and female 'health, fertility and faithfulness' qualities. I suspect Richard Branson's business success may rest on the public's perception of his 'good provider' characteristics. Angela Lamb is the model used by Scottish Widows (who provide financial services such as pensions, mortgages etc.). Scottish Widows claim that she embodies "Scottish Widows' strength, reliability, integrity, innovation and heritage." I suspect that health, fertility and faithfulness are qualities we look for in institutions that we trust to look after our financial affairs as well as our long term mates!. I am assured that Robbie Williams represents 'good genes' by one lady I asked.]

The idea that there are fundamental differences between the reproductive strategies of males and females is an example of what evolutionary psychologists call universals . Universals are behaviour patterns that do not vary greatly between individuals.

But this view does not stand up to closer analysis. You have probably seen news reports of strangers who meet on aircraft and engage in sexual behaviour that lands them in court - (e.g The Independent, 6/04/2000, p 9). In the UK there have been several programs showing the behaviour of young people on holiday where there is clear evidence of women as well as men seeking sex with multiple partners.

This diagram shows overlap in the willingness of men and women to have sex with an attractive, healthy and emotionally undemanding stranger ( Symons & Ellis1989).

Similarly, Gangestad & Simpson (2000) re-examined Buss & Schmitts (1993) report on male and female desire for multiple sex partners discussed above . Clearly the sexes differ on this measure, but there is considerable overlap between the sexes - and variation within the sexes - on the desire to have sex with multiple partners. Thus some women wish to have multiple partners, and some men wish to have relatively few sexual partners. Clearly there are not universal differences between males and females in these behaviours. Why did this variability evolve? Does it make sense in terms of an individuals inclusive fitness? Gangestad & Simpson (2000) suggest that which strategy is adopted may be conditional on other factors in the person's environment.

For example, if a man was relatively unattractive to women, it would increase his inclusive fitness if he helped to raise his children rather than invest time and energy pursuing other reluctant mating opportunities. From a woman's point of view, it may be to her advantage to seek a succession of short-term mates so that her child benefits from their good genes, and a long term mate so that her child benefits from his parental investment. The best strategy thus becomes a matter of the individual carrying out a cost-benefit analysis on each of the sexual strategies at their disposal.

Detecting Desirable Partner Characteristics

Paternal investment involves giving time and physical resources to a child

Men and women may have evolved ways of detecting characteristics in each other that predict good short-term and long-term mates.

  • Short term mates are selected on the basis of physical characteristics
  • Long term mates are selected on the basis of psychological characteristics

Evolutionary psychology views this decision making process as being unconscious . Remember that one of the principles of evolutionary psychology is that complex behaviours that impact on inclusive fitness are unconscious.

Good short-term mates are men with signs of healthiness and the ability to withstand diseases that will be passed on to the woman's child. This is called good gene sexual selection (GGSS). What are the external signs of good-genes? Currently there is considerable interest in fluctuating asymmetry . This refers to the degree to which a person's features differ between each side of their body. For example, do you have problems finding shoes that fit because one foot is bigger than the other? Are your ears a different shape? These are all examples of asymmetries.

The characteristics valued in long-term mates may include honesty, reliability,loyalty - good parenting qualities. Evolutionary psychologists are interested in the psychology of cheating . Emotions may have evolved as part of our reaction to cheating.

Gangestad & Simpson (2000) suggest that women's decision to seek short or long term mates may depend on environmental circumstances at the time they mate. If the child is going to be born into a world dominated by death and disease it would make sense to mate with a man rich in good genes. On the other hand, if a woman is in an environment where she has difficulty obtaining resources she should favour mating with men of high status and power who exhibit good parenting skills.

Consequently, according to these authors, the environment controls what tactic women adopt - short or long term mating, and men in turn adjust their mating strategy to female demands.

Point to ponder: "I only want you for your sperm" How would you test the hypothesis that fluctuating asymmetry is associated with human mating behaviour?

Beauty & Symmetry

eye asymmetry in womanMen rate the following features as attractive in a woman:

  • symmetrical face and body. In this image there is a very slight size asymmetry between the woman's eyes, possibly due to ptosis - drooping of upper eyelid
  • full lips and small noses
  • a waist to hip ratio (WHR) of about 0.7 (the hour glass figure)

These features are associated with a strong immune system, high estrogen level, developmental stability and youthfulness i.e. they signal youth and high fertility.

When women are presented with diagrams of males with waist to hip ratios between 0.7 and 1.0, they preferred the figure with a WHR of 0.9 ( Singh , 1995)

You probably find the pictures on the left hand side of this display more attractive than those on the right. The male and female pictures on the left have been artificially constructed using a computer which is fed 32 human faces and averages them to produce an average face. One explanation for this preference is that the image exploits our preference for symmetrical faces. Symmetrical faces are though to signal ' healthiness ' to the viewer. It strikes me that one effect of the way these faces are constructed is to remove minor skin blemishes - the spots and wrinkles we all have. A clear complexion may be a contributory factor in attractiveness. How would you test this hypothesis?

The odd thing is that 6 month old babies also show this preference for averaged faces. They spent more time looking at the pictures on the left. Evolutionary psychologists use this type of evidence to argue for the presence of universal modules in the human brain that have evolved as part of our mate selection strategy. You can read about this research in Discover 21/2 February 2000 which is available online.

Preferred Partner Age

'Men grow cold as girls grow old'. Kenrick & Keefe ( Behavioural and Brain Sciences , 15, 75-133, 1992) provide evidence that is consistent with the idea that men are looking for youthfulness in their partners, whereas women seek mates who are older and presumably control greater resources. They examiner advertisements in a newspaper placed by men and women who were seeking romantic partners. The advertiser indicated their own age, and the age of the partner they were looking for. As men get older they are looking for younger and younger partners. Women of all ages, on the other hand, were prepared to accept replies from potentipartners 10 years their senior.

Gangestad & Simpson suggest that women have short-term and long-term mating strategies that are influenced by a male's genetic fitness and his willingness to help in child-rearing.

The Human Family: Marriage

Females are a rate-limiting resource which limits male reproductive success. Much human behaviour involves negotiating reproductive contracts between men and women. Parents can influence their reproductive success by facilitating the reproductive success of their own offspring. This is seen in societies in which parents ensure their sons reproductive success by buying them wives and leaving them them resources in the form of inheritances.

In some societies, the groom and his family transfer resources (labour or goods) to the bride's family. These resources are called bridewealth .

In others, the bride's family gives a dowry to the groom or his family.

Evolutionary psychology predicts that - because males compete for access to females - bridewealth payments should be more common than dowries.

This figure (adapted from Daly & Wilson (Homicide, New York, Aldine, 1988) is a cross-cultural comparison of economic arrangements surrounding marriage and shows that bridewealth is much commoner than dowry payment which is only practised in 22 societies.

Furthermore, bridewealth payment occurs in about 90% of societies where one man marries several women (polygynous societies). In nearly all polygynous societies, the system of inheritance of parents' wealth favours sons over daughters (Alcock, 1998 op. cit. ).

Alcock discuses societies in which parents favour their daughters over sons in terms of giving them dowries and inheritances. This seems to occur in situations where the dowry will secure their daughter a high-status husband who will use his resources to support their grandchildren. Sons are not favoured by these resource-poor parents because the son's reproductive success is less likely. Alcock argues that this flexibility is a reflection of our ability to select a strategy that will maximize fitness payoff within a given set of environmental circumstances.

The Human Family: Parental Care

Evolutionary psychology predicts that men should bias investment of time and other resources in favour of their own children. These diagrams (adapted from Daly & Wilson (Homicide, New York, Aldine, 1988) show that step children are at greater risk of child abuse and murder, than children living with both their natural parents. These tragedies have been interpreted as a maladaptive side effect of a universal psychological mechanism. You should note that the actual numbers of children murdered by step parents is extremely low. Evolutionary psychologists are not arguing that step parents are 'programmed' to kill step children. What they are saying is that we do have a bias to care for our own children, but that this mechanism can become distorted under certain circumstances.

When a baby is born, relatives and friends invariably comment on how the child resembles other members of the family - the 'he has his father's chin' syndrome. Christenfield and Hill (1995) report that strangers presented with a picture of a one year old and photographs of the child's mother, father and unrelated adults, spotted the child's father at above chance levels, but were unable to identify its mother. An evolutionary explanation of this result would be that a child's facial characteristics inherited from the father are preferentially expressed to reassure daddy that the child is his.

The Human Family: Ensuring Paternity

Source: Diamond, The Third Chimpanzee , Harper, New York, 1992

Source: Personality and Individual Differences , 28, 929-963, 2000

Except in cases of rape and forced marriages, women decide who fathers their children. In contrast, men have relatively little control over this important aspect of reproductive success. Human female fertility is a 'silent' event, so men can never be sure that they have fathered the children born to their partners.

In orang-utans and gorillas there is a marked difference in size between males and females. In these animals the male is much larger than the female. In contrast, there is very little sexual dimorphism in the sizes of chimpanzees and humans. Orang-utans and gorillas tend to form polygynous groups - one dominant male with a 'harem' of females that he mates with and protects from other males. Human males cannot rely on physical strength to prevent other males mating with their partners.

Consequently attempts to restrict access to a partner's sexual favours is a universal human male behaviour. Marriage, sexual jealousy, and violence by cuckolded males are viewed by evolutionary psychologists as examples of 'mate guarding' .

Mechanisms may have evolved in the human male that reduce the possibility that another man has fertilized their partner's eggs (coloured pink in the diagram). One of these mechanisms is sperm competition in which the man produces a superabundance of sperm (coloured blue) to reduce the chance that his partner is fertilized by the sperm from another man (coloured yellow) that she has copulated with in his absence.

An alternative explanation for this finding ( see Alcock, op. cit . p 628) would be that men who spend time away from their regular partners might have more contact with women who could be potential mating partners. Sperm production increases to cope with these opportunities. This could potentially increase the man's reproductive success.

The large size of the human testes may be adaptation to produce a large quantity of sperm to increase the chances that a man is the father of the children produced by his mate. The adaptive value of the relatively large human penis is unknown.

The Human Family: Divorce

These results can be interpreted in terms of evolutionary psychology.

  • Men are concerned about paternity and therefore will seek a divorce if their wife commits adultery.
  • Women will seek to end their marriage if their husband's cruelty signals that he is not acting as a good provider for his wife and children.

According to Buckle et al (1996), over 90% of divorces in England and Wales between 1974 and 1989 were initiated by women under 25 years old. The statistics show that divorces amongst young couples is more likely to be initiated by the wife, but as the couple gets older the husband becomes more likely to initiate divorce proceedings. The evolutionary explanation for this would be that a young woman who realizes that she has made a mistake needs to get out of the relationship as quickly as possible so that she can have children whilst she is still young enough with another partner. In contrast because men can continue to father children until they are at least 65, older men can divorce and raise a family with a new younger partner.

Source: Buckle et al , 1996

Why Men Rape

In an extremely controversial book Thornhill and Palmer have argued that rape can be explained in terms of evolutionary psychology. The most controversial aspect of their thesis is that men rape because of sexual desire, and not because they want to control and dominate women - the explanation put forward by the feminist writer Susan Brownmiller in 1975.
Thornhill & Palmer view rape as a way in which some men can enhance their inclusive fitness . They view womens' reaction to rape as a reaction to a threat to their inclusive fitness. They argue that

  • in contrast to murder, most rape victims are of childbearing age - see diagram opposite
  • married women and women of childbearing age suffer more distress after rape than unmarried, older or younger victims
  • rape victims suffer less emotional distress when they are subjected to more violence
  • in many cultures rape is considered as a crime against the victim's husband

    Note that this is an explanation of rape, not a justification for it.

    The authors present two hypotheses about the link between rape and human evolution:

    1. rape was favoured by natural selection because it increased mens' reproductive success
    2. rape is a by-product for obtaining multiple mates without commitment

    The diagram is redrawn from Thornhill &Thornhil l, Ethology and Sociobiology , 4, 137-173, 1983.

Altruism & Reciprocity

Professor Sir Magdi Yacoub -
the world renowned heart
surgeon surrounded by his team

Altruism poses a major problem for a theory of evolution based on natural selection. If individuals are in competition to survive, why has a system which benefits others survived the fires of natural selection? One explanation put forward by Trivers in 1971 was reciprocal altruism - 'You scratch my back and I'll scratch yours'. Another explanation was that altruism favoured the benefactor's inclusive fitness (Hamilton, 1964) . According to this view altruism could have evolved because the altruistic act allowed relatives who shared some of the benefactor's genes to survive, thus increasing the benefactor's inclusive fitness. These concepts have been useful in trying to understand the life of bees, wasps and ants. Most members of these species are sterile - they cannot reproduce sexually - their lives involve caring for a 'queen' who lays eggs within the colony, and her offspring. Wilson showed that this lifestyle made sense if we considered the genetic relationship between the queen and her sterile workers.

Nicky Hayes (1994, p22) provides a very clear description of how the genetic makeup of these insects supports the argument that a worker's inclusive fitness is enhanced by by helping her mother (the queen) to raise more offspring.

In humans kin selection is equivalent to nepotism . Mother Teresa and dedicated health workers like Magdi Yacoub are often quoted as examples that disprove explanations of altruism in terms of inclusive fitness. What do you think? Are they simply statistical aberrations?

Can evolutionary psychology explain charity contributions ?

'Mother Love'

I have put the phrase - Mother Love - in quotations to emphasise that the equivalent male phrase is 'A father's love for his child' rather than 'Father's love'. There is no special phrase to describe the love received from uncles, aunts and grandparents. Perhaps mother's love is special because only she is absolutely certain that a child will benefit her inclusive fitness.

The diagram shows the extent to which we are related to our close relatives in terms of a shared genetic inheritance. It has been suggested that altruism involves a cost-benefit tradeoff : We will help a distant relative if the cost to us is low, whereas we will endure greater cost for a close relative.

Contributions to charity have been explained in this way. For example, appeals for natural disasters in foreign countries often stress the great benefit to complete strangers if we make a relatively small contribution.

Many charities give small badges to people who make a contribution - this could be an example of capitalising on reciprocal atruism. People who are judged as being helpful may themselves benefit from being helped by other people. Of course the symbol may simply be a way of advertising yourself as a 'good-provider'

Desmond Morris (1977) explains war, patriotism and other extreme forms of self-sacrifice in terms of our evolutionary past in which we lived in tribes where everyone was genetically related to some extent.

'Culture' and Mate Selection

Up to now we have emphasized the role of stable , evolved, universal factors in controlling reproductive behaviours. But there is evidence that mate choice may be less stable than suggested so far.

Galef (1995) conduct an interesting experiment which appears to show social learning in a group of rats - what we would call the development of a ' culture '.

Rats were given two identical foods which only differed in flavour by the addition of either cayenne pepper or Japanese horseradish.

This was a complex experiment and I am only going to describe the results from one experimental treatment.

Rats were trained in groups of 4 to prefer horseradish by making them slightly ill after eating the food flavoured with cayenne pepper. The diagram shows a decline in the the amount of cayenne flavoured food eaten over the 14 days of the experiment.

No surprise there, except that the majority of rats in the experiment had never experienced illness after eating cayenne flavoured food. They had acquired the behaviour from rats 'in the know'.

If you examine the identification letters of the rats taking part in the experiment you will see that - starting on day 5 - the four original rats were replaced - one each day - by a new rat that had never been made ill after eating cayenne. By day 8 all the original rats had left the study , but their experience lived on. Indeed by day 11 there wasn't a single rat remaining in the study who had ever been in contact with a rat from the original group of 4 rats who had experienced illness after eating cayenne. Spooky!

One explanation is that rats eat what they smell on their friends. These rats smelt horseradish rather than cayenne on each other. Therefore the tendency to eat horseradish was passed on from rat to rat. Remember that rats - because of their scavenging way of life - need to be very careful about what they eat to avoid being poisoned by decomposed food. "If you can smell it on your friend, it must be OK, because it didn't kill him."

What has this got to do with mate choice? Westneat et al (2000) build on Galef's work and review research investigating the possibility that a female's choice of mate can influence the choices of those around her. It is extremely difficult to separate out competing hypotheses in this research area. For example, male behaviour can change after mating, and this may increase the attractiveness of the male to the female who has observed him mate. Similarly watching mating behaviour may affect the observer's willingness to mate.

One possibility in humans is that a preference for a particular type of mate is learnt as we grow up. Our parents, friends and relatives give use good - or bad - examples of parental behaviour . Thus we may favour relationships with - or without - the characteristics of adults that we have observed during childhood.

Young men seem quick to label some young girls as 'slags' or 'sluts' on the basis of their supposed promiscuity. From what we know about the effect of paternal uncertainty. Perhaps this aspect of 'male culture' can be understood in terms of males concerns over paternity mate choice


Adaptation:The gradual accumulation of inherited characteristics across generations, that give living organisms the best chances of surviving and reproducing in their environment (ie. characteristics that confer high inclusive fitness). The underlying mechanism of evolution by natural selection.

Altruism: disinterested and selfless concern for the well-being of others. Altruism has a fitness cost for the benefactor and a fitness benefit to the recipient.

Darwinism: theory of evolution driven by natural selection. Researchers who study Darwin's theories of evolution are often referred to as Darwinists.

Evolution: comes from the Latin word meaning the 'unrolling of a scroll'. Refers to the adaptive change that occurs over many generations. A change in the gene pool across generations. Scientists believe that over millions of years, simple organisms have developed into more complex ones, each better adapted to its environment that its predecessor. Charles Darwin proposed a means of evolution called natural selection, or the 'survival of the fittest'.

Evolutionary Psychology: a field of psychology concerned with the adaptive value of human behaviours. Uses knowledge and principles drawn from evolutionary biology.

Evolutionary Fitness: the ability of an organism to cope with environmental challenges, and to successfully hand on genes. Natural selection can also be said to occur because of difference in fitness within a population.

Fitness: often measured as the number of offspring produced by an individual that survive and reproduce themselves. Fitness reflects the genes contributed by an individual to their descendants. Humans effect their fitness through successful mating- called direct fitness , or by helping relatives (with whom they share genes) to reproduce - indirect fitness .

Inclusive fitness= direct fitness + indirect fitness

Group Selection: a theory put forward by Wynne-Edwards in 1962 that evolution by natural selection occurs on the scale of the group , as opposed to the individual. According to this theory, groups or species in which individual members are prepared to sacrifice their own welfare, and even life for the benefit of the group, are less likely to become extinct than a group in which individuals put their own interests first. This theory is no longer as popular as it was in the 1960's and 70's.

Manis used on this page to refer to male and female humans.

Monogamous: having only one sexual partner, or mate, at any one time.

Natural Selection: difference in reproductive capability. Thought to be the main process of evolution by natural selection, whereby organisms that are better adapted to their environment tend to survive and produce more offspring. The most common process of natural selection is to remove 'unfit' variants of an organism as they arise via mutation of genes across generations, producing new organisms with a different genetic makeup. First proposed by Charles Darwin, as outlined in his book, The Origin Species. Often referred to as 'survival of the fittest'.

Parental investment: in any species the parent (male or female) that invests the most time, energy, and resources on its offspring will be the choosier mate

Sexual Selection: sexual selection is natural selection operating on characteristics that contribute to an organism's mating success but not necessarily to their survival, eg. the male peacock's lurid tail. Can occur in two ways, namely: certain characteristics or traits may increase the ability to compete with individuals of the same sex for access to mates, while other traits increase the ability to attract individuals of the opposite sex.

Sociobiology: the scientific study of the biological (especially evolutionary and ecological) aspects of the social behaviour in animals and humans.

Seminar discussion topics

View the PBS video "Sweaty T-Shirts and Human Mate Choice" which is available online(if necessary, borrow headphones from the technicians office ) and consider the following points:

  • What is evolutionary psychology
  • What behaviors are studied by evolutionary psychologists?
  • Give several examples of human 'universals'.
  • What is a smell?
  • What is the effect of body odor on sexual attractiveness?
  • Design a follow-up experiment based on the study described in the video to investigate the role of odor in  sexual attractiveness.

Blair_Witch_Project_1.jpg (3207 bytes) Blair_witch_project_2.jpg (3281 bytes)It strikes me that some of the most interesting questions about human behaviour tend to involve things we take for granted. For example "Why are we afraid of the dark?" The 'common sense' answer is because we can't see well in the dark.  But actually it's quite rare to go into the countryside at night and not be able to see well enough to move about relatively easily.  Nevertheless going alone at night into the countryside can be a really scarry experience. The film "Blair Witch Project" may evoke memories of camping and being frozen with fear after listening to the rustling of leaves or animals in the night.

Here are some interesting questions posed by Gregory Carey (1998 chapter 15 ) which is available onlineabout human behaviours that we take for granted, or invite common sense answers:

  • Why are small children cute?
  • Why go through childbirth?
  • Why have sex?
  • Why have sex with other people?

Recommended reading:

  • Alcock , Animal Behaviour: An Evolutionary Approach , Sinauer, Sunderland, 1998. Alcock presents a balanced, interesting, and clearly presented discussion on current thinking about the evolution of human behaviour in chapter 16 of this book.
  • Cartwright, Evolution and Human Behaviour ,Macmillan Press, Basingstoke, 2000.
  • Daly &  Wilson, Human evolutionary psychology and animal behaviour, Animal Behaviour , 57, 509-519, 1999.
  • Hayes (Principles of Comparative Psychology, Lawrence Erlbaum, Hove (UK), 1994
  • Morris, Manwatching: A Field Guide to Human Behaviour, Triad Panther, St Albans, 1977
  • Mace,Evolutionary ecology of human life history, Animal Behaviour , 59, 1-10, 2000.

Online text resources:

  • Carey (1998) Human Genetics for the Social Sciences Rough Draft Chapters
  • Center for Evolutionary Psychologyat the University of California Santa Barbara
  • CogPrints electronic archive of papers in Evolutionary Psychology
  • Cosmides and Tooby Evolutionary Psychology: A Primer
  • Cosmides (2001) in an interview  was asked a series of questions including:
    • Can you explain what evolutionary psychology is, and how knowing that "our modern skulls house a stone age mind" can help us to understand modern humans?
    • Some experts criticize the evolutionary approach because it generates only "after the fact" ( or "just-so" ) explanations, and that for any trait you can always make up an evolutionary explanation. What is your response to this criticism? Can the evolutionary approach help to generate testable novel predictions about animal or human behavior?
    • Social scientists criticize the evolutionary approach to human behavior, because it is too closely tied to biology. That has led in the past, they say, to abhorrent political projects, including racism and extermination policies, and can also lead to genetic determinism, seen as reductionist and incompatible with a humanist view of humankind. What do you respond to that?
    • Evolutionary psychology challenges the Standard Social Science Model’s basic claim about human nature, namely, that the human mind is able to "learn" whatever external (cultural) patterns are presented to it. Could you explain in what sense postmodernism and structuralism (to name a few theories), would not be compatible with an evolutionary view of our behavior?
    • You have said that you foresee a time when the term "evolutionary psychology" will be abridged to plain "psychology", because there will be no differences between them. Can you elaborate on that and tell us what is next, in terms of challenges and practical implications, for evolutionary psychology?

Online video resources :

  • The Biology of Beauty
    "Anders Pape Møller and Randy Thornhill work respectively in Paris and Albuquerque, New Mexico. They are part of a new wave of scientists currently changing our ideas of beauty and where it comes from. Beauty is part of the animal world too, although no artist or designer has had any influence on it. It is beauty made through evolution. The provocative thesis is that that humans' conception of beauty is based in our biological nature. In short, when we are visiting an exhibition or watching a movie in the cinema, we are still just animals! "

    There is a very provocative section at the end of this video in which Moller & Thornhill give an evolutionary explanation for sex discrimination in academic jobs. You may want to consider their arguments in the light of Richard Dawkins lecture: " Our big brains can overcome our selfish genes " given at the Royal Institution, in London 12 February 2002

  • PBS (2001)  "Sweaty T-Shirts and Human Mate Choice"  video available online

Online audio resources :

  • Dr. Leda Cosmides, UCSB, Evolutionary Psychology Dr. John Tooby, UCSB, Anthropology Dept. Has Natural Selection Shaped How Humans Reason?
    "The study of the human mind has recently been moved into the natural sciences through biology, computer science, and allied disciplines, and the result has been the revelation of a wholly new and surprising picture of human nature. Instead of the human mind being a blank slate governed by a few general purpose principles of reasoning and learning, it is full of "reasoning instincts" and "innate knowledge" -- that is, it resembles a network of dedicated computers each specialized to solve a different type of problem, each running under its own richly coded, distinctly nonstandard logic. The programs that comprise the human mind (or brain) were selected for not because of their generality, but because of their specialized success in solving the actual array of problems that our ancestors faced during their evolution, such as navigating the social world, reasoning about macroscopic rigid objects as tools, "computing" or perceiving beauty, foraging, understanding the biological world, and so on. Much of this research has taken place at UCSB, and in the talk we will show how "logic probes" have been used to map these reasoning instincts. "
    The early part of this audio tape is very useful, but unfortunately the discussion of cheater detection on the Wason Selection task is unclear because the overheads shown to the audience are not provided.
  • The Descent of Man. A four part series on Australian radio including interviews with David Buss, William Hamilton, Sarah Hrdy, Stephen Pinker, Matt Ridley, and John Tooby.
  • Evolutionary psychology. Segment  from Talk of the Nation Thursday, March 04, 1999.
    "Throughout this century, advocates of women's liberation have confronted a persistent notion: that women occupy an inferior place in society simply as a result of their biology. In recent years, evolutionary psychologists claimed that women have evolved physically and psychologically to be weaker, less assertive, and monogamous, while men are naturally stronger, aggressive, and promiscuous. But recent historical and biological research has produced some evidence that may turn this conventional wisdom upside down.   Ray Suarez and guests for a look at evolutionary psychology, and the new findings that challenge its assumptions. "
  • Sarah Hrdy: "Mother Nature: A History of Mothers, Infants, and Natural Selection"
  • X ChromosomeSegment fromTalk of the Nation Friday, April 30, 1999.
    "Evolutionary psychology, trying to pinpoint the evolutionary roots of human behavior, typically says that women aren't promiscuous or aggressive, and they're programmed to find a mate who's a good provider. But does science agree with those conclusions?"

References :

  • Boaz N.T, and Almquist, A.J. Essentials of Biological Anthropology , Prentice-Hall, New Jersey, 1999.
  • Buckle et al, Ethology and Sociobiology , 17, 363-377, 1996
  • Buss and Barnes, Journal of Personality and Social Psychology , 50, 559-570, 1986.
  • Carey (1998) Chapter 15:Introduction to Evolutionary PsychologyIn Human Genetics for the Social Sciences Rough Draft Chapters
  • Christenfield and Hill , Nature , 378, 669, 1995.
  • Galef, BG, & C Allen, Animal Behaviour , 50, 705-717, 1995
  • Kenrick et al, Journal of Personality , 58, 97-116, 1990
  • Singh, Journal of Personality and Social Psychology , 69, 1089, 1995.
  • Symons & Ellis In Rasa, Vogel & Voland (Eds), The Sociobiology of Sexual and Reproductive Strategies , Chapman & Hall, London, 1989
  • Thompson, W.R., Proc. Ass. Res. nerv. ment. Dis . 33, 20-9-231, 1954
  • Westneat, D.F., et al., Animal Behaviour , 59, 467-476, 2000
  • Wilson, E.O, Sociobiology the New Synthesis, Cambridge, Mass..: Harvard University Press, 1975.