salmonlogo.gif (14637 bytes)

Evolutionary Psychology: The 'Emperor Eugenics' in new clothes?
Author Paul Kenyon
Visit the SALMON Bookshop for recommended books on this topic


Evolutionary psychology views much of human behaviour as the product of evolutionary forces which shaped the survival and reproductive success of our ancestors. It challenges the dominant view within psychology that the human mind is blank at birth, and is filled as the result of experiences during the individual's lifetime.

Evolutionary psychology has been accused of resurrecting biological determinism; of supporting the view that our behaviour is written in our genes.

Biological determinism has a dark history - born as social darwinism and let loose on the world as eugenics which sought to increase desirable characteristics in the human population by selective breeding. Eugenic's methods included: cutting off your head or your balls!

This lecture will argue that there is a fundamental difference between the 'biological determinism' of eugenics and evolutionary psychology.

The lecture also introduces two apparent weaknesses in contemporary evolutionary psychology:

The lecture ends by suggesting that the application of Tinbergen's four questions about behaviour: 'How has it evolved?',' How does it develop?' 'What are its immediate causes?' and 'What is its function?', may rid evolutionary psychology of the charge of crude biological determinism.

The birth of evolutionary psychology

Jefferson's introduction  to the American Declaration of Independence begins with these famous words:

"A declaration by the Representatives of the UNITED STATES OF AMERICA, in General Congress assembled.

When in the course of human events it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth the separate and equal station to which the laws of nature and of nature's god entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain inalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness."
[emphasis added]

indepence-declaration-usa.jpg (88985 bytes)
First Page of Jefferson's Draft of the Declaration of Independence


Until relatively recently psychology was dominated by the Standard Social Science Model which echoes the belief that we are all equal at birth. According to this 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 father of behaviourism, John Watson, made this startling claim about human nature in 1924:

"give me a dozen healthy infants, well -formed, and my own specified world to bring them up in and I'll guarantee to take anyone at random and train them to become any type of specialist I might select - doctor, lawyer, artist, merchant-chief, and, yes, even beggar man and thief , regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors"

Watson was exaggerating and he knew it, because he added:
"I am going beyond my facts and I admit it, but so have the advocates of the contrary and they have been doing it for many thousands of years"

(see Boakes, 1984, pp226)

In his recent book "The Blank Slate: The Modern Denial of Human Nature", Steven Pinker (2002) points out the social impact of  the view that humans are born as blank slates:

"The Blank Slate has also served as a sacred scripture for political and ethical beliefs. According to the doctrine, any differences we see among races, ethnic groups, sexes, and individuals come not from differences in their innate constitution but from differences in their experiences. Change the experiences—by reforming parenting, education, the media, and social rewards— and you can change the person. Underachievement, poverty, and antisocial behavior can be ameliorated; indeed, it is irresponsible not to do so."

Steven Pinker discusses his book "The Blank Slate: The Modern Denial of Human Nature" in  this video (authors@MIT 2002)

In the 1960s and 1970s most psychologists came to accept that behaviour develops as an interaction between factors in an animal's environment, as well as biological predispositions.

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. The term sociobiology has now been replaced by the phrase 'evolutionary psychology'. A good place to begin reading about this new area is Cosmides and Tooby "Evolutionary Psychology: A Primer" which is available online.


According to evolutionary psychology, human behaviour has evolved, it is adapted to meet the survival and reproductive challenges faced by our ancestors, and it is controlled by modular structures whose operation is unconscious.

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
  • The statement
    "Modern skulls house a stone age mind"
    sums up a central idea in evolutionary psychology. Evolutionary psychologists believe that evolutionary processes 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.
  • The human brain consists of specialized neural circuits - modules - designed by evolution to unconsciously solve problems that our ancestors faced during our evolutionary history.
  • Most of what goes on in the mind is unconscious .

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'.

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

Evolutionary psychology is controversial

Evolutionary psychology is controversial; it confronts the standard social science model and it has generated socio-political debate in the wider community. Some of this criticism is well-founded, but some is based on misunderstanding.

Evolutionary psychology ...

  • not saying that all adaptive behaviour is present at birth. Teeth and breasts have evolved and are not present at birth, but nobody would argue that people learn to have these features. These features appear as a result of maturation at a time when the provide adaptive advantage. The same argument holds for the development of behaviours.
  • not saying that behaviour is genetically determined. The idea that behaviour is caused by genes without environmental influence is simply false. The environment plays a crucial role in influencing how we develop and react. All behavioural traits result from an interaction between genes and environment. For example, if IQ was genetically determined we would expect identical twins raised apart or together to show identical IQ. They do not. If IQ was environmentally determined we would expect a close correlation between the IQ of parents and their adopted children. We do not find this either. (See Alcock, 2001, p 57-59)

The benefits of evolutionary psychology

Let me put this crudely, starkly and provocatively - the scientific study of human behaviour is fragmented into several virtually independent subdisciplines. Some of these subdisciplines have ignored 150 years of scientific research which flowed from Darwin's theory of evolution.
  • Contemporary psychology is fragmented - evolutionary psychology offers unification
  • Contemporary psychology lacks an overarching theory - evolutionary psychology is based on Darwin's theory of evolution
  • Contemporary psychology is neither a science nor a branch of the arts - evolutionary psychology offers integration of psychology with the established life sciences which are all based on Darwin's theory of evolution

Why does contemporary psychology ignore Darwin? Because psychology got badly burnt when it flirted with social darwinism - a popular misrepresentation of Darwin's theory of evolution through natural selection - in the guise of eugenics. And as we shall see shortly some research into human behaviour is still exploited by modern advocates of eugenics.

Four evolutionary processes in a nutshell

Natural selection
  • Darwin was struck by the fact that all organisms produce considerably more offspring than are needed to replace the parents. For example, a pair of spawning salmon are estimated to produce 5,000 fertilized eggs, if all these eggs developed into adult salmon our rivers would be choked with salmon. Clearly there is massive mortality between egg and adult organism.
  • Despite this tendency to a progressive increase in numbers, the numbers of a given species remains more or less constant
  • From these two factors it follows that there is a 'struggle for existence' - some offspring survive, others perish
  • Offspring are not 'carbon-copies' clones of their parents; there is widespread heritable variation
  • Darwin suggested the novel idea that the fittest tend to survive, the unfit perish. He called this process natural selection
  • Consequently this leads to adaptive improvement over the generations - evolution

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.

In essence, sexual selection can operate through two mechanisms:

  • intersexual selection: competition between members of the opposite sex, e.g.
    • females and males choosing to mate with 'attractive' mates = 'selective mate choice'
    • 'male sexual proprietariness' over females to protect against undetected cuckoldry  
  • intrasexual selection: competition between members of the same sex e.g.
    • males competing with each other for access to female
    • females competing with each other for males

Parental investment

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.

Hamilton's kin selection theory

The existence of altruism posed a significant challenge to Darwin's theory of evolution through natural and sexual selection. For example:

  • Why do we co-operate with one another?
  • Why do humans and other animals make self-sacrifices that benefit other members of their species?

Hamilton proposed that genes can spread by benefiting other carriers of the same gene. This additional mechanism underlying evolution is called inclusive fitness or kin selection theory.


Abuses of evolutionary theory

Social Darwinism

The use of evolutionary principles to explain human behaviour - Social Darwinism - was championed by the British philosopher Herbert Spencer who coined the phrase 'survival of the fittest' which implies that only the ruthless will survive.

spencer.jpg (4246 bytes) gives this summary of his views:

".. progress in society was to be achieved by a 'genuine liberalism' which maximized individual liberty and minimized interference from the State; vaccination, and care for the infirm or insane, only served to promote the regression of the human race; economic and social differences between races, sexes or classes were part of the natural order, a necessary part of evolution."

(Boakes, 1984; emphasis added)

Eugenics and selective breeding

The term eugenics refers to a set of methods that are designed to increase desirable characteristics in the human population by selective breeding. For example, individuals with desirable characteristics are encouraged to have children, whilst those with undesirable characteristics are sterilized.

This form is an Order for Sexual Sterilization of an inmate in a state hospital in Virginia (USA).

If "the Board finds that the said inmate is [either] insane, idiotic, imbecile, feeble-minded [or] epileptic and by the laws of heredity is the probable potential parent of socially inadequate offsprings likewise afflicted; that the said inmate may be sexually sterilized without detriment to his/her general health, and that the welfare of the inmate and of society will be improved by such sterilization." [emphasis added]

The 'inmate' would be sterilized 30 days after the form was signed.

sterilization-form.jpg (75508 bytes)

Where did the terms 'idiotic' and 'imbecile' come from? - psychology!

Intelligence tests - a potential eugenic tool?

alfred-binet.gif (11635 bytes)As psychologists you will be familiar with the use of intelligence tests to reveal individual differences. IQ tests were invented by the French psychologist Alfred Binet in 1905 to select children for academic or vocational schools. There are echoes of this approach to education in the British system of grammar and secondary modern schools which have only relatively recently been replaced by comprehensive schools by most local educational authorities.

IQ scores range from 0 to 200 and fit a bell-shaped curve, with an average IQ of 100. Normal intelligence ranges from 86 to 115. At one time, terms like "moron," "imbecile," and "idiot" described persons with IQs below 86, while "bright" and "genius" were used for scores above 115.


binet-test-materials.jpg (95412 bytes)


You are probably aware of the controversy surrounding the British psychologist Burt's research into intelligence. Burt claimed that IQ was 80% inherited and 20% environmentally determined. This breakdown of IQ into a small contribution from nurture but a large contribution from nature could be used to justify the belief that intelligence was to a large extent fixed at birth, biologically determined and essentially immutable.

Subsequent analysis of his papers indicates that Burt fabricated some of his most important data (see Hearnshaw, 1979).

Eugenics and IQ

Eugenicists employed IQ and other tests to compare different racial and ethnic groups. In the 1920's some American psychologists (who were mostly WASPs - White Anglo Saxon Protestants) attempted to use the results of IQ tests to influence congress which was debating the introduction of legislation to restrict immigration from southern and eastern Europe. This attempt to use psychology to influence legislation was opposed by scientists who had suffered from racial discrimination on account of their 'non-Aryan' background. But the influence of social darwinism and eugenics persisted - and had horrific consequences - in Europe during the second world war when millions of people were murdered on account of their 'race', behaviour or beliefs.

Revulsion at the consequences of eugenics probably influenced psychologists to embrace behaviourism and the Standard Social Science Model. The message that 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 was consistent with the social and political climate in democracies after World War II.

But the view that aspects of human behaviour were genetically determined keeps resurfacing. In 1994 Herrnstein and Murray published the controversial (see Bouchard and Dorfman, 1995) book "The Bell Curve" which discussed the relationship between IQ and socioeconomic status. Herrnstein and Murray argue that:

In an interview Dr. Robert Gordon of Johns Hopkins University stated in response to the question "Do you think that people with low IQs should be paid to be sterilized?" replied:

"I think that's a rather shocking proposal! I would prefer not paying people. [Laughter] I do think that if we could persuade people with lower IQs not to have as many children as they are having, there would be obvious benefits. As far as black people are concerned, I don't know. If they care about the future of their race, they might be willing to go to some trouble to accomplish that. I think it's right to give them the proper information in order for them to do that."

(From: Papavasiliou (nd) The Racial Genetics of Intelligence: The Gadfly Interview with Dr. Robert Gordon. Available online )

Apart from the condescending, patronizing tone of the answer, Gordon gives a clear cut example of how genetics and psychological research can be misused to advocate a social policy based on eugenics.

Herrnstein and Murray met with a hail of criticism including the generally accepted views that

Nevertheless IQ and the social environment do interact in important ways. For example, in a review of Herrnstein and Murray  book "The Bell Curve" Bouchard commented:

"For separated, divorced, or never married White mothers with very low IQs, the probability of being in poverty is almost 70 percent. For the same group of mothers with very high IQs, the risk of poverty is about 10 percent. For married mothers, however, the range is from under 20 percent to near zero. IQ is influential, but marriage is clearly more important. Thus poverty among children is strongly associated with the marital status of their mothers." (Bouchard, 1995)

I think Bouchard may be implying that an apparently environmental factor - marriage - is more important than a genetic factor - IQ - in determining whether or not a child will be raised in poverty. But, as we shall see, evolutionary explanations of marriage (long and short term mating strategies) and child care (parental investment) lie at the heart of evolutionary psychology.

Eugenics and family life

Although evolutionary psychology has made a number of useful contributions to our understanding of reproductive and parental behaviours, this is potentially controversial territory. In the 1930s eugenics was also concerned with these topics as shown on the cover of this book which discusses:
"The privileges and duties of bringing children into the world, choosing a mate, the science of reproduction, pure sex-love, how to have perfect children, womanhood, prenatal influence, the effects of environment on children, the gift of motherhood, how the human body can be made immune to disease, physical and mental training of children, the power of the mind, the successful mother."

For this reason evolutionary psychologists enter these waters with extreme caution.

eugenics1.jpg (67434 bytes)


Is evolutionary psychology eugenics in new clothes?

Evolutionary psychology may have attracted criticism and suspicion from some psychologists because it appears to resurrect old issues to do with eugenics and genetic determinism in a new suit of clothes. However eugenics is concerned about the differences between people and about placing value judements on those differences. Evolutionary psychology is concerned with universals - behaviours that are common to all humans across all cultures.

Biology has taken massive strides during the last 50 years:

The Human Genome Project

Chromosomes are found in the nuclei of most cells in the body (see Patterson,1999). Chromosomes are made of DNA (deoxyribonucleic acid). DNA consists of four different bases: adenine, cytosine, guanine and thymine. In 1953 Watson and Crick worked out the structure of DNA - the double-helix. DNA is like a spiral staircase. The side pieces consist of long chains of sugar and phosphate. The 'treads' of the staircase are made of two interlocking bases. But there are only two types of tread because the four bases can only pair in two ways:
  • adenine can only bond to thymine
  • guanine can only bond to cytosine

This staircase of interlocking bases is called the 'genetic code' or genome. Humans have 24 distinct chromosomes (separate staircases). Each chromosome consists of 50 million to 250 million base pairs. Sections of chromosome are called genes. Genes make up only 2% of the human genome. Humans contain 30 to 100 thousand genes (Kreek in Yudell and DeSalle, 2002, p97). Each cell of the body (apart from red blood cells) contains a complete copy of the genome.

The human genome project has unravelled the staircase of interlocking bases in order to read the genetic code (See Yudell and DeSalle 2002).
  • 99% of the DNA of chimpanzees and humans is identical (Hood in Yudell and DeSalle, 2002, p 71)
  • 99.9% of the DNA of any two randomly selected humans is identical.
  • Thus only 0.1% of the DNA of any two randomly selected individuals is different

Humans and other mammals have approximately 3 billion base pairs in their DNA.

There are 3 million base pair differences in the DNA of any two randomly selected individuals.

To put this in perspective a bacteria or virus has only 3 million base pairs.

  • There are 1.5 million base pair differences between the DNA of a mother and her child
  • There are 2.25 million base pair differences between the DNA of a grandmother and her grandchild
  • There are 2.625 million base pair differences between the DNA of biological first cousins (i.e. on the child's mother's side of the family)
  • There are 2.906 million base pair differences between the DNA of biological second cousins (i.e. on the child's mother's side of the family)
  • There are 3 million base pair differences in the DNA of any two randomly selected individuals

Genes control the production of proteins by cells in the body. Proteins are large complex molecules made up of chains of amino acids. Within a gene each specific sequence of three bases controls the addition of one amino acid to the growing protein chain.

By clicking on these links you can view slides in the display area which show:
  • the relationship between cells, chromosomes, DNA, genes and proteins
  • the role of genes in protein synthesis
  • how variation in base pairs may or may not produce changes in protein synthesis
  • how variation in base pairs may or may not produce changes in protein synthesis leading to disease
  • how abnormalities in one representative chromosome can lead to a wide range of illnesses
genes-to-humans-reduced.jpg (65946 bytes)
Images created by the U.S. Department of Energy Human Genome Program's Human Genome Management Information System (HGMIS)
presented here by courtesy of  the U.S. Department of Energy Human Genome Program or U.S. Department of Energy Genomes to Life Program
see website at

Genotype and phenotype

The genotype describes the genes inherited by an organism. Phenotype refers to an individual's anatomical structure, physiology and behaviour. The phenotype refers to everything that can be easily observed and measured about an plant, animal or human being.

Our phenotype is the product of

I deliberately used the word 'our' in the phrase "Our phenotype is the product of " because as psychologists we are interested in human behaviour. But debates about human phenotype are based on the science of genetics. A quick glance through a genetics textbook (e.g. Suzuki et al, 1981) reveals that genetics is founded on the study of plants and simpler animals such as the fruit fly Drosophila. The next section gives a flavour of these studies (see Lewontin (2000) Chapter 1 ).

It is relatively easy to study how the same genotype reacts to different environments in plants.

Some species of plant can be cut into several pieces that will grow if they are put into soil. In this experiment (Clausen et al. 1958 see Suzuki et al, 1981 p18) seven Achillea plants were each cut into three separate sections.

The three plants in each column have identical genotypes. Each parent plant (#4.....#16) has a unique genotype.

Consider the plants that resulted from parent plant #4. The clones grew well at low and high altitude, but at medium elevation this genotype failed to thrive.

Maybe the conditions are not suitable for growing Achillea at 1400 meters. But this explanation is not likely because the cutting from parent #24 thrived at this level compared to the performance of this genotype at low and high altitude.

What does this experiment tell us about genotype. Quite clearly the phenotype of each cloned plant depends on the environment in which it grows.

Also it is impossible to make any firm conclusions about altitude as an environmental factor in this study. For example, some of the plants grew well at the intermediate site (#23,24) where others fared less well (#4,16).

The norm of reaction

This graph is based on the data presented in the picture above. Individual lines on the graph shows how the three cuttings taken from one plant grew at various altitudes. Each line is called the 'norm of reaction' .The term 'norm of reaction' refers to how a particular genotype (from a parent plant) develops into a phenotype (cloned offspring) as a function of the environment in which the clone develops.

It is immediately obvious that identical genetic material produces different phenotypes.

This result is not peculiar to plants. Similar results have been seen in the fruit fly (see Lewontin, 2000)

Suzuki et al (1981) draw the following conclusion from this type of experiment:

Can you identify examples on the graph that illustrate these point?


What is the consequence of 'norms of reaction' for human behaviour? Imagine that a political proposed a perfectly plausible proposal to modify the environment to improve childrens' intelligence. Would it work? Well it might, but again it might not if intelligence is the product of each child's genes and environment.

I have used the norms of reaction from the Achillea experiment in this diagram.

The child with a genotype represented by the red line would benefit from the 'enriched' and 'hyper-enriched' environments. But the other child would suffer a progressive loss in IQ score if raised outside the normal environment.

The point of this example is not to suggest that we should abandon attempts to improve our childrens' environment, but to suggest that we need to be aware that outcomes may not always be as expected.

Incidentally the phenotypes in his hypothetical example have been given very high IQs in order to turn-up the 'ethical screws'.

The evidence presented thus far suggests that the way we are and what we do, our physical appearance and behaviour - our phenotype - is the result of a complex interaction between our genome, our environment and 'je ne sais quoi' - random factors. It is thankfully uncontrollable and unpredictable. But there may be things about ourselves that are a result of where we came from - our evolutionary past.

The application of evolutionary concepts to human behaviour

Hamilton's kin selection theory

We have seen that kin share many common genes. Genes can spread by benefiting other carriers of the same gene. Hamilton proposed the inclusive fitness or kin selection theory to explain altruism or self-sacrifice.

In an altruistic encounter there is:

The probability that the altruist and the recipient share a gene is called the coefficient of relatedness ( r ). The diagram shows the extent to which we share genes with our relatives. The value of r varies between 0 and 1. On average we share half of our genes with our brothers, sisters and children ( r=0.5 ), and a quarter of our genes are identical with those of our grandchildren, nephews and nieces ( r=0.25 )

According to Hamilton's Rule altruism pays off if rb>c . In other words, shared genes will profit if the cost to the altruist is less than the benefit to the recipient multiplied by the probability that the recipient shares genes with the donor.

Costs and benefits are expressed in units of fitness or reproductive success with values between 0 and 1.

For the sake of argument assume you have spare food that you could give to your brother to feed him and his children.

We can test if your altruism would benefit kin selection by putting these values into Hamilton's Rule rb>c where:

You might wonder why b and c are not always equal. Why not use the spare food you have to increase your own reproductive success? Well there is a limit to how much you can eat. If you have an abundance of food and your brother is starving, the cost to you of sharing is small, but it may be a matter of life or death to your brother and his children.

Kin selection theory in action

You may not have a twin (r = 1.0), but you probably feel more related to
  • your mother, father, brother(s) and sister(s) (r = 0.5) than to
  • your grandmothers, grandfathers, niece(s) and nephew(s) (r = 0.25) or
  • your cousin(s) (r = 0.125) or
  • non-relatives (r = 0.00)

Redrawn from Study 1: Burnstein, et al (1994).

Burnstein et al (1994) and Petrinovich et al (1993) asked people to imagine how they would react in life-and-death situations in which they could prevent the death of a relative.

For example imagine that a train is running out of control down a track that branches. You are in charge of the points at a track junction. If the train goes down the left track it will kill one of your relatives. If you send the train down the other track it will kill two strangers. The choice is yours!

The results reflect what you may have predicted; we are more likely to help a near relative (e.g. brother or sister (r=0.50) than a stranger (r=0.00) in a life-and-death situation.

"The cricket field problem"

Individuals can ensure the survival of a proportion of their genes by reproducing and helping their relatives to reproduce.


belding-squirrels-juvenile.jpg (14191 bytes) Mateo (2002) studied the ability of Belding's ground squirrels to discriminate between close relatives on the basis of odours produced by facial scent glands. Ground squirrels live in burrows. Mateo placed odour-impregnated cubes at burrow entrances. She measured the amount of time emerging squirrel spent sniffing these cubes. Her results show that a Belding's ground squirrel is able to recognize as kin a relative it has never encountered before.

This discrimination is exquisitely sensitive; squirrels are able to distinguish between relatives where the difference in coefficients of relatedness is very small [ e.g. 'r' =0.06; Mateo, 2002, Figure 2b]. See Segelken (2002) for Cornell's press release on this research.

Belding's ground squirrel mother and offspring picture credit: J. Mateo/Cornell University

I have a problem with the application of kin selection to explain human behaviour. Compared to animals (see Pfennig, 2002 ), humans seem to be very poor at 'kin recognition'. The human language contains surprisingly few words to describe people that are genetically related to us. For example:

I call this the 'cricket field' problem because our language is so rich when it comes to describing the location of people on a cricket field. apparently there are at least 23 named positions that a fielder can occupy during a cricket match.

We have no 'innate / unconscious' way of recognizing relatives. For example, at family gatherings I have to be introduced to distant relatives.

s_mat5.gif (15318 bytes)

But humans do have the potential to recognize each other through chemical processes. One of the most surprising abilities of new mothers is their ability to recognize their child on the basis of smell. This diagram shows the percentage of women who successfully recognized the odour of their baby as a function of how long they had been exposed to their child (Kaitz,1987).

The video "That's My Baby" shows that soon after birth, mothers can recognize their baby by smell and touch but not sight

Parental investment

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.

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."

"Barkus is willing, but Peggoty is shy" with apologies to Charles Dickens author of 'David Copperfield'

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:
rasputin1.gif (11044 bytes)
  • 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.

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 over their lifetime. This opens up the possibility that a male may be cuckolded and consequently waste his parental investment.

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 women. Buss (1999) distinguishes between different types of explanations for female short-term mating that have some experimental support in the human and animal literature:

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).
Boleyn-Mary.jpg (3146 bytes) The historical novel 'The Other Boleyn Girl' by  Philippa Gregory -  the subject of a recent BBC TV programme - deals with the relationship between Mary Boleyn and Henry VIII . At a time when Henry was married to Catherine of Aragon and Mary Boleyn was married to William Carey, her father and brother (Thomas and George Boleyn) persuaded Mary, against her wishes, to enter a sexual relationship with the king - Henry VIII - to secure the Boleyn family's position at court. After the birth of a son, Henry broke off his relationship with Mary, established a new relationship with her sister Anne, divorced his wife Catherine and married Anne Boleyn. Anne failed to produce a male heir and was executed on charges of participating in extra-marital sexual relationships. Henry VIII married Jane Seymour and Mary Boleyn lived in relative obscurity with William Stafford, whom she married after the death of her first husband. HenryVIII.jpg (5737 bytes)

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),
    • 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.
Queen_Victoria__Albert_1854.jpg (7838 bytes)

Four ways of looking at behaviour

One way of thinking about behavioural questions is to consider how they relate to stages on a species' journey through time, from the distant past into the future.

  • The evolution question: How has a behaviour evolved in the species? Is it shown by closely related species?
  • The development question: How does a behaviour develop during the individual's lifetime? Does the individual learn the behaviour? Does s(he) practice the behaviour? Is the behaviour performed perfectly the first time it is performed?
  • The cause question: What causes the particular behaviour? Are there internal factors (e.g. hormones) that influence the behaviour?
  • The function question: Why is the behaviour performed in a particular way? How do the behaviour contribute to the reproductive success of the individual, and ultimately the survival of the species?

Online resources

Webcasts --Online Audio and Video Files
about Genetics and the Human Genome Project


Copyright Dr. C.A.P. Kenyon 1994-2006