Human Reproductive Behaviour

Annotations:

• Reproductive success is at the very heart of the evolutionary process. 
• Among early humans, those who failed to mate also failed to become ancestors.
• For our ancestors, successful mating was a complex business, involving selecting the right mate, out-competing rivals and then engaging in all the right behaviours for successful conception and child-rearing.
• It follows then, that modern-day humans have a similarly complex array of psychological adaptations specifically dedicated to the task of mating.

1. Darwin, 1871

   Annotations:

   • "Sexual selection depends not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for possession of the other sex."
2. The Nature of Sexual Selection

   Annotations:

   • An important feature of most sexually reproducing species is that males are more brightly coloured than their female counterparts (e.g. peacock's tail)
   • One would expect such disadvantageous traits not to be naturally selected - unless they enhanced reproductive success in some way.
   • To explain this, Darwin came up with his theory of sexual selection, describing two processes through which it took place.
   1. Intersexual selection (mate choice)

      Annotations:

      • This form of selection involves the preferences of one sex for members of the opposite sex who possess certain qualities.
      • For example, if females prefer tall males, over time there would be an increase in the number of tall males in the population.
      • The preferences of one sex, therefore, determine the areas in which the other sex must compete. This may be in terms of plumage (peacock) or economic resources (human).
      • These indicators reveal traits that could be passed on to offspring (i.e. selection for "good genes") as well as information about the chances of the mate being able to give protection and support to offspring (i.e. selection for "good parents").
      • Human beings are perceptually "pre-programmed" to attend to displays of these important indicators, which in turn increases their willingness to mate with the individual who possesses them.
   2. Darwin, 1874
   3. Intrasexual selection (mate competition)

      Annotations:

      • Members of one sex (usually males) compete with each other for access to members of the other sex.
      • The victors are able to mate and so pass on their genes, whereas the losers do not. Whatever traits lead to success in these same-sex contests will be passed on to the next generation.
3. Short-term mating preferences

   Annotations:

   • Human beings possess a menu of different mating strategies, some of which evolved specifically for short-term mating success.
   • According to Parental Investment theory, men evolved a greater desire for casual sex, and would ideally seek sex earlier in a relationship.
   • Female behaviour would not be subjected to the same evolutionary pressures.
   1. Buss, 2007

      Annotations:

      • Over the period of one year, for example, a male who managed to impregnate a large number of females would have passed on more copies of his genes than a less successful male.
      • On the other hand, a female who had sex with the same number of men in the same time period would only conceive once.
      • The less time a man permits to elapse before he has sexual intercourse with a woman, the larger the number of women he can impregnate in a given time.
   2. Buss and Schmitt, 1993

      Annotations:

      • In contrast to woman, men appear to lower their standards in the context of short-term mating opportunities and then show a marked decrease in attraction following sex.
      • This is an evolved adaptation to bring about a hasty departure which prevents them spending too long with one woman.
4. Long-term mating preferences

   Annotations:

   • In long-term mating, both sexes typically invest heavily in any offspring. As a consequence of this, sexual selection should favour high levels of choosiness in both sexes.
   • Poor long-term mate choice could be disastrous for both sexes because they would have wasted valuable resources.
   • As women have an obligatory biological investment in their children, they are predicted to be very particular about their choice of mate.
   • This means being attracted to males who are able to  physically protect and invest resources in her and her children, as well as show promise as a good parent and be sufficiently compatible to ensure minimal costs.
   • However, people do not give away their resources indiscriminately, therefore males would be most attracted to females who display signals of fertility, an indication of reproductive value.
   • Buss' research explored sex differences in long-term mate choice and found universal trends in male and female preferences.
   1. Buss, 2003
5. Sex differences

   Annotations:

   • Women more than men desired mates who were "good financial prospects". This translated into a desire for men with resources, or qualities that were linked to resource acquisition, such as ambition.
   • Men placed more importance on physical attractiveness. Research has consistently shown that physical appearance provides a wealth of cues to a woman's health and hence her fertility and reproductive value.
   • Men universally wanted mates who were younger than them - an indication that men valued increased fertility in potential mates.
   • Both sexes wanted mated who were intelligent (linked to skill at parenting), kind (linked to an interest in long-term relationship), and dependable (linked to willingness to help a mate in times of trouble).
   1. Buss, 1989

      Annotations:

      • Buss explored what males and females looked for in a  marriage partner. The study involved over 10,000 people from 37 different cultures, including a wide diversity of ethnic, religious, political and economic groups.
6. Logic?

   Annotations:

   • Why do mechanisms for mate choice evolve? Being choosy requires time and energy, and the costs of mate choice can even impair survival in some cases.
   • The rationale behind sexual selection is that random mating is essentially stupid mating. IT pays to be choosy, as the genetic quality of a mate will determine half the genetic quality of any offspring.
   • Low-quality mates (e.g. those who are unattractive and unhealthy) will be more likely to produce unattractive, unhealthy offspring. By joining forces with an attractive, high-quality mate, offspring are higher quality and an individual's genes are much more likely to be passed on.
7. Menstrual cycle

   Annotations:

   • Research from Penton-Voak et al. suggests that far from being constant, female mate choice varies across the menstrual cycle.
   • They found that women chose a slightly feminised version of a male face as "most attractive" for a long-term relationship. 
   • However, for a short-term sexual relationship, during the high conception risk phase of menstrual cycle, the preferred face shape was more masculinised. What does this tell us about mate choice?
   • Sexual selection may well have favoured females who pursue a mixed mating strategy under certain conditions. A female might choose a main partner whose feminised appearance suggests kindness and cooperation in paternal care, but might also copulate with a male with a more masculine appearance when conception is most likely.
   • Such males are likely to have higher levels of the sex hormone testosterone, which suppresses the immune system. A male who is healthy despite this must, therefore, have a highly efficient immune system - a very valuable characteristic to pass on to offspring.
   1. Penton-Voak et al, 1999
8. Male preference for younger women

   Annotations:

   • One of the most striking conclusions from Buss's study of 37 cultures was that men have a distinct preference for younger women, a finding consistent with the theory of sexual selection because the younger the woman, the greater the fertility.
   • However, some critics have tried to explain this preference in terms of social power - younger women are easier to control, and therefore are preferred as mates.
   1. Kenrick et al, 1996

      Annotations:

      • Kenrick effectively rejected the hypothesis for social power. 
      • They found that teenage males are most attracted to women who are five years older than them, despite the fact such women usually show no interest in them, and are certainly not more easily controlled by adolescent males.
9. Short term mating strategies

   Annotations:

   • Men and women experimenters approached total strangers on a college campus and said, "Hi, I've been noticing you around campus and I find you very attractive." 
   • Then they asked them one of three questions: 1) Would you go on a date with me? 2) Would you go back to my apartment with me? 3) Would you have sex with me? 
   • Of the females approached, 50% agreed to go on a date with the man, 6% to go back to his apartment, and 0% to have sex.
   • Of the males approached, 50% agreed to go on a date, 69% were willing to go back to her apartment, and 75% agreed to have sex.
   • These sex differences, which have been replicated in subsequent studies, provide compelling evidence that men have evolved psychological mechanisms to ensure success in short-term mating.
   • These include a desire for sexual variety, the tendency to let little time elapse before seeking sexual intercourse, and a willingness to consent to sex with strangers.
   1. Clarke and Hatfield, 1989
10. Importance of fertility

    Annotations:

    • We have seen how important fertility is in mate choice, but how does a male know when a female is "in oestrus" - i.e. sexually receptive?
    • In most mammalian species the female gives out a variety of visual or chemical signals, which encourage the male to mate.
    • In human females, however, oestrus is hidden, although some research suggests that women near the most fertile point of their menstrual cycle are more attractive to males.
    1. Miller et al, 2007

       Annotations:

       • A recent study in the US confirmed this. Researchers calculated the tips earned by lap dancers at varying stages of their menstrual cycle.
       • Those girls who were in the fertile oestrus phase of their cycle earned almost twice the value of tips compared to girls who were not in oestrus.
11. Mate choice in real life

    Annotations:

    • Studies such as Buss's survey of mate choice might suffer from a serious problem of validity - i.e. they give us an indication of expressed preferences rather than being a reflection of what actually happens in real life.
    • However, many real-life studies support these mate-choice hypotheses.
    1. Buss, 1989

       Annotations:

       • A study of actual marriages in 29 cultures confirmed that men do choose younger women. In fact, men who divorce and remarry tend to marry women who are increasingly younger than they are.
12. Gender bias

    Annotations:

    • Although research consistently reports that men more than women have a desire for a variety of sexual partners and a greater willingness for casual sex, men could never have evolved this desire in the absence of willing females.
    • Every time a man has sex with a willing new partner, the woman is also having sex with a new partner. Despite the fact that short-term mating carries a considerable potential cost to the woman, there must also be some benefits.
    1. Greiling and Buss, 2000

       Annotations:

       • Greiling and Buss suggest that she could profit in a number of ways, including using short-term mating as a way of leaving a poor quality relationship (mate switching) or as a way of producing more genetically diverse offspring.

Annotations:

• Parental investment (PI) is defined as "any investment by a parent in an offspring of that parent's ability to invest in any other offspring (alive or yet to be born)". Trivers, 1972.

1. Trivers, 1972

   Annotations:

   • At the heart of Trivers' theory is the fact that in most species males and females do not invest equally. 
   • Females' initial investment is far greater because female gametes (eggs) are less numerous and more costly to produce than male gametes (sperm).
   • A female can have only a limited number of offspring, whereas a male can (potentially) have a virtually unlimited number.
2. King Moulay Ismail the Bloodthirsty

   Annotations:

   • A medieval ruler of Morocco fathered over 800 children during his life, whereas the world record for a woman is 69 children, which included 16 sets of twins, 7 sets of triplets and 4 sets of quadruplets.
   • As a result of this biological inequity, females must be much choosier concerning potential mates. Males compete for quantity of females, whereas females select for quality of males and their resources.
3. Sex differences

   Annotations:

   • The most obvious sex difference in human parental investment is that human males can opt out of parental investment in a way that females cannot. 
   • By expending a relatively large part of their reproductive effort of courtship and mating, males of most species can afford to devote rather little, in comparison, to parental care.
   1. Why do females invest more?

      Annotations:

      • As brain size increased in response to adaptive pressures among our ancestors, this resulted in a more difficult childbirth because of the enlargement of the skull.
      • To compensate for this, childbirth in humans occurs earlier in development, meaning that human infants are born relatively immature compared to other animals.
      • In common with other mammals, human females breastfeed their young, and so are more burdened by the extended period of childcare that results from this prolonged immaturity.
      • Human mothers therefore not only make the greater prenatal contribution of resources (through pregnancy) but also make the larger postnatal contribution as well.
   2. Daly and Wilson, 1978
   3. Costs of maternal investment

      Annotations:

      • In human females, the costs of childcare are especially high. For our female ancestors, the minimum parental investment would have been a nine-month pregnancy followed by years of feeding and carrying.
      • The minimum paternal investment on the other hand would have been a few moments of copulation and a teaspoonful of semen. Symons, 1979.
      • The result is an enormous difference in the potential maximum reproductive success of the sexes, so making random mating all the more costly for human females.
      1. Symons, 1979
   4. Cuckoldry

      Annotations:

      • When males do invest parentally (e.g. through their resources) they are under pressure to protect themselves from the possibility of cuckoldry (i.e. investing in offspring that are not their own).
      1. Miller, 1998

         Annotations:

         • Because human males make a considerable investment in their children, they have a greater concern than females about the fidelity of their mates.
         • As a result, they try to ensure that their care is not misdirected towards non-relatives - e.g. through adultery laws that define the offence in terms of the woman's marital status rather than the man's.
   5. Sexual and emotional jealousy

      Annotations:

      • The possibility of sexual infidelity posed different adaptive problems for males and females.
      • A man whose mate was unfaithful risked investing in offspring that were not his own, whereas a woman whose mate was unfaithful risked the diversion of resources away from her and the family.
      1. Buss, 1995

         Annotations:

         • Sexual jealousy, therefore, may have evolved as a solution to these problems.
         • Men are more jealous of the sexual act (to avoid cuckoldry) while women are more jealous of the shift in emotional focus (and consequent loss of resources).
4. Maternal investment

   Annotations:

   • There are two consequences of the high costs of maternal investment. First, infant dependency means females want male providers.
   • Second, the expense of childrearing means that females want to ensure good quality offspring so they don't waste their efforts.
   • One way to ahcieve this is to marry a man who has god resources and is caring, but shop around for good genes through extramarital affairs with "studs" - attractive men advertising good genes but no resources.
   1. Baker and Bellis, 1990

      Annotations:

      • Although accurate data for mistaken paternity are notoriously elusive, there is some evidence from a magazine survey of over 2700 UK women.
      • From the results of this survey, Baker and Bellis estimated that as many as 14% of the population were products of extra-marital matings.
5. Sex differences in jealousy

   Annotations:

   • In line with PI theory predictions about sex differences in type of jealousy, Buss et al. found that male US students indicated more concern about sexual infidelity, whereas female students expressed more concern about emotional infidelity.
   • This was supported by physiological responses when respondents asked to imagine scenes of sexual or emotional infidelity - the men showed much more distress for sexual than emotional infidelity.
   1. Buss et al, 1992
6. Shared parental care

   Annotations:

   • In humans, joint parental care is desirable because of the high costs of successful reproduction.
   • In any situation where males can increase the success of childrearing, it will pay them to do so. This means that in humans, males do restrict their reproductive opportunities and invest more in each individual offspring.
   1. Dunbar, 1995
7. Sex differences
   1. Gove, 2008

      Annotations:

      • In 2008, Conservative MP Michael Gove claimed that "lads' mags" such as Zoo and Nuts reinforced a "shallow approach" to women, and linked them to a rise in feckless fatherhood and family breakdown.
      • Could this be the case, or might male attitudes to parenting actually be shaped more by biological forces (as predicted by PI theory)?
   2. Geher et al, 2007

      Annotations:

      • Geher et al. studied 91 non-parent heterosexual undergraduates. 
      • Each completed a parental investment perception scale, which included statement such as: "I believe that I am very prepared to raise a child at this time in my life."
      • They were additionally exposed to various parenting related scenarios such as, "You are the parent of a three-year-old girl who has an ear infection. Your plans for the day have completely changed, as you now have to look after her."
      • Although there were no sex differences in self-report responses to parenting on the parental investment perception scale, there were clear differences in ANS arousal to the different parenting scenarios.
      • Males showed significantly increased heart rate when presented with scenarios that emphasised the costs of parenting (e.g. that they would be unable to work).
      • Researchers concluded that, consistent with predictions from PI theory, males are biologically less prepared than females to confront issues associated with parenting.