Evolutionary Explanations For Human Behaviour

Sexual Selection Reproductive success is the basis of the evolutionary process. Among early humans, and for us today, those who failed to mate, failed to pass on their genes. For those who did, the process involved selecting the right mate by out-competing rivals, mating at a time which would ensure a child was conceived and then behaving in a way that would ensure the child would survive long enough to pass on its genes too. It follows then, that we, as modern-day humans also have a repertoire of psychological behaviours which give the best possible chance for our genes to be passed on.

The Nature of Sexual Selection In most sexually reproducing species the males are much more brightly coloured than the females even when you would think that this was a disadvantage. (Think of the peacock’s tail and it’s pathetic flying ability!) Clearly then, these features enhance reproductive success. Charles Darwin (1874) came up with the theory of sexual selection and the two processes through which it took place.

Natural Selection V Sexual Selection Natural Selection – Darwin – Survival of the fittest – BUT – how does this explain why Peacocks have such large, bright plumage and tails? – Wouldn ’ t they be more noticeable and slower.

Intrasexual Selection (mate competition) Members of one sex (usually males) compete with each other for access to members of the opposite sex. The winners mate and so pass on their genes, whereas the losers do not. So whatever traits lead to successful mating in these contests will be passed on to the next generation.

Intersexual Selection (mate choice) This involves the preferences of one sex for certain traits in the opposite sex. For example, if females prefer tall males, over time there would be an increase in the number of tall males in the population. So therefore the preferences of one sex determine the areas in which the opposite sex must compete. It may be plumage in peacocks and economic resources in humans. So these indicators (be they plumage, tallness etc ) become signals to the opposite sex that the bird/animal/human has “good genes” likely a) to produce successful offspring b) to be able to give protection and support to offspring.

Commentary The logic of sexual selection – why have mechanisms for mate choice evolved? After all choosiness requires time and energy. It is because it pays to be choosy because the genetic quality of a mate will determine the genetic quality of the offspring. Remember, reproductive success is at the very heart of the evolutionary process – low quality mates (those who are unattractive and unhealthy will be more likely to produce unattractive, unhealthy offspring. By joining forces with an attractive, high-quality mate, the offspring are higher quality and so the individual’s genes are much more likely to be passed on.

Sexual selection & fashion Sexual attractiveness is determined by characteristics that suggest that the person (male or female) is a ‘safe genetic bet’ Fashion, in evolutionary terms, is a ‘con trick’ to suggest or exaggerate favourable characteristics

Sexual selection & fashion Bustle Dress ©2005 University of Utah

Sexual selection & fashion Zoot Suit ©2006 Suavecito Apparel Co.

Sexual selection & fashion Rara Skirt ©1984 Korova Records

Sexual selection & fashion Corset ©2006 The Charleston Museum

Sexual selection & fashion Cutaway Coat ©1995 BBC

Sexual selection & fashion Eyeliner ©2006 LeMor

Short-term Mating Preferences Men appear to have a number of evolutionary strategies with regard to sex, one of which has evolved for short-term mating success and is based on the Parental Investment Theory, which we come to in detail in the next chapter. The idea is that because men are competing for quantity of females in order to pass on more copies of their genes than a less successful rival, they would want to have sex earlier in a relationship. The less time a man waits before having sex, the larger the number of women he can impregnate in a given time. Women however, are not under this evolutionary pressure as no matter how many men they have sex with, there can only have one pregnancy at a time.

Issue Issue – Gender Bias in Sort-Term Mating ButBut – It takes two to tango! Every time a man has sex with a willing partner the woman is also having sex with a new partner so men’s greater willingness to have sex with multiple partners could not have evolved without willing females. Clearly there must be some benefits for women. Greiling and Buss 2000, suggest that women profit in many ways, including using short-term mating as a way of leaving poor-quality relationships or as a way of producing more genetically diverse offspring.

More on Short-term Mating Preferences According to Buss and Schmitt, 1993, men “lower their standards” regarding short-term mating opportunities and show a marked decrease in attraction following sex. The researchers argue that this is an evolved adaptation to bring about a hasty departure which prevents them spending to much time with a woman. HA!!!!!! “So little time, so many women to impregnate”(RN) And don’t’ quote me!!!!

Research – Short-Term Mating Strategies Clarke and Hatfield 1989 – Students on a college campus were approached by men and women experimenters who said:- “Hi, I’ve been noticing you around campus and I find you very attractive”. The students were then asked: 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? The subtle approach then!!

This is what happened Females – 50% agreed to go on a date 6% agreed to go back to his flat 0% agreed to have sex Males - 50% agreed to go on a date 69% agreed to go back to her flat 75% agreed to have sex These results gave been replicated in other studies and seem to provide compelling evidence that men have evolved psychological mechanisms to ensure success in short-term mating – these include a) a desire for sexual variety, b) the tendency to let little time elapse before seeking sex and c) a willingness to consent to sex with strangers.

Long-term Mating Preferences Since parental investment is high in long- term mating, sexual selection would predict that both men and women would be choosy. Poor long-term mate choice would mean wasted resources and opportunities.

According to Buss 2003 Woman would be attracted to males who According to Buss 2003 Woman would be attracted to males who:- a) are able to invest resources in her and the children b) are able to physically protect her and the children c) show signs that they’ll be a good parent d) are compatible enough to ensure minimal costs to her and the children. And what about the men? Well in order not to give away resources indiscriminately, men would be most attracted to females who display signs of fertility – an indication of their reproductive value. The following research by Buss found universal trends in men and women.

Research: Sex Differences in Human Mate Preferences. Buss 1989 Study involving 10,000 people from 37 widely diverse cultures. The following was found. Women desired men with financial ambition.(linked to the ability to support a family) Men were more concerned with physical attractiveness and mates who were younger than them. (Both potential cues of fertility and reproductive value) Both sexes desired intelligence, kindness and dependability – all of which are linked to the qualities needed for long-term investment in children.

Commentary: Male Preferences for Younger Women. BUTOne of the most striking conclusions of Buss’s study was that men have a distinct preference for younger women, which is a finding consistent with the theory of Sexual Selection because the younger the woman, the greater the fertility. BUT 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. However Kenrick et al 1996 found that teenage males were most attracted to women who were 5 years older than them despite the fact that these women showed no interest in them and were clearly not more easily controlled by adolescent males!

Research Methodology: Mate-Choice in Real Life Do studies such as Buss’s suffer from a lack of Validity? - i.e. they may indicate preferences but they’re not a reflection of what happens in real life. Apparently a study by Buss 1989 of actual marriages in 29 cultures backs it up. This research confirms that men do choose younger women. Not only that, but men who divorce and remarry tend to marry women who are increasingly younger than they are.

Research: The Importance of Fertility Miller et al 2007 Clearly fertility is very important in mate choice, but how does a male know when a female is “in oestrus” – fertile? Most female mammals give out visual or chemical signals which encourage the male to mate. However in human females oestrus is apparently hidden, but this study indicates that woman are most attractive to men at the most fertile point of their menstrual cycle. Researchers calculated the tips earned by lap dancers at varying stages of their cycle. Those who were in the most fertile part of their cycle earned almost twice the value of tips compared to girls who were not.

Commentary: Mate choice and the Menstrual Cycle Interestingly, according to research by Penton- Voak et al 1999, female choice of a mate varies according to her stage in her cycle. Woman chose a more feminine version of a male face as most attractive for a long-term relationship. But during the high conception stage of her cycle the preferred shape of face was more masculinised.

What does this tell us about mate choice? Sexual selection my well have favoured females who pursued a “mix mating” strategy. A female might choose a partner whose feminised appearance suggests kindness and cooperation for parental care – But she might also copulate with a male of more masculine appearance when conception is most likely because he probably has higher levels of the sex hormone testosterone. This dampens the immune system so if he’s well despite this, he must have a very robust immune system – a very valuable characteristic to pass on to offspring!

Homework Discuss the relationship between sexual selection and human reproductive behaviour 9 marks + 16 marks