The Relationship Between Sexual Selection and Human Reproductive Behaviou

The relationship between sexual selection and human reproductive behaviour

Sexual selection is a special type of natural selection that is concerned with an organisms ability to successfully reproduce. Survival is no guarantee of passing gene variants to the next generation, that can only happen if the animal reproduces.

Males usually compete to mate with females

According to Trivers (1972), "Where one sex invests considerably more than the other, members of the latter will compete among themselves to mate with members of the former."

In most species, females invest considerably more than males; however, in humans, there is a large variation in the amount that males invest. Some males invest a great deal, while others invest nothing.

Parental Investment and Sexual Selection

Females usually contribute more to the physical development their offspring. In humans a new born baby is hundreds of billions of times heavier than the fertilised ovum from which it develops. All of the additional weight is provided by the mother (Coen, 1999). Male humans provide only the sperm, which is the smallest cell in the human body.

However, to ensure that their progeny survive to reproductive age, humans also provide food, shelter, protection, education etc.

Nevertheless, the more time, effort and resources that parents invest in their children, the less they can invest in further reproduction. Therefore, there is a trade off between parental investment and reproduction; either many children and very little investment or fewer children with greater investment. Females do not really have a choice, however, because of their limited reproductive potential (see box 1). This means that females need to be more choosy in their mate selection than males. Nevertheless, when males adopt a long term mating strategy of forming a relationship and investing in few offspring, they also need to make wise mating decisions if their genes are to survive.

Sexual selection, therefore, often takes the form of female mate choice, whereby characteristics that females find attractive are those that are selected. Nevertheless, sexual selection in humans can also operate through male mate choice - males adopting a long term mating strategy may be choosy because they are limited in the number of offspring that the relationship can produce. Recent research by Finkel and Eastwick (2009) has supported the notion that males are choosier than females in certain social situations.

Evolutionary Psychology and Mate Choice

Genes can affect behaviour

According to evolutionary psychologists, the human brain has evolved "mental modules" to solve adaptive problems, such as avoiding predators, eating the right food and finding mates. These modules have evolved because the genes that produce them increased our ancestors chances of surviving and reproducing.

Mate Selection Modules

Sexual selection may be influenced by mental modules that influence the characteristics that are prefered in a mate; for example, females prefer symmetrical males with waist-hip ratios of 0.9 and immune systems that are different to their own, they also prefer males who have accumulated resources and are kind and generous.

Good Genes v Bad Genes

Good genes are those that increase the probability of survival and reproduction, while bad genes are those that reduce the probability of survival and reproduction. If a person mates with someone with good genes, then their children will have a good chance of surviving and passing on their genes again. If a person mates with someone with harmful genes, their children will be less likely to survive and pass on their genes. Evolutionary Psychologists believe that humans have evolved mental modules that enable us to identify potential mates with good genes and avoid those with harmful genes

How Mental Modules Identify Good Genes

Symmetry

Gangstead and Thornhill (1993) claim that both males and females prefer mates who are symmetrical. In their study, they measured the body parts of a number of people (length and breath of feet, hands, arms, ears etc.) and asked participants to rate these people for attractiveness. They found a significant correlation between attractiveness rating and overall symmetry. More symmetrical people were judged as more attractive.

Waist-hip ratio

Evolutionary psychologists also claim that males have solved the problem of identifying healthy, fertile females through an evolved preference for females with a waist-hip ratio of 0.7. The waist-hip ratio is calculated by dividing the narrowest measurement around the waist by widest measurement around the largest protrusion of the buttocks.

Devindra Singh (1993) found that although cultures vary in what they consider as the ideal weight for women, they are consistent in rating the 0.7 waist-hip ratio as the most attractive.

Waist-hip ratio is an accurate measure of the reproductive and hormonal status of women, with 0.7 being indicative of the most fertile. Obesity related diseases also vary with waist-hip ratio: higher WHR indicates increased risk of death in women independently of weight (Badcock, 2000).

Some studies have, however, found cultural differences in waist-hip ratio preference; for example Dixson, Dixson & Anderson (2007) found a preference for a WHR of 0.6 in China, and Marlowe & Wetsman (2001) have found preferences for WHRs of 0.8 or 0.9 in parts of South America and Africa. These studies, however, used WHRs measured from the front or rear rather than measurements taken from around the waist and hips. When similar studies have been carried out in these cultures using WHR measured around the waist and hips they have found the 0.7 ratios to be most attractive. This is probably because African women tend to store fat on their buttocks, while Chinese women store more fat on their hips. This means that narrower hips in African women and wider hips in Chinese women indicate the optimal 0.7 total WHR (Marlowe, Apicella & Reed, 2005).

The role of scent in identifying compatible genes

According to Wedekind et al. (1995) women are able to use their sense of smell to detect genetic differences in men. Female participants were asked to rate the attractiveness of T-shirts that had been worn by men for 48 hours. It was found that participants found the odours of t-shirts worn by men who had Major Histocompatibility Complexes (MHCs) that were different from their own. The Major Compatibility Complex is part of the genome that helps the immune system distinguish between itself and pathogens. Mating with someone with a different MHC to oneself will produce offspring with a more robust immune system because they would inherit a more diverse MHC.

Females who were taking oral contraceptives, however, showed a preference for MHCs that were similar to their own. Oral contraceptives mimic the hormonal conditions of pregnancy. and it is thought that during pregnancy females prefer to be with members of their own family who are likely to provide care and resources.

How Mental Modules Identify Males Willing to Invest

Human males, compared with other species, show a relatively high level of male parental investment. In most species of mammal, the males do not care for their young at all. One explanation of this is related to the size of the human brain relative to body size. Because humans have such large brains when they are born, they are relatively immature compared to other species. This means that human babies require a great parental care and, therefore, are more likely to survive and reproduce if they are cared for by two parents. Nevertheless, it may be in the interests of some, but not all men to use a short term mating strategy: invest less and reproduce more.

Mating Strategies

Mating strategies can be either short-term or long term. A Short-term strategy involves mating, but not developing a long term relationship, whereas a long-term strategy involves the formation of a lasting monogamous relationship. Because of the high reproductive potential of males compared to females, it makes sense for some males to pursue a short term mating strategy. However, there is a large variance in male mating success. Some males are able to attract a large number of females, but this means that there are less available for others. Therefore, some males are unsuccessful in finding a mate.

Some males, therefore, pursue a short term strategy, while others are more likely to have more reproductive success by forming one long-term relationship and investing more in a few offspring.

Females, on the other hand, have a limited reproductive potential and are more likely to benefit from a long term strategy of maintaining a relationship with a father who invests in the offspring than a short term strategy where the father does not invest. The problem for females is identifying males who are using a long-term mating strategy and would be a good father. Males using a long term strategy are going to have relatively few offspring and, therefore, also need to find a mate who will be a good parent.

Identifying Good Parents

Because having two parents is so crucial to the survival of an infant, natural selection should favour those who are able to identify a mate who will make a good parent. Evolutionary psychologists suggest that evolution has solved this problem by giving males and females preferences for partners with behavioural characteristics that demonstrate parenting ability. These characteristics include kindness, patience, generosity and trustworthiness. People who choose partners with these characteristics are more likely to look after their children and less likely to cheat in relationships. This is supported by evidence from a large cross cultural survey by David Buss (1989) [see below], which found that both males and females place equal value on partners with attractive personalities.

There is also research that shows that shows both male and female participants were able to predict the mating strategy of members of the opposite sex from facial features. Boothroyd et al. (2008) gave questionnaires to participants to measure the extent to which they were either ‘restricted’ or ‘unrestricted’ in their sexual behaviour and took photographs of these participants. She then showed the photographs to another group of participants and gave them a set of questions, such as:

• How many sexual partners would this person have in a given year? • How likely is this person to have a one night stand? • How likely is this person to fantasise about someone other than their current partner? • How likely is this person to think sex without love is okay?
The researchers then created facial composites from the participants who were most ‘unrestricted’ and those who were the most ‘restricted’. Another group of participants were shown the pairs of composites and asked to “choose the individual that you feel is more open to short-term relationships, one night stands and the idea of sex without love”.

The results showed that female participants were able to successfully distinguish between the restricted and unrestricted composites. Male participants were also able to identify the restricted and unrestricted females; however, they were less successful than the female participants. Females also found the ‘restricted’ composites to be more attractive, which suggests that they prefer facial features that suggest commitment to long term relationships.

This supports the idea that humans have an evolved ability to identify partners who are likely to increase the survival of offspring; however, the results could also be explained as the effects of learning: the participants may have learnt from experience that people with particular facial features are more likely to have liberal attitudes towards casual sex.

Men With Resources

Willingness to stay in a long term relationship to bring up children is not the only factor in male investment in offspring. The amount of resources that the male has to invest in offspring has also been shown to be important.

David Buss (1989) carried out a cross-cultural survey of 10,037 people from 33 countries. He found that women placed twice as much value on financial prospects in a potential partner than males did in a female partner. He also found that females rate wealth, industriousness and ambition more highly than males, while males rate youth and health as more important.

According to Buss (1994):

Because ancestral women faced the tremendous burden of internal fertilisation, a nine month gestation and lactation, they would have benefited tremendously by selecting mates who possessed resources.

Buss has also found that females generally prefer men who are older than themselves (3.32 years older, on average), while males prefer younger females (2.66 years younger, on average). This is consistent with the idea that females seek resources in a partner, as males generally get better at acquiring resources as they age.

The male preference for younger females can be explained by the fact that younger women tend to be more fertile than older women. This also explains why men are attracted to smooth skin, good muscle tone, shiny hair and full lips as these are indicators of youth and fertility.

Research into lonely hearts advertisements also supports the hypothesis that females have evolved to prefer males who possess resources. Dunbar (1995) carried out a content analysis of dating advertisements in the personal columns of newspapers. He found that females sought financial security more often than men, whereas men were more likely to offer financial security; men were more likely to seek physical beauty and women were more likely to advertise it.

Fidelity

According to Evolutionary Psychologists, men and women both tend to look for partners who are likely to be faithful to them; however, the risks of an unfaithful partner are different in each sex. If a man is unfaithful, then the risk is that his resources will be diverted to the other woman and her children.

If a woman is unfaithful, there is a risk of cuckoldry: that she may become pregnant and the male will unknowingly be investing his resources in offspring who do not possess his genes.

If evolutionary theory is correct it would be expected that jealousy would manifest itself in different ways in males and females: females should be more concerned with the male developing a relationship with another woman and males should be more concerned with the female copulating with another man.

Research has supported this evolutionary view. David Buss (1992) carried out a”self-report” study where male and female participants were asked to imagine a romantic relationship where their partner is either having sex with someone else, or falling in love with someone else” and say which of the two would cause them the greatest distress. Female participants reported they would be more upset by the emotional infidelity more often than male participants, while sexual infidelity tended to be more upsetting for males than females. Buss also used physiological measures as a more objective way of testing the level of jealousy in each scenario. As predicted, male participants had higher heart rates and galvanic skin responses when contemplating sexual infidelity, while emotional infidelity produced the largest physiological response in females.

Buss’ (1992) study can be criticised, however, as the participants were American college students. College students are usually young and have less experience of relationships than older people, so the sample may not adequately represent the larger population. Another problem with the study is that participants may have made inferences about what the infidelity means. Males may have infered, for example, that if a woman is having sex with someone, they are probably ‘in love’ since women are less likely to have casual sex; females on the other hand may have infered that if a man has fallen in love with someone they are probably also having sex. This means that it is difficult to draw firm conclusions from this research.