Relationships: Sexual Selection and Human Reproductive Behaviour

Evolutionary approaches explain human behaviour in terms of adaptiveness and reproductive success. These approaches argue that if a behavioural feature (for example, aggression) has been genetically inherited by one generation from another, then it must have a specific value for human species; it might either help humans adapt better to the environment and survive (natural selection) or might help to attract a mate and have healthy offspring (sexual selection).

Anisogamy

Part of the sexual selection explanation of human romantic relationships includes explaining differences in partner preferences between males and females. One explanation comes from the concept of anisogamy – differences between male and female sex cells.

Males’ sex cells (sperm) is produced in large quantities, quickly replenished and created continuously from puberty to old age. On the contrary, females’ sex cells (eggs or ova) take a lot of energy to produce, are created in limited numbers during specific time intervals and their production only lasts for a certain number of fertile years. These differences mean that males and females need to use different strategies to achieve reproductive success. Before the invention of DNA testing, males could never be sure that a particular child is theirs, so the reproductively successful strategy for a male would involve having sex with, and impregnating, as many women as possible. For women, however, the energetically expensive process of producing an egg and then carrying a child in the womb for nine months would mean that she needs a partner who will be committed to the relationship in the long run and provide resources for her and the child, ensuring the child’s survival.

These differences in mating strategies were demonstrated by David Buss (1989), who surveyed over 10,000 adults in 33 countries. Buss found that females universally put more importance on resource-related characteristics in a partner, such as ambition, high intelligence and good financial prospects. Males, however, preferred younger mates and put more value on signs of a female’s ability to reproduce, such as attractiveness and modesty.

Sexual Selection and Mate Choice

The principles of sexual selection described above mean that males and females use different strategies to select a suitable mate.

Since human females do not advertise their fertility openly, unlike some animal species (for example, redness and swelling of genitalia of female baboons), males have evolved to pay attention to other signs in a human female’s appearance that show her ability to produce healthy offspring. As was mentioned above, Buss (1989) has discovered that males universally put importance on attractive and healthy looks and youth, which are signs of fertility.

Further evidence comes from research carried out by Devindra Singh (1993, 2002) who studied preferred waist-to-hip ratio as a sign of female fertility. Studying the measurements of waist-to-hip ratio of the winners of the Miss America contest for a decade, she found that men generally found any waist and hip sizes attractive, as long as a ratio between them is about 0.7. A female having larger hips and a slim waist achieves this ratio, and men unconsciously interpret this as a sign that the woman is fertile but not currently pregnant.

Women, on the other hand, have adapted to look for the signs of male’s ability to provide resources and protect themselves and a child.

For example, Waynforth and Dunbar (1995) researched ‘lonely hearts’ columns in American newspapers, and discovered that women tended to describe themselves in terms of physical attractiveness and youth (‘exciting, flirty, curvy’). Men, on the other hand, advertised their resources and intelligence more than women did.

Intra-Sexual and Inter-Sexual Selection

Anisogamy can also explain the existence of two types of sexual selection: inter-sexual selection and intra-sexual selection.  Inter-sexual selection is sometimes referred to as ‘female choice', because it's based on the idea that due to the greater investment of time, energy and resources required from a female to raise a child, females need to be more careful when choosing a partner. They need to be sure that their partner will provide the right genetic fit and will be willing to provide resources to support the female and the child.

Female choosiness was illustrated by the study conducted by Clark and Hatfield (1989). They asked male and female student volunteers to approach opposite sex students individually on a university campus, asking the same question: ‘I’ve noticed you around the campus. I find you very attractive. Will you go to bed with me tonight?’  They found marked gender differences in the responses: 75% of male students agreed; however, not a single female said ‘yes’.

Intra-sexual selection, on the other hand, is a preferred male strategy. It refers to the evolutionarily developed features that allow a male to compete with other males for a female mate. The winner of this competition reproduces and passes on to his offspring the genes that contributed to his success. For example, a physically stronger and larger male will be able to fight off his competitors for access to the female, so he will produce physically stronger sons.

Intra-sexual selection also can explain the differences in the body size and physical appearance between males and females (this is known as physical dimorphism). As males need to compete with other males for an access to a fertile mate, sexual selection favours physically strong and aggressive males. However, females don’t need to physically compete for a mate, meaning that physical strength and aggression will hold no evolutionary advantage for them.

Extension: Paternal Certainty and Jealousy

As mentioned above, males have much less certainty than females that the child they are raising is theirs. According to Buss (1995), this fact can explain the difference in jealousy between males and females: males tend to be more jealous of their partner’s sexual infidelity, because this could result in raising someone else's child; females, on the other hand, are more jealous of their partner’s emotional infidelity, as this may result in withdrawing of resources from the female and the child and puts the child’s survival at risk.

This idea was supported by Buss et al. (1992): male students showed greater distress (measured by galvanic skin response) when asked to imagine partner’s sexual infidelity, while women were more distressed by thoughts of emotional infidelity.