Sexual Selection

We tend to think that we choose our partners freely, but could it be that thousands of years of evolution compel us to choose certain partners? Due to the fusion of male and female reproductive cells (or anisogamy), biology and evolution greatly influences human reproductive behaviour.

It was Darwin who made this suggestion. Let's delve deeper into why evolution might influence reproductive behaviour.

A couple holding hands, Pixabay

Anisogamy and sexual selection

What is anisogamy? What is its relationship with sexual selection and human reproductive behaviour? Let's find out.

What is anisogamy?

Scientists have used anisogamy to explain sexual selection and human reproductive behaviour on a biological level.

Whilst men produce large amounts of sperm from puberty to old age women only produce a finite amount of eggs for a finite amount of time. This means that from a biological perspective, women choose their sexual partners carefully but men don't need to be picky when it comes to choosing a mate as they have plenty of sperm to go around and having sex with lots of women increases their chance of reproducing.

How does anisogamy influence sexual selection and human reproductive behaviour?

For men, a successful reproductive strategy would be to impregnate as many women as possible or to have sex with the same woman as many times as possible to ensure offspring.

For women, the process of having a child takes a lot of time and energy, therefore they would be selective when choosing a mate as they need someone who will stick around during the pregnancy and care for her and the child after.

Studies supporting anisogamy as a factor affecting attraction

Let's now look at some studies supporting anisogamy as a factor affecting attraction.

Buss (1989)

In 1989, psychologist David Buss surveyed the sexual selection and reproductive behavior of over 10,000 adults' in 33 countries. He found that in all countries, females placed more importance on resource-related characteristics such as ambition, high intelligence, and good financial prospects. In contrast, men preferred younger partners and placed more value on attractiveness and modesty, values that reflected a woman's ability to reproduce. This shows the idea of anisogamy influences mate choice as men focus more on reproduction as they are able to reproduce more, whereas women seek to invest in a mate as they have a finite amount of time and reproductive cells to reproduce.

Sperm surrounding an egg, Flixr

Singh (1993, 2002)

In 1993 and 2002, Devindra Singh studied the preferred waist to hip ratio of men as a sign of female fertility. When studying the waist to hip ratio of Miss America winners over a decade, she found that men generally find a waist to hip ratio of 0.7 attractive (large hips and a slim waist). She argued that men unconsciously take this as a sign that a woman is fertile but not pregnant so attraction is motivated by the evolutionary desire to reproduce.

Waynforth and Dunbar (1995)

Waynforth and Dunbar reviewed 'lonely hearts' columns in American newspapers and found that whilst women described themselves in terms of physical attractiveness and youth, men advertised their resources and intelligence. Again, this reflects the idea that male and female sex cell production affects whom they choose as a mate. Women tend to have fewer resources, so try to invest in a solid partner with resources whereas men can reproduce much more frequently, so they aim for physical attractiveness.

Buss (1995) and Buss et al. (1992)

Buss investigated jealousy in males and females and found that women tended to be jealous of emotional intimacy where men feared sexual intimacy. Buss stated that this supports biological explanations of reproductive behaviour as men fear sexual intimacy more due to the possibility of a woman having another man's child. Women, on the other hand, fear emotional intimacy as this puts their security and resources at risk.

Inter-sexual and intra-sexual selection

Previously we have described how biological processes and differences can influence partner choice during reproduction through anisogamy. However, this can be further explained through inter-sexual and intra-sexual selection.

• Inter-sexual selection: sometimes referred to as 'female choice', inter-sexual selection refers to the way in which women choose male partners. Women selectively choose partners based on qualities that indicate they will provide for mother and baby. This is because having a child is a great investment of time and energy for women.

• Intra-sexual selection: this refers to the way in which men must have evolutionarily desirable characteristics, such as good health and resources in order to compete for the females' attention.

Intra-sexual selection may also explain physical differences between men and women. Men are generally stronger and bigger as they need to maintain these evolutionarily desirable characteristics to be selected by women (if men were bigger and stronger they were more likely to survive and provide). Women, on the other hand, choose mates and therefore do not need to have such traits to enable them to survive or reproduce.

Studies into inter-sexual and intra-sexual selection

Clark and Hatfield (1989) investigated differences between male and female reproductive behaviour on university campuses. They asked male and female students to approach the opposite sex and ask, 'I've noticed you around campus and find you very attractive, will you go to bed with me tonight?'. They found that whilst 75% of men agreed, no women said yes. This supports the idea that women are much more selective and engage in inter-sexual selection (carefully picking mates) whereas men are intra-sexual (picked by women).

Disadvantages of the evolutionary approach to inter-sexual selection

This theory fails to account for major social and cultural changes which may affect reproductive behaviour.

This means that women do not have to invest as deeply in finding the perfect mate and men do not have to spread their seed so widely to be sure they have a child. Therefore, nowadays mate choice is likely influenced by other factors than just evolutionary preferences.