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Sexual Selection

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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.

As women have fewer reproductive cells, they are more selective when it comes to selecting a sexual partner as they must choose someone who can nurture and protect them and their limited offspring.

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

Sexual Selection couple holding hands StudySmarterA 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?

Anisogamy refers to the fusion of different male and female reproductive cells (sperm and eggs) during reproduction.

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.

Sexual Selection sperm egg StudySmarterSperm 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.

Today women are not dependent on men for survival and men can tell if a child is theirs or not through DNA testing.

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.

Sexual Selection - Key takeaways

  • Some psychologists think that human reproductive behaviour and sexual selection is influenced by an evolutionary need for survival.

  • However, this need has changed nowadays. This means that it cannot account for all attractions.

  • Anisogamy refers to the fusion of different male and female cells (sperm and eggs) during reproduction.

  • Intra-sexual selection is most common for women and refers to selectively choosing a sexual partner.

  • Inter-sexual selection is most common for men and refers to attempts to be chosen as a sexual partner by the opposite sex.

  • Studies such as Buss and Waynforth and Dunbar's support this explanation of human reproductive behaviour.

Frequently Asked Questions about Sexual Selection

Sexual selection refers to an evolutionary explanation for partner preference, suggesting that attributes that suggest a better chance of survival (eg physical fitness) are passed on through genes, ensuring people pick partners with the best chance of survival for themselves and their offspring. This is part of human reproductive behaviour, which refers to how and why humans reproduce.

Sexual selection refers to an evolutionary explanation for partner preference, suggesting that attributes that suggest a better chance of survival so this affects how and who people seek in partners. 

Human reproductive behaviour refers to how people search for potential partners and engage in relationships. 

Sexual selection refers to an evolutionary explanation for partner preference, suggesting that attributes that suggest a better chance of survival

Intrasexual selection is when potential mates compete for a partner, so a good example is when animals display their colours or plumes to attract mates. 

Final Sexual Selection Quiz

Question

How can anisogamy explain human reproduction?

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Answer

Anisogamy refers to the fusion of different between male and female cells during reproduction. If women only produce a finite amount of eggs in a set period of time, they are more selective of their partners, whereas men's surplus of sperm and ability to reproduce over a long period of time means that they are less selective and aim to be chosen by females.

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Question

Evolutionarily speaking, which sex is most likely to engage in inter-sexual selection?

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Answer

Women. They have fewer reproductive cells (eggs) and it takes much more time and energy to produce offspring meaning they need to carefully choose their sexual partners.

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Question

What did Buss discover in his 1989 study? 


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Answer

He found that in all countries, females place 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 reflect a woman's ability to reproduce.

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Question

Evaluate evolutionary explanations of human reproduction. 


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Answer

There is research to support evolutionary explanations such as Waynforth's and Dunbar and Buss's studies. However, it remains the case that much has changed since humans evolved and humans do not need to rely on their sexual partners for survival, meaning the theory lacks temporal validity.

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Question

Give an example of an evolutionarily desirable characteristic for sexual selection?


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Answer

In men, strength and physical fitness are desirable as they indicate they can protect and provide for offspring.

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Question

 Why are evolutionary explanations of sexual selection disputed? 


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Answer

They are not applicable today. We no longer need evolutionary characteristics to survive. Eg women don't need men for resources

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Question

In Clark and Hatfield's study, how many men agreed to sex?


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Answer

75%

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In Clark and Hatfield's study, how many women agreed to sex?


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Answer

None

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Question

Which sex is more likely to engage in intra-sexual selection?


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Answer

Men

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Question

What is term is given to the fusion of different cells during reproduction?


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Answer

Anisogamy

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Question

What mode of sexual selection is also known as 'female choice'?


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Answer

Inter-sexual selection 

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Question

 Briefly describe Waynforth and Dunbar's study 


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Answer

Waynforth and Dunbar reviewed 'lonely hearts' columns in American newspapers and found that whilst women described themselves in terms of youth and physical attraction, men described themselves in terms of resources. This supports evolutionary theories of attraction which suggest that women seek partners who will support them, while men want to reproduce as much as possible.

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Question

Why would waist to hip ratio be a factor affecting sexual selection?


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Answer

It can be an indicator that a woman is not pregnant (if she has a small waist) meaning she could be a potential mate.

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What is an evolutionary desirable characteristic in men?


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Answer

Having resources

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Question

Who did Singh use in their study of waist to hip ratio?


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Answer

Miss America winners

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