Evolutionary Explanations for Partner Preferences (AQA A-Level Psychology): Revision Notes
Sexual Selection
Sexual selection provides an evolutionary explanation for human partner preferences. Charles Darwin observed that evolution favours the development of characteristics that are attractive to the opposite sex, even if these traits might seem disadvantageous in other ways. These attractive features increase reproductive fitness by improving an individual's chances of finding a mate and passing on their genes.
Key definitions
Sexual selection is an evolutionary explanation of partner preference. Attributes or behaviours that increase reproductive success are passed on and may become exaggerated over successive generations of offspring.
Human reproductive behaviour refers to any behaviours that relate to opportunities to reproduce and increase the survival chances of our genes. This includes the evolutionary mechanisms underlying our partner preferences, such as mate choice and mate competition.
Anisogamy and different mating strategies
Anisogamy is the fundamental concept underlying all sexual selection theory. It refers to the differences between male and female sex cells (gametes) and explains why males and females have evolved different mating strategies.
These differences are striking in humans and most other animals. Male gametes (sperm) are extremely small, highly mobile, produced continuously in vast numbers from puberty to old age, and require little energy to produce. In contrast, female gametes (ova) are large, static, produced at intervals for a limited number of fertile years, and require enormous energy investment.
This fundamental difference has important consequences for mating behaviour. There is no shortage of fertile males, but fertile females represent a rare 'resource'. Anisogamy gives rise to two different types of sexual selection, leading to distinct male and female mating strategies.
Inter-sexual selection
Inter-sexual selection occurs between the sexes and represents the preferred strategy of females - quality over quantity. Since ova are rarer than sperm and require greater energy investment, females must be particularly choosy about their mate selection. As Trivers (1972) emphasised, females make a greater investment of time, commitment and resources before, during and after birth.
Both sexes risk losing out if they invest resources in poor-quality partners, but the consequences of making wrong choices are more serious for females than males. This leads females to adopt an optimal mating strategy of selecting genetically fit partners who can provide resources.
Female preferences for attractive male characteristics determine which features are passed on to offspring. This creates evolutionary pressure that shapes male traits over generations.
For example, if height is considered attractive, successful generations of females will mate with tall males and produce taller sons over time, with daughters developing greater preferences for tall partners. This creates a runaway process, as described by Ronald Fisher (1930) in his sexy sons hypothesis. The idea is that females mate with males who possess desirable characteristics, increasing the likelihood that their male offspring will inherit these 'sexy' traits.
Intra-sexual selection
Intra-sexual selection represents the preferred strategy of males - quantity over quality. This refers to competition between males to gain access to females for reproduction. The winner of this competition gets to mate and pass on the characteristics that contributed to his success.
Dimorphism (physical differences between males and females) has evolved as a consequence of this male competition. In humans, males are typically larger than females because size provides advantages in physical competition between males. Females do not compete for reproductive rights in the same way, so there is no evolutionary pressure favouring larger body size in women.
Intra-sexual selection creates both physical and psychological differences between males and females. These differences reflect the optimal reproductive strategies for each sex.
Intra-sexual selection also creates behavioural and psychological differences. Males may benefit from behaving aggressively to acquire fertile females and protect them from competing males. Since males can produce vast numbers of sperm with minimal energy cost, their optimal reproductive strategy involves mating with as many fertile females as possible. This explains male preferences for youth and fertility indicators, as well as reduced post-reproductive investment compared to females.
Research evidence
Support for anisogamy-based preferences
Research Study: Buss (1989) Cross-Cultural Partner Preferences
Method: Comprehensive survey of over 10,000 adults across 33 countries, examining age preferences and attributes important in partner selection.
Key Findings:
- Female participants placed greater value on resource-related characteristics such as good financial prospects, ambition and industriousness
- Males valued reproductive capacity indicators like good looks and chastity
- Males showed stronger preferences for younger mates than females did
Conclusion: Results support predictions derived from sexual selection theory and apply across different cultures, suggesting universal human preferences.
These findings reflect sex differences in mating strategies resulting from anisogamy. The results support predictions derived from sexual selection theory and can be applied across vastly different cultures, suggesting universal human preferences that are not primarily dependent on cultural influences.
Support for inter-sexual selection
Research Study: Clark & Hatfield (1989) Campus Approach Study
Method: Male and female psychology students approached other students individually with the proposition: "I have been noticing you around campus. I find you to be very attractive. Would you go to bed with me tonight?"
Key Findings:
- Not a single female student agreed to the request
- 75% of males immediately agreed
Conclusion: Demonstrates that females are considerably more selective than males when choosing sexual partners.
This supports evolutionary theory by demonstrating that females are considerably more selective than males when choosing sexual partners, while males have evolved different strategies to ensure reproductive success.
Support from waist-hip ratio research
Research Study: Singh (1993, 2002) Waist-Hip Ratio Preferences
Method: Studied male preferences for female body shapes that signal fertility across different body sizes.
Key Findings:
- Males show consistent preferences for a waist-hip ratio of approximately 0.7
- This preference exists regardless of overall body size
- The 0.7 ratio represents an 'honest signal' indicating fertility but not current pregnancy
Conclusion: Males have evolved sensitivity to indicators of female fertility, supporting their optimal mating strategy.
This research demonstrates that males have evolved sensitivity to indicators of female fertility, which supports their optimal mating strategy of detecting and mating with fertile females to maximise their reproductive success.
Support from lonely hearts research
Research Study: Waynforth & Dunbar (1995) Lonely Hearts Analysis
Method: Analysed lonely hearts advertisements in American newspapers to examine what qualities men and women offered and sought in partners.
Key Findings:
- Women more frequently offered physical attractiveness and youth indicators ('flirty, exciting, curvy, sexy')
- Men more often offered resources ('successful, fit, mature, ambitious')
- Men sought relative youth and physical attractiveness in partners
Conclusion: Supports sexual selection predictions about different male and female mating preferences.
These findings support sexual selection predictions that females prefer males who can provide protection and resources for raising offspring, while males prefer females who show reliable indicators of fertility and reproductive potential.
Evaluation
Strengths
Strong Empirical Support
The theory receives strong empirical support from multiple research studies across different cultures and time periods. Cross-cultural consistency in findings suggests that partner preferences have evolutionary rather than purely cultural origins. The research uses diverse methodologies, from large-scale surveys to historical document analysis, providing converging evidence for evolutionary predictions.
Limitations
Cultural and Social Influences
Sexual selection theory has been criticised for ignoring social and cultural influences on partner preferences. Partner preferences over the past century have been influenced by rapidly changing social norms regarding sexual behaviour, which develop much faster than evolutionary timescales would suggest.
Changing Social Roles
Women's increased participation in the workplace means they are no longer dependent on men for resources (despite ongoing inequalities in earning power). Berenczkeei et al. (1997) argue that this social change has consequences for women's mate preferences, which may no longer be resource-oriented.
Chang et al. (2011) compared partner preferences in China over 25 years and found that some preferences had changed while others remained constant, corresponding with major social changes during that period.
Mate preferences are therefore the outcome of both evolutionary and cultural influences. Any theory that fails to account for both provides only a limited explanation of human behaviour.
Key Points to Remember:
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Sexual selection explains partner preferences through evolutionary advantages - traits that increase reproductive success become more common over generations
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Anisogamy (differences between sperm and ova) creates different optimal mating strategies for males and females
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Inter-sexual selection represents female strategy - choosiness and 'quality over quality' due to greater reproductive investment
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Intra-sexual selection represents male strategy - competition and 'quantity over quality' due to low-cost sperm production
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Research evidence from multiple studies and cultures supports evolutionary predictions, but cultural factors also play important roles in shaping modern partner preferences