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One of the clearest examples of psychological influences on population structure is the phenomenon of mate preference. Mate preference occurs when members of one sex prefer to mate with individuals who have certain phenotypes. Evolution occurs when these preferences result in actual differences in fitness—the preferred phenotype does, in fact, leave more gene copies than the less preferred phenotypes.
One account of how the peacock’s tail evolved invokes mate preferences. We have all witnessed the display of a male peacock during which he fans his long tail out into a semicircle of brightly coloured “eyes.” The reason for the display, however, has nothing to do with human esthetics— the male simply uses this display to attract a female and mate with her.
The story says that at some point in time, female peacocks developed a preference for males with large and colourful displays. Hence, they preferentially mated with males of these phenotypes. The males passed their genes on for large, colourful displays while the females passed their own genes on for preference for large, colourful displays.
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We humans also have mate preferences, most of which show strong cross-cultural agreement as demonstrated by the work of evolutionary psychologist David Buss. When people from different cultures are asked to rate or rank traits in terms of preference in a potential spouse, almost all place a “nice person” as the number one quality.
Kindness, sincerity, and compassion consistently rate high. Intelligence is second. Not all traits, however, show cross-cultural uniformity. The desirability for premarital chastity in a spouse varies considerably from one culture to another.
Buss’s research clearly demonstrates that on average different cultures agree on the traits that are desirable, neutral, or not desirable in a mate. But do people actually do what they say? And do mate preferences result in real differences in fitness?
Here, the evidence is mixed. If males and females have similar mate preferences, if these preferences are strong, and if they are actually acted upon, then there should be strong spousal correlations for preferred traits. For example, if a woman—let us call her Diane—is a very nice person, highly intelligent and physically attractive, she should be a highly sought after potential spouse.
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Because many different men are pursuing Diane, she can have her own choice of a partner. Because Diane has her own mate preferences and has a choice of Tom, Dick, and Harry, she is likely to select a male who is also nice, smart, and good looking.
A woman lacking Diane’s attributes is less likely to attract someone like Diane’s husband and is more likely to mate with a guy of her own level of mate desirability. As a consequence, we should find positive correlations among spouses for niceness, intelligence, and physical attractiveness.
Empirically we do find significant spousal correlations for intelligence and cognitive ability (about 40) and for physical attraction (about 30). The problem comes with niceness. Virtually all the empirical data on spouses show that they are completely uncorrelated on the majority of personality traits! Consequently, contemporary personality inventories fail to tap the construct of “niceness,” and/or there is really no spousal correlation in the first place. Other evidence suggests that there is no spousal correlation.
When people are asked about mate preferences, almost all agree that “personality” is the most important issue before even intelligence and physical attractiveness. In addition to niceness, people also express preference for a mate who is happy, outgoing, active, and talkative.
These are attributes of the dimension called extraversion or positive affect and no studies have reported significant spousal correlations for this trait. People also express preferences against having a mate who is anxious, high strung, and worrying. These attributes comprise the dimension of emotional stability, negative affect, or neuroticism, and once again, there is no spousal similarity for this trait.
Hence, some aspects of mate preference might reflect discrepancies between self-report and actual behaviour. People may think and even genuinely feel that an outgoing mate is more desirable than a shy one, but in the actual, day-to-day encounters with a specific person, the overt and concrete behaviours that constitute extroversion do not matter much in choosing a mate.
Similarly, the failure to rate social and political attitudes highly may be due to our own under appreciation of these traits. We may think and even genuinely feel that they are arbitrary behaviours of little consequence, but when faced with a person whose attitudes are very different from our own, then attitudes become an issue in mate choice.