Evolutionary psychologists have taken three distinct perspectives on the study of emotions, such as fear , rage, and jealousy . One view , represented by the work of Paul Ekman, is to examine whether facial expressions of emotion are interpreted in the same ways across cultures, on the assumption that universality is one criterion for adaptation (Ekman, 1973, 1992a, 1992b). In other words, if all humans share an adaptation, such as smiling to express happiness, that adaptation is likely to be a core part of human nature. A second evolutionary view is that emotions are adaptive psychological mechanisms that signal various "fitness a fordances" in the social environment
(Ketelaar, 1995). According to this perspective, emotions guide the person toward goals that would have conferred fitness in ancestral environments (e.g., the pleasur one feels having one's status rise within a group) or to avoid conditions that would have interfered with fitness (e.g., getting beaten up or abused). A third evolutionary perspective on social emotions is the "manipulation hypothesis," which suggests that emotions are designed to exploit the psychological mechanisms of other people. For example, expressions of rage might be designed to make a verbal threat more credible than the same threat made without displaying rage.
All these evolutionary perspectives on emotions hinge on the proposition that they are universal and universally recognized in the same way . Ekman (1973, 1992a, 1992b) pioneered the cross-cultural study of emotions. He assembled pictures of several different faces, each of which showed one of seven emotions: happiness, disgust, anger, fear, surprise, sadness, and contempt. When these pictures were shown to subjects in Japan, Chile, Argentina, Brazil, and the United States, all showed tremendous agreement on which emotions corresponded to which face. Subsequent research has confirmed the universal recognition of these emotional expressions in Ital , Scotland, Estonia, Greece, Germany, Hong Kong, Sumatra, and Turkey (Ekman et al., 1987).
Especially impressive is the study of the Fore of New Guinea—a cultural group with practically no contact with outsiders. They spoke no English, had seen no TV
or movies, and had never lived with Caucasians. Nonetheless, the Fore also showed the universal pairing of emotions and faces. Subsequent research has also shown the universality of the facial expression of contempt (Ekman et al., 1987). Although only the most preliminary aspects of the evolutionary psychology of emotions have been studied, Ekman's work suggests that emotions, as central components of personality , are universally expressed and recognized, thus fulfilling an important criterion fo adaptation. They are good candidates for evolved components of human nature.
We have reviewed only a few hypotheses about the components of human nature from an evolutionary perspective—the need to belong, social anxiety about ostracism, the urge to help, and the universality of emotions. An evolutionary perspective may shed light on many other possible components of human nature, such as childhood fears of loud noises, darkness, spiders, and strangers; emotions such as anger , envy, passion, and love; the universality of play among children; retaliation and revenge for perceived personal violations; status striving; psychological pain on the loss of status and reputation; and perhaps many more. Human nature, however , represents only one level of personality analysis. We now turn to the second level—sex dif ferences.
Evolutionary psychology predicts that males and females will be the same or similar in all the domains in which the sexes have faced the same or similar adaptive problems. Both sexes have sweat glands because both sexes have faced the adaptive problem of thermal regulation. Both sexes have similar (although not identical) taste preferences for fat, sugar , salt, and particular amino acids because both sexes have faced similar (although not identical) food consumption problems.
In other domains, men and women have faced substantially dif ferent adaptive problems over human evolutionary history. In the physical realm, for example, women have faced the problem of childbirth; men have not. Women, therefore, have evolved particular adaptations that are lacking in men, such as mechanisms for producing labor contractions through the release of oxytocin into the bloodstream.
Men and women have also faced dif ferent information-processing problems in some adaptive domains. Because fertilization occurs internally within the woman, for example, men have faced the adaptive problem of uncertainty of paternity in their of f-spring. Men who failed to solve this problem risked investing resources in children who were not their own. We are all descendants of a long line of ancestral men whose characteristics led them to behave in ways that increased their likelihood of paternity and decreased the odds of investing in children who were presumed to be theirs but whose genetic fathers were other men.
This does not imply , of course, that men were or are consciously aware of the adaptive problem of compromised paternity . A man does not think, "Oh, if my wife has sex with someone else, then my certainty that I'm the genetic father will be jeopardized, and this will endanger the replication of my genes; I'm really mad." Or , if a man's wife is taking birth-control pills, he does not think, "W ell, because Joan is taking the pill, it doesn't really matter whether she has sex with other men; after all, my certainty in paternity is secure." Instead, jealousy is a blind passion, just as our hunger for sweets and craving for companionship are blind passions. The blind "wisdom" ofjealousy is passed down to us over millions of years by our successful forebears (Buss, 2000a).
Women faced the problem of securing a reliable or replenishable supply of resources to carry them through pregnancy and lactation, especially when food resources were scarce (such as during droughts and harsh winters). We are all descendants of a long and unbroken line of women who successfully solved this adaptive challenge—for example, by preferring mates who showed the ability to accrue resources and the willingness to channel them toward particular women (Buss, 2003). The women who failed to solve this problem failed to survive, imperiled the survival chances of their children, and hence failed to become our ancestors.
Evolutionary-predicted sex differ ences hold that the sexes will dif fer in precisely those domains where women and men have faced dif ferent sorts of adaptive problems (Buss, 2004). To an evolutionary psychologist, the likelihood that the sexes are psychologically identical in domains in which they have recurrently confronted different adaptive problems over the long expanse of human evolutionary history is essentially zero (Symons, 1992). The key question, therefore, is not "Are men and women different psychologically?" Rather, the key questions about sex dif ferences, from an evolutionary psychological perspective, are the following:
1. In what domains have women and men faced dif ferent adaptive problems?
2. What are the sex-differentiated psychological mechanisms of women and men that have evolved in response to these sex-dif ferentiated adaptive problems?
3. Which social, cultural, and contextual inputs af fect the magnitude of expressed sex differences?
This section reviews some of the key domains in which the sexes have been predicted to differ: aggression, jealousy, desire for sexual variety , and mate preferences.
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