Evolution and Close Relationships
For Simpson & Dovidio APA Handbook
University of Minnesota
Martie G. Haselton
University of California Los Angeles
Joshua M. Ackerman
Massachusetts Institute of Technology
Throughout history humans have faced critical challenges that included finding a mate, keeping that mate, caring for kin, forming coalitions, and gaining some status. Solving each of these ancestral challenges involved forming a different type of social relationship. An evolutionary perspective suggests that there is a set of fundamentally different types of close relationships associated with different evolutionary challenges. These include: (1) mate attraction (e.g., dating couples), (2) mate retention (e.g., married couples), (3) kin care (e.g., family members), (4) coalition formation (e.g., friends), (5) status (e.g., workplace relationships). Each type of ancestral challenge is associated with different kinds of evolutionary opportunities and costs, suggesting that different types of relationships may be governed by a different relationship-specific psychology. In this chapter, we review the principles of evolutionary psychology and their implications for close relationships.
Although some animals spend most of their lives as hermits, humans have always lived in groups. The human brain has evolved for social relationships (Kenrick, Griskevicius, Neuberg, & Schaller, 2010; Kenrick, Neuberg, & White, in press). But while people are born ready to love and relate to other people, relationships differ in several important ways. For example, the words “I love you” can be spoken by a parent to a newborn baby, by a young man to a woman he met yesterday at a beach resort in Mexico, and by a heterosexual woman to her best female friend. Yet parental love is not the same as romantic love, which is still different from Platonic love between friends.
In this chapter, we consider social relationships from an evolutionary perspective. This perspective contends that, throughout history, humans have faced a set of core ancestral challenges, which include attracting a mate, keeping that mate, caring for kin, forming coalitions, and attaining status. An evolutionary perspective suggests each type of challenge can be solved by forming different types of relationships. These relationships types include: (1) dating couples, (2) married couples, (3) family members, (4) friends, (5) and co-workers (see Table 1). Each type of relationship is associated with different kinds of evolutionary opportunities and costs, meaning that people need different things from different types of intimate others and must provide different things to those intimate others. These needs and provisions vary systematically depending on the type of relationship.
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In this chapter, we first briefly review what it means to take an evolutionary perspective, reviewing some foundational principles. We then discuss each of the five types of relationships, reviewing relevant theory and findings. Finally, we discuss emerging themes and future directions in the study of social relationships from an evolutionary perspective.
An Evolutionary Approach to Relationships
A modern evolutionary approach is based on the seminal work of Charles Darwin. This approach suggests that, just as the forces of natural selection can shape morphological features, so can those forces shape psychological and behavioral tendencies. An evolutionary approach maintains that human and non-human animals inherit brains and bodies equipped to behave in ways that are fitted to the demands of the environments within which their ancestors evolved. Just as human morphological features—opposable thumbs, larynxes, and upright postures—have been shaped by evolutionary pressures, humans inherited brains specially designed to solve recurrent problems in the ancestral world (Barrett, Dunbar, & Lycett, 2002; Buss, 1995; Neuberg, Kenrick, & Schaller, 2010; Tooby & Cosmides, 1992). For example, along with the larynx, humans also inherited a brain designed to easily learn to communicate using language. Although the specific words and sounds of a language might differ across cultures, all languages share an underlying universal structure as a result of the evolved human mechanisms for language (Pinker, 1994). Below we review two key distinguishing features of a modern evolutionary approach.
The Mind Evolved to Solve Adaptive Problems of the Ancestral Past
An evolutionary approach does not assume that humans or other organisms inherit the capacity to determine in advance which behaviors will enhance fitness. People do not proceed through life deliberating the reproductive consequences of each decision. Instead, natural selection, operating over millennia, endows individuals with a psychology designed to increase the probability of solving recurrent adaptive challenges. All of our ancestors confronted a set of problems that had to be solved in order to survive and reproduce, such as making friends, gaining status, attracting a mate, keeping a mate, and caring for family. The brain is designed to solve adaptive challenges, whereby the cumulative solutions to these challenges together enhanced fitness over evolutionary history.
Although the modern environment is in many ways different from ancestral environments, humans still encounter problems the brain is designed to solve. Each problem has always been uniquely different. Solutions to one problem failed to successfully solve other problems. For example, solutions to the adaptive problems of attracting a mate (e.g., approaching genetically fit individuals) generally could not be used as solutions to the adaptive problem of gaining status (e.g., approaching people with power or prestige). Because different evolutionary problems required qualitatively different solutions, an evolutionary psychological perspective suggests that the brain houses multiple domain-specific psychological mechanisms geared toward solving different adaptive problems (Tooby & Cosmides, 1992; Barrett & Kurzban, 2006).
The domain-specific view of the mind is importantly different from the “domain-general” view that dominated the social sciences in the twentieth century. This traditional view posits that the brain has a single executive system operating according to a few domain-general rules such as seek rewards, avoid punishments, or maximize utility. Such views initially appeal to parsimony, but an abundance of research on learning and cognition in both animals and humans challenged views of the brain that are completely domain-general. Zoologists, biologists, and ecologists have uncovered a wealth of specialized behavioral and cognitive mechanisms in animals peculiarly suited to solving different types of adaptive challenges (Garcia & Koelling, 1966; Wilcoxon, Dragoin, & Kral, 1972). For example, birds use distinct, domain-specific neuropsychological systems for learning and remembering information about species songs, poisonous foods, and spatial position of food caches (Sherry & Shachter, 1987). Similarly, humans use distinct, domain-specific systems and neural architectures for dealing with different types of adaptive problems (Klein, Cosmides, Tooby, & Chance, 2002; Barrett & Kurzban, 2006). Thus, because different types of social relationships helped to solve fundamentally different types of adaptive challenges, an evolutionary perspective posits that different types of social relationships function according to different rules – in the ancestral past and today.
Multiple Levels of Explanation for Behavior
An evolutionary perspective draws an important distinction between ultimate and proximate explanations for behavior (see Tinbergen, 1963). Psychologists typically have been concerned with proximate explanations for behavior, which focus on the relatively immediate triggers (causes) of action. For example, when considering why so many people seek romantic relationships, the primarily proximate reasons often include sex, companionship, love, happiness, pleasure, resources, and support. An evolutionary approach, however, also asks why people evolved to want sex, companionship, and love, and why these things provide so much pleasure in the first place. The ultimate reason why so many people enter romantic relationships is because they enhanced our ancestors’ reproductive fitness during evolutionary history.
Proximate and ultimate explanations are not in competition with each other. Instead, they are complementary. Because human behavior is the product of brain activity and the brain is an evolved organ, behaviors are likely to have both ultimate and proximate explanations. For example, people seek romantic relationships because they provide pleasure (a proximate reason) and because they enhanced reproductive fitness in ancestral environments (an ultimate reason). Both of these explanations are correct. Each one provides insights into the same behaviors, but at different levels of analysis (see Simpson & Gangestad, 2001).
Sometimes there are obvious connections between the different levels of analysis. But connections between different levels of analysis are not always obvious. Consider why birds migrate each year. A proximate explanation is that birds migrate because days are getting shorter—the immediate cue triggering migration. The ultimate reason for such migration, however, is survival and reproduction: The distribution of desirable food and mating sites varied seasonally throughout the evolutionary history of birds. There are two key implications here: (1) Animals, including humans, need not be consciously aware of the ultimate functions of their behaviors that were forged over evolutionary time, and (2) the connection between ultimate functions and immediate goals is often indirect and non-obvious.
An explanation at one level of analysis must be compatible with explanations at other levels. Positing a proximate mechanism that would have reliably lead people to make functionally maladaptive decisions (such as Freud’s death instinct) is problematic (Tooby & Cosmides, 1992). Evolutionary psychologists typically advance hypotheses about links between proximate mechanisms and ultimate functions. In deriving those hypotheses, however, psychologists adopting an evolutionary perspective attempt to take into account pertinent findings from evolutionary biology and/or anthropology. Psychologists can derive hypotheses about proximate causes and development without thinking in evolutionary terms, but disregarding evidence and theory derived from research on other cultures and other species can lead to hypotheses that are incompatible with other levels of analysis. For example, psychologists during the last century often assumed that most sex differences in social behavior (such as differences in violent aggression) were products of “American culture,” unaware that similar differences were found in other cultures across time, and even in other species (see Daly & Wilson, 1988; Kenrick, Trost, & Sundie, 2004).
In order to achieve compatibility between levels of explanation, evolutionarily-minded researchers often adopt an engineering focus in order to detail the functioning of potentially adaptive behaviors at multiple causal levels. Tooby and Cosmides (1992) recommended five central components of an adaptation analysis: (1) identification of an adaptive target (a proposed biologically successful outcome), (2) background conditions (a description of the relevant ancestral environment in which the feature likely emerged), (3) a design (a detailed depiction of the components and boundaries of the feature), (4) a performance examination (a description of the actions and outcomes of the feature), and (5) a performance evaluation (an assessment of how well the design has met the adaptive target). An analysis such as this one highlights both ultimate function and proximate mechanism as well as their necessary interplay.
At its core, an evolutionary approach is concerned with how various behaviors and mental constructs facilitated reproductive success. This success arises from differential reproduction (either by out-producing conspecifics or inhibiting those conspecifics’ own production), and thus an evolutionary approach involves many principles related to aspects of reproduction. Before turning to the specific domains that make up human relationship psychology, we introduce some foundational principles that have broad relevance to multiple relationship domains.
Inclusive fitness. A key, often misunderstood, aspect of an evolutionary perspective is that it is focused on reproductive success at the genetic level, not at the level of the individual. Thus, what matters is the aggregate success of individuals who share genes in common. Inclusive fitness refers to the mechanisms that might facilitate this aggregate success, such as those for biological kin recognition and altruism among genetic relatives (Hamilton, 1964; Lieberman, Tooby, & Cosmides, 2007; Park & Ackerman, 2011). Inclusive fitness theory does not predict that people always prefer or help their relatives; rather, it predicts that psychological mechanisms that tended to increase the reproductive success of relatives will evolve. These mechanisms, of course, function at a proximate level and are susceptible to erroneous or novel inputs, which can produce behaviors that seem maladaptive despite their foundation in inclusive fitness. Interestingly, this theory also predicts non-affiliative tendencies between relatives that might impair genetic success, such as incest aversion (Westermarck, 1891).
Parental investment. After conceiving offspring, organisms face the issue of whether or not to invest in the development of that offspring. This investment typically carries costs such as limiting future reproduction opportunities and loss of resources that might be used for the self or for others (such as other genetic relatives). Parents engage in investment because it supports the physical and mental maturation of offspring, which in turn promotes future genetic propagation (of shared genes). The “decision” of how much to invest, typically made unconsciously, is rooted in the cost-benefit trade-offs that parents face as a function of factors such as what resources parents have, what offspring need (e.g., due to their health or environmental pressures), what relatives need (e.g., other offspring), and the likelihood that a parent shares genes with the offspring.
Importantly, organisms face these trade-offs not only after conception, but prior to conception as well. Some costs can be predicted in advance, and organisms often act to minimize them. Within mammals, there is a natural division in parental investment because females gestate the young within their bodies (for almost 2 years in the case of elephants, and for 9 months in the case of humans), and they then nurse them afterwards (sometimes for several years). Thus, females have a higher minimal obligatory parental investment than do males (Trivers, 1972). Males could, in theory, contribute little more than sperm to their offspring, which is the typical pattern for over 90 percent of mammalian species (Geary, 2000).
Sexual selection. The established differences in prospective costs for females and males set the stage for a number of additional, behavioral sex differences. Within a species, the sex that invests less in offspring tends to compete for mating opportunities with the higher-investing sex. Because mammalian females nearly always pay a higher price for reproduction, whereas males might contribute little or nothing to offspring care, females are relatively more selective in their choice of mates (Trivers, 1972). This reflects sexual selection, which refers to the relative success of traits that assist in mating (by helping to either attract the opposite-sex or to compete with one’s own sex for mates). Darwin developed the idea of sexual selection to address the fact that one sex is often larger, more colorful, and more competitive than the other (i.e., sexually dimorphic). A peacock’s bright feathers increase his chances of attracting peahens as mates, while at the same time making him more susceptible to detection by predators. The increased developmental effort necessary to produce these feathers, and the associated increased risk of predation, make such indicators honest or costly signals (Zahavi, 1975; Grafen, 1990). Females do not need ostentatious ornamentation displays to the same extent as males because they make a higher investment in offspring and, therefore, are choosier about their mating partners, who must compete with other males to be chosen as mates.
Male investment, however, varies across species. To the extent that male investment in offspring increases, the degree of sexual dimorphism is reduced, as in many bird species in which both parents devote effort to nest-building and offspring care (Cockburn, 2006). In rare cases, a male actually invests more resources in the offspring than does the female, as in the case of bird species such as phalaropes—a type of sandpiper in which the female leaves the male to tend the eggs while she searches for another mating opportunity (Colwell & Oring, 1989). Consistent with parental investment theory, sex differences in morphology and behavior reverse for such species (see Trivers, 1985).
Because all the usual mammalian constraints on gestation and nursing apply to humans, several broad sex differences—greater female mating selectivity and greater male intra-sexual competition—also apply to humans (e.g., Clark & Hatfield, 1989; Wilson & Daly, 1985). One indirect consequence of greater female selectivity is slower sexual maturity in males (Geary, 1998). The reason for the maturational delay among males in dimorphic species is that it takes longer for males to reach a size at which they can successfully compete with other males for females. In line with this observation, human males typically reach sexual maturity much later than females, and attain a somewhat larger size than do females.
Types of Close Relationships and Associated Ancestral Challenges
Throughout history, humans have faced a set of core ancestral challenges, each of which can be solved by forming different types of relationships. These evolutionary challenges include: (1) mate attraction (e.g., dating couples), (2) mate retention (e.g., married couples), (3) kin care (e.g., family members), (4) coalition formation (e.g., friends), (5) status (e.g., workplace relationships) (see Table 1). Below we discuss how each type of challenge is associated with different kinds of evolutionary opportunities and costs, which have important ramifications for the workings of relationships that help solve difference evolutionary challenges.
Dating Couples: Challenge of Mate Attraction
Differential reproduction is at the center of natural selection. Hence, decisions about mating are, from a functional perspective, crucially important. There has been an abundance of evolution-inspired research on mate selection. Selecting a mate involves three separable sets of questions: (1) which type of relationship is the partner being considered for (e.g., short-term versus long-term)? (2) what are the characteristics of the potential mate (e.g., his/her physical attractiveness, social status)?; and (3) How does the potential mate’s characteristics meet my desires and needs?” The characteristics desired in a short-term mate are different than those desired in a long-term mate (e.g., Buss & Schmitt, 1993; Fletcher et al., 2004; Li & Kenrick, 2006). Because of differences in obligatory parental investment (with women being required to invest more in potential offspring), men and women differ in their preferences for both type of relationship and the characteristics they desire in a mate (e.g., Kenrick et al., 1990; Shackelford et al., 2004; Wiederman & Hurd, 1999). There are also sex differences in the criteria for mate choice, with women prioritizing status and resources more than men, and with men prioritizing physical attractiveness more than women (Buss, 1989; Kenrick et al., 1993; Li & Kenrick, 2006). Human males and females often cooperate in raising offspring, and raising offspring requires some similar characteristics in men and women (e.g., cooperativeness, generosity, sense of humor, etc.). Consequently, the sex differences in mate selection criteria exist alongside a number of sex similarities (Kenrick et al., 1990, 1993; Li & Kenrick, 2006).
Romantic Relationship Preferences
In humans, romantic relationship preferences are often characterized along the dimension of short-term and long-term relationships. People tend to seek out romantic partners for short-term romantic (typically sexual) encounters, or they look for more longer-term relationships that are more likely to provide stability, support, and parental investment. The short-term approach tends to be marked by increased openness to multiple mating partners. The long-term approach is characterized by sexual exclusivity (lack of openness) or the existence of extra-pair liaisons (some degree of openness to certain kinds of partners). A great deal of within-sex variation exists in human mating strategies, with both men and women varying in their courtship strategies in ways linked to either different developmental trajectories (Belsky, Steinberg, & Draper, 1991) or local environmental conditions (Gangestad & Simpson, 2000). Men who adopt an unrestricted (sexually open) mating strategy, for example, are larger, more physically attractive, and more competitive on average compared to men who adopt a restricted strategy, characterized by higher investment and greater monogamy (Boothroyd, Jones, Burt, DeBruine, & Perrett, 2008; Thornhill & Gangestad, 1994).
Compared with people who have a restricted orientation, individuals with an unrestricted orientation had relatively more partners in the past, including one-night stands, and are more likely to view their opposite-sex friends as potential sexual partners (Bleske-Rechek & Buss, 2001). They also intend to have relatively more partners in the future, begin having sex earlier in any given relationship, are more likely to carry on multiple relationships at one time, and feel less investment, commitment, love, and interdependence with their current partners (Simpson & Gangestad, 1992).
Ecological and cultural factors influence tendencies toward restricted versus unrestricted mating, but men are universally more inclined toward unrestrictedness (Schmitt et al., 2003). Numerous studies have demonstrated that women are more reticent about entering short-term relationships and more selective about the minimum characteristics they will accept in a partner for such relationships (e.g., Clark & Hatfield, 1989; Kenrick et al., 1990; Schmitt et al., 2012). When asked about their regrets in life, men are much more likely to wish they had slept with more partners, whereas women wish they had tried harder to avoid getting involved with losers (Roese et al., 2006). When asked about casual sex experiences in the past, women are far more likely than men to say that they regretted them (Galperin et al., 2012). One survey of 16,288 people from around the world suggests that the sex difference in the desire for sexual variety is universal and medium to large in magnitude (ds ranging from .31 to 1.20; Schmitt et al., 2003).
The sex difference in interest in casual sex has implications for other aspects of relationships. For example, people sometimes deceive others about the extent of their interest in forming a long-term relationship in order to induce a partner to have sex. Women tend to be bothered much more than men by such deception (sex differences ranging from ds of .67 to 1.69; Haselton et al., 2005). Compared with men, women are also more skeptical about interpreting a man’s compliments, gifts, touch, and even confessions of love as evidence of commitment (Ackerman, Griskevicius, & Li, 2011; Haselton & Buss, 2000). Because women are reticent about sexual opportunities, men tend not to miss possible signs of sexual interest (Haselton & Buss, 2000; Haselton & Funder, 2006). Compared with women, men are more likely to interpret a woman’s compliments, gifts, or touch, and love confessions as a signal of true sexual desire (Ackerman et al., 2011; Haselton, 2003; Haselton & Buss, 2000). In one study, college students were asked to judge whether faces in photographs showed subtle signs of “suppressing” any underlying feelings. In reality, all the faces had been carefully picked to be emotionally neutral. After watching a film clip that put them into a romantic frame of mind, men projected their sexual feelings onto the photos, but only the photos of beautiful women. Women’s romantic feelings, on the other hand, did not cloud their judgments (Maner et al., 2005).
Romantic Partner Preferences
Just as people prefer different types of romantic relationships, they also seek different types of romantic partners. Sometimes this search is tied to their particular relationship preferences. For example, unrestricted people tend to choose partners who are socially visible and attractive (Simpson & Gangestad, 1992). Restricted individuals prefer partners with traits linked to good parenting, such as responsibility, affection, stability, and faithfulness (Simpson & Gangestad, 1992). The preference for certain partner characteristics is also tied to other individual and ecological factors. One of the most prevalent of these in the research literature has been an individual’s biological sex (Gangestad & Simpson, 2000).
Women’s Relative Preference for Status
The sex difference in minimal obligatory parental investment leads female mammals to be more choosey when picking mates, and it leads males to compete with one another to demonstrate their relative viability and superiority as mates. This generalization applies to humans as well. Despite the fact that human males contribute to offspring care, human females still make a higher physiological investment in gestation and nursing, and they typically provide more direct care for children than do men. Because men do not contribute resources directly from their bodies to the offspring, evolutionary theorists argue that ancestral women sought high-status men who could provide resources and protection (Gangestad & Simpson, 2000; Gangestad & Thornhill, 1998).
Numerous studies have found that women place more emphasis on status than do men when selecting partners. For example, in one study, women preferred a physically unattractive but well-dressed man to a handsome burger flipper (Townsend & Levy, 1990). Another study found that women were more attracted to a man who made money in business over one who just got lucky, suggesting that it is the ability to generate future resources that is attractive to women (Hanko, Master, & Sabini, 2004). Women’s singles ads are, compared with men’s, more likely to require status or wealth in a man; conversely, men taking out singles advertisements are more likely to advertise any status or wealth they possess (Rajecki, Bledsoe, & Rasmussen, 1991; Wiederman, 1993). Women also respond more to men who advertise their income and education levels, whereas men reading women’s ads pay little attention a woman’s status (Baize & Schroeder, 1995). A study of 37 different cultures found the same trends around the world (Buss, 1989). Similar to American women, Japanese, Zambian, and Yugoslavian women rate good financial prospects in a mate as more important than do men in those countries (Buss, 1989). Women around the world also tend to seek and to marry somewhat older men, who generally have more resources and social status (Buss, 1989; Kenrick & Keefe, 1992).
Men compete for women’s attention not only by fighting and struggling for social status, but also by signaling that they have desirable characteristics such as attractiveness, health, intelligence, sense of humor, and creativity (e.g., Griskevicius et al., 2006; Maner & Ackerman, in press). Wilbur and Campbell (2011) found that, in dating contexts, men report being more likely to show off their sense of humor, whereas women evaluate men as potential partners on these kinds of displays. These researchers also found that women evaluate nonhumorous online dating profiles as much less desirable (men also preferred a partner with a good sense of humor, but not as strongly as women). In a related study, Bressler, Martin, and Balshine (2006) found that women prefer men who demonstrate their sense of humor, whereas men prefer women who are receptive to their own humor.
Men’s Relative Preference for Reproductive Resources
Given women’s provision of direct physical resources to their offspring, it would have been advantageous for ancestral men to seek out cues of health and reproductive potential, such as youth and physical attractiveness, in their mates (Pawlowski & Dunbar, 1999). Indeed, men’s age preferences in mates are consistent with the general tendency for men to prioritize cues to fertility over cues to status. Women’s fertility peaks in their mid-twenties (Dunson, Colombo, & Baird, 2002). Older men tend to be attracted to younger women, men in their twenties are attracted to women around their own age, and teenage men are attracted to slightly older women (Buunk et al., 2002; Kenrick & Keefe, 1992).
As noted above, men advertise, and women request, financial resources in singles advertisements. On the other side of the bargain, men evaluating potential dates place more emphasis on physical appearance (Li, Bailey, Kenrick, Linsenmeier, 2002; Shaw & Steers, 2001). Other findings suggest that being seen with a physically attractive member of the opposite-sex improves the social impression made by a man, but has no effect on the impression made by a woman (Sigall & Landy, 1973). Indeed, to say that a man is physically attractive is to say he shows signs of social dominance, such as a strong chin and mature features, whereas a physically attractive woman shows signs not of dominance, but of youthfulness and fertility (Singh, 1993).
Both sexes would be most happy with a partner who is high on all desirable dimensions—physically attractive, wealthy, charming, agreeable, and so on (Fletcher, Simpson, Thomas, & Giles, 1999). However, most people are not in a position to attract a partner who is perfect in every way, so they must compromise and make trade-offs. When forced to compromise in choosing a long-term partner, men and women make very different choices. Women prioritize social status and give up good looks; men prioritize attractiveness and give up wealth (Li et al., 2002). In choosing a casual sexual partner, on the other hand, women shift their priority to physical attractiveness (Fletcher et al., 2004; Li & Kenrick, 2006).
Hormonal Effects on Mating Strategies
Several decades ago, many scientists believed that hormonal influences were irrelevant to human sexual behavior (e.g., Simon & Gagnon, 1984). However, numerous studies have refuted that viewpoint, and suggest that sex hormones have many of the same functions in humans as they do in other mammals (Leitenberg & Henning, 1995; Regan & Berscheid, 1999). For instance, injecting testosterone into men who have malfunctioning testes leads them to increase their sexual fantasies, and stopping the injections leads to a drop in fantasies (Regan & Berscheid, 1999). Likewise, injections of testosterone increase sexual desire and fantasy in women (Sherwin, Gelfand, & Brender, 1985). Men involved in short-term relationships have higher levels of testosterone, whereas married men have lower levels (Gray et al., 2004). Increases in testosterone might also facilitate intrasexual competition for mates (Mazur & Booth, 1998), supporting the view that this hormone is intricately involved in many aspects of sexuality and initial mating effort.
Several findings also suggest that the rules of mate selection change when women are in the ovulatory phase of their menstrual cycles and, hence, are most fertile (e.g., Pillsworth & Haselton, 2006b; Thornhill & Gangestad, 2008). At these times, women show increased preferences for men with sexy traits such as masculine faces (DeBruine et al., 2010) and competitive behavior (Gangestad, Simpson, Cousins, Garver-Apgar, & Christensen, 2004). Additionally, women report greater attraction to men other than their primary partners, particularly when their primary partners lack the sexy traits these women prefer most strongly near ovulation (Haselton & Gangestad, 2006; Pillsworth & Haselton, 2006a; Gangestad, Thornhill, & Garver-Apgar, 2005).
Other Factors Affecting Variation in Mating Strategies
Other findings suggest that variations in mating strategies can influence the characteristics people seek out in others. Unrestricted women—who are inclined to have short-term sexual relationships—tend to prefer masculine men (Wayneforth, Delwadia, & Camm, 2005). When women are considering men for short-term sexual relationships, they are similar to men in that they give priority to physical attractiveness over other characteristics that might be more desirable in a long-term mate (Fletcher et al., 2004; Li & Kenrick, 2006). These findings suggest that some women some of the time play an alternative mating strategy of seeking a man whose characteristics indicate good genes, even if that means compromising on getting a man who will stay around and invest in offspring (see Gangestad & Simpson, 2000).
Correlational evidence shows that women who state they are in control of their own resources, and presumably less dependent on a man for assistance with offspring care, place greater emphasis on attractiveness in mates than women who do not feel in control of their own resources (Moore, Cassidy, Law Smith, & Perrett, 2006). Recent experimental research shows that women placed in positions of power over a male stranger have more sexual thoughts and perceive greater sexual interest from the man (Kunstman & Maner, 2011). Together, these findings suggest that women in non-traditional gender roles – perhaps those women high in occupational prestige – display a mating psychology shifted toward that of men.
Finally, research has also pointed to variation in the basic process of mate attraction (Maner & Ackerman, in press). We typically assume that people seek out and interact with mate prospects in interpersonal bubbles, away from third-party influences. When social environments are taken into account, research has almost exclusively considered third parties as potential competitors (e.g., Buss, 1989; Maner, Miller, Rouby, & Gailliot, 2009; Shackelford et al., 2005). Yet, cross-species findings have indicated that animals sometimes cooperate in their courtship pursuits (e.g., Krakauer, 2005; Smuts & Smuts, 1993). Some evidence suggests that people do so as well, with women cooperating more strongly to build mating quality control thresholds and barriers to unwanted advances, and with men cooperating more strongly to overcome these thresholds and interpersonally break down these barriers (Ackerman & Kenrick, 2009; Bleske-Rechek & Buss, 2001).
Married Couples: Challenge of Mate Retention
Once a romantic couple forms, relationship cognition shifts from a mate attraction to a mate retention mindset. Clearly, continuing to perform many of the behaviors and decisions that attracted romantic partners would damage one’s chances of maintaining a long-term relationship. For example, a man who spent lavishly in order to display status and attract women might undermine his long-term relationship success by persisting in this behavior after marriage. Instead, other problems emerge in this relationship domain, including maintenance of bonds, dealing with the threat of infidelity, and potentially, child care.
The joint care of two parents was probably crucial to the survival of human children throughout evolutionary history (Hrdy, 1999). As a point of comparison, newborn chimpanzees are able to hold onto their mothers’ backs as their mothers forage for food in the forest, they are weaned at about four years of age, and they are self-sufficient by about five years of age. Chimpanzee mothers have only one offspring about every five and a half years, rarely caring for more than one dependent offspring at a time (Lancaster et al., 2000). In contrast, human children are born helpless. They are unable to lift their own heads or unfold their hands until almost the third month of life, they often share parental care with dependent siblings, and they remain dependent on their caregivers for a much longer period of time than any other primate offspring. Among modern hunter-gatherers, children cannot personally acquire as many calories as they need to consume to survive until approximately 15 years of age (Hill & Hurtado, 1996; Lee & Kramer, 2002). The needs of human children might explain the fact that, across cultures, men and women fall in love (Jankoviak & Fischer, 1992) and form long-term cooperative relationships in which both parents contribute to the offspring’s welfare (Daly & Wilson, 1983; Geary, 1998). A key adaptive problem for both sexes is maintaining these mating bonds with desirable and cooperative partners (Buss, 2007; Hazan & Diamond, 2000).
The process of maintaining these bonds might have exploited existing physiological systems designed for attachment between mammalian mothers and infants (Brown & Brown, 2006; Hazan & Shaver, 1994). The hormone oxytocin is secreted in greater quantities by women, and it is believed to promote maternal bonding (Feldman, Weller, Zagoory-Sharon, & Levine, 2007). Oxytocin is also linked to sexual receptivity, increased genital lubrication in women, and orgasm in both sexes (Salonia et al., 2005). Other neuropsychological evidence suggests that oxytocin might play a role in connecting love and sex, explaining why the two are more closely interconnected for women than for men (Diamond, 2004).
Certain lower-order processes might also facilitate mate retention. Attention to desirable alternative partners can lower relationship commitment (Kenrick, Neuberg, Zierk, & Krones, 1994), and some research suggests that people maintain commitments partly by changing their visual attention to and perceptions of attractive alternatives (Johnson & Rusbult, 1989; Lydon, Fitzsimmons, & Naidoo, 2003; Gonzaga, Haselton, Smurda, & Poore, 2008; Maner, Rouby, & Gonzaga, 2008). In one study of this phenomenon, Simpson, Gangestad, and Lerma (1990) asked students to judge advertisements from magazines such as Cosmopolitan, Gentleman’s Quarterly, and Time. Included in the series were several photographs of attractive members of the opposite-sex. College men and women involved in dating relationships, in contrast to those not involved, found the models significantly less physically and sexually attractive. In another study, participants saw a profile of a highly attractive member of the opposite-sex and learned that this person was currently available (Lydon et al., 1999). Half were also told that this person had expressed a romantic interest in them. How participants responded depended on how committed they were to their current relationship. When the attractive person expressed romantic interest, less committed participants increased in their attraction to the person, but more committed participants became less attracted to the person. These findings indicate that being in a loving relationship leads to a defensive change in perception—seeing potentially threatening alternatives as less desirable. Benefitting long-term bonds, this inattention to attractive alternatives can make people more content with the relationship they currently have (Miller, 1997).
The Threat of Infidelity
On a less rosy side, relationship maintenance also involves defending against incursions by interlopers, competitors who might be romantically attracted to one’s partner (Shackelford & Goetz, 2007). The threat of infidelity can elicit an array of changes in behavior, such as increased visual attention to potential interlopers (Maner, Miller, Rouby, & Gailliot, 2009) and feelings of sexual jealousy if others have desirable characteristics, such as dominance in a man and physical attractiveness in a woman (Buunk & Dijkstra, 2004). Men and women are highly sensitive to what the other sex finds desirable, with jealous women paying more attention to a potential rival’s waist, hips, and hair, and men checking out a rival’s shoulders (Buunk & Dijkstra, 2005). If one’s romantic partner seems to be showing interest in, or involvement with, potential interlopers, more extensive, defensive responses might occur, including emotional and physical violence directed either at the partner (Shackelford, Goetz, Buss, Euler, & Hoier, 2005) or at the potential mate poacher (Campbell & Ellis, 2005).
Additionally, there is a sex difference in the dangers posed by infidelity linked to life history differences between males and females. Women bear children and are always absolutely sure that a given child is their own biological offspring. For men, it is less clear; indeed, a recent meta-analysis reported that approximately 1 in 25 children are genetically unrelated to their purported birth fathers (Bellis, Hughes, Hughes, & Ashton, 2005). If a man’s partner was unfaithful, he might unknowingly invest substantial resources to raising another person’s child – an act that had high fitness costs throughout evolutionary history. Given the possibility of paternal uncertainty, men might be particularly prone to jealousy about a partner’s sexual liaisons. Conversely, because the father’s resources and support are often critical to raising a child successfully (Geary, 2000; Steiglitz, Gurven, Kaplan, & Winking, in press), a woman stands to lose considerably if her partner falls in love with another woman. This problem of paternal investment suggests that women might be especially concerned about a male partner’s emotional rather than sexual attachments.
To test these ideas, Buss, Larsen, Westen, and Semelroth (1992) asked subjects to imagine either that their long-term partner was falling in love and forming a deep emotional attachment to another person, or having sexual intercourse with that person. The majority of the men reported they would be more distressed by the sexual infidelity. However, approximately 80% of the women said they would be more upset by the emotional attachment. Similar sex differences in the triggers of jealousy have since been found in Korea, Japan, Germany, Netherlands, and Sweden (Buss et al., 1999; Buunk et al., 1996).
However, there has been controversy about the extent of the sex difference in jealousy and its theoretical meaning. Some psychologists have argued that the sex difference depends on the particular method used to measure jealousy (e.g., DeSteno et al., 2002), but other researchers find the same sex difference using very different methods (Pietrzak, Laird, Stevens, & Thompson, 2002; Shackelford, LeBlanc, & Drass, 2000). In another critique of the paternal uncertainty hypothesis, Harris (2003) argued that the sex difference in jealousy-linked homicides is simply another manifestation of the general tendency of men to be more violent than women. Consistent with this idea, she reported cross-cultural data demonstrating that although women are less likely to kill, 16% of killings by women are motivated by jealousy compared to about 12% of killings by men. Most psychologists involved in this controversy agree that jealousy is a powerful emotion that is likely to have some adaptive function, but they disagree about whether there is a specific sex difference in the triggers for jealousy per se. One alternative is that both sexes are equally upset by either sexual or emotional infidelity because, ancestrally, the survival of human infants required a close bond between both parents (DeSteno et al., 2002; Harris, 2003). Challenging the idea that the sexes are equal in their responses to infidelity, a recent meta-analysis of the published and unpublished literature shows that sex differences in responses to hypothetical jealousy scenarios and experienced jealousy events are robust and consistent across the methodologies used to investigate them (Sagarin et al., in press).
Monogamy and Polygyny Across Cultures
The vast majority of societies allow a man to marry multiple wives, whereas only about half of 1% of human societies allow polyandrous unions between a woman and multiple men. Regardless of whether a society permits polygamy or not, most of the marriages in all societies are monogamous. But if our species is generally inclined toward monogamy, why are any societies and any marriages within those societies nonmonogamous?
Traditional Tibetans are one of the world’s few polyandrous societies, with one woman often marrying a group of brothers. The harsh conditions of life in the high Himalayan desert have made it difficult for a single man and a woman to survive alone. Tibetan families in which one man marries one woman have fewer surviving children than do families in which brothers pool their resources and share a wife (Crook & Crook, 1988). By sharing one wife, brothers can preserve the family estate, which would not support even one family if it were subdivided each generation. If all the children are girls, the polyandrous pattern switches to a polygynous one, with several sisters marrying one man and passing the family estate on to the sons from that marriage. Hence, Tibetan polyandry appears to be an economically-based strategy by which a limited pool of resources must be channeled into a very focused family line.
Economic resources may also explain the link between social status and polygynous marriage. Men are especially likely to take multiple wives when several conditions converge: (1) a steep social hierarchy, (2) a generally rich environment so one family can accumulate wealth, and (3) occasional famines so the poor face the possible danger of starvation (Crook & Crook, 1988). Under these circumstances, a woman who joins a large, wealthy family reaps benefits, even if she must share her husband with other women. Compared with marrying a poorer man with no additional wives, marrying into a wealthy family can provide a more satisfactory buffer against famine along with the chance of greater wealth for her children in times of plenty. Interestingly, this is the same pattern found in birds such as the indigo bunting: males who attract more than one female are those that control resource-rich territories (Orians, 1969).