Douglas T. Kenrick, Jon K. Maner, and Norman P. Li
Social psychology holds a central place in human evolution. Indeed, we are here today because our ancestors were intuitive masters of social psychological principles. The engine driving evolution by natural selection is differential reproduction. As a minimum, reproduction in any sexually reproducing species involves negotiating at least one critical social interaction. But successful reproduction involves much more than sexual intercourse. To reproduce successfully, our ancestors were required to master a diverse array of social tasks: making friends, negotiating status hierarchies, maintaining long-term relationships, and taking care of one's children.
Although the first social psychology textbook adopted an explicitly evolutionary perspective (McDougall, 1908), the perspective was ignored by social psychologists for most of the 20th century (Kenrick, 2011). However, many of the field's leaders have begun incorporating evolutionary perspectives into their research (Kenrick & Cohen, 2012).
Adaptationist reasoning—bolstered by cognitive, behavioral, cross-cultural, and neurophysiological evidence (Bugental, 2000; Buss, 1999; Fiske, 1992; Kenrick, Li, & Butner, 2003; Panksepp, 1982; Plutchik, 1980)—suggests that much of human behavior may be organized around a relatively small set of fundamental motives, each linked to a major adaptive challenge posed by ancestral environments. We organize this chapter around seven key motivational domains of social life—coalition formation, self-protection, disease avoidance, status, mate choice, mate retention, and parental care (Kenrick, Griskevicius, Neuberg, & Schaller, 2010; Kenrick, Neuberg, Griskevicius, Becker, & Schaller, 2010). For each domain, we consider some cognitive and behavioral decision rules that may have evolved to allow people to achieve fundamental motives.
For most of human history, our ancestors lived in small, highly interdependent groups (Caporael, 1997; Sedikides & Skowronski, 1997). Successful cooperation among group members greatly increased each person's probability of surviving, prospering, and eventually reproducing. This was particularly true during times of need (e.g., food shortages) (Hill & Hurtado, 1996).
Social psychologists have focused primarily on proximate functions of participating in cooperative alliances. For example, other members of our groups provide us with several valuable types of social support (Reif & Singer, 2000). Emotional support from friends and family is linked to a vast array of positive health outcomes: People with a support network are more resistant to disease, less upset by stressful events, and live longer (e.g., Buunk & Verhoeven, 1991). Second, group members share information, which is especially valuable in uncertain or ambiguous situations (Orive, 1988). Third, group members exchange material goods and resources. Social psychologists have delineated different types of exchange rules (e.g., equity versus communal sharing) that commonly guide patterns of social exchange (e.g., Clark & Chrisman, 1994; Fiske, 1992; Kenrick & Griskevicius, 2013).
Cooperative alliances have costs as well as benefits: It requires time and resources to provide support for others, and raises dangers of being exploited in one-sided exchanges. Alliances also involve direct competition for food, social status, mates, and other resources (Alcock, 2013; Hill & Hurtado, 1996). From an evolutionary perspective, the trade-offs are more favorable to the extent that one forms alliances with others who share one's genes, or who share a history and expected future of reciprocal exchanges.
Social psychologists traditionally focused relatively little attention on the differences between interactions among kin versus nonkin (Daly, Salmon, & Wilson, 1997). However, cooperative relationships in traditional societies have most frequently involved individuals who were genetically related. From the perspective of Inclusive Fitness Theory (Hamilton, 1964), it is easy to see why people align themselves with their kin: Benefits shared with a kin member implies indirect genetic benefits to oneself, and costs exacted on the self by kin are also indirect costs to them (Kenrick, Sundie, & Kurzban, 2008). Consistently, research with humans and other species suggests lower thresholds for engaging in various types of cooperative behavior among neighbors who are closely related (Burnstein, Crandall, & Kitayama, 1994; Essock-Vitale & McGuire, 1985; Neyer & Lang, 2003; Segal & Hershberger, 1999). Indeed, people are more likely to cooperate with someone is if his or her morphological features make the person merely appear to be genetically related (DeBruine, 2002; Rushton, 1989).
Given the importance of investing energy in relationships with kin, why would people form coalitions with nonkin? Theories of reciprocal altruism provide one answer (Axelrod & Hamilton, 1981; Cosmides & Tooby, 1992; Trivers, 1971). According to these theories, our ancestors would have benefited from cooperating with others to the extent that those other people were likely to reciprocate. In this way, each member of a reciprocal exchange relationship reaps benefits over the long term. Indeed, whereas cooperation is less contingent on past history of reciprocation among close kin, sharing between progressively less related individuals becomes more linked to a history of reciprocal sharing (Fiske, 1992; Trivers, 1971).
Because people cannot see into the future, they cooperate with group members based on the probability that those group members will someday reciprocate. Hence, it pays to attend to signs that a member of one's group is not a good bet for future reciprocation or is likely to draw more resources from the group than he or she is willing to give back. Indeed, evidence suggests that people are quite vigilant to potential deceit and evidence of cheating (Cosmides & Tooby, 1992; Delton, Cosmides, Robertson, Guemo, & Tooby, 2012; Mealey, Daood, & Krage, 1996). People are generally more likely to trust friends, and women in particular are generally more cooperative toward friends (Ackerman, Kenrick, & Schaller, 2007). Activating concerns about group competition, though, increases men's cooperativeness with members of their groups (van Vugt, de Cremer, & Jannsen, 2008).
Given the importance of maintaining strong relationships with kin and nonkin, people are very vigilant to cues signaling that they might be at risk for being excluded from their group. Ostracism, rejection, and other forms of social exclusion can be highly aversive (Williams & Nida, 2011).The pain of social exclusion can produce a range of destructive consequences, including depression and aggressive behavior (Twenge & Campbell, 2003). However, when given the opportunity, people who feel rejected also sometimes try hard to reconnect with others, acting prosocially and turning optimistically toward others as sources of social affiliation (Maner, DeWall, Baumeister, & Schaller, 2007). Such responses are consistent with evolutionary logic. Being part of a group has been essential for survival throughout most of human history, and thus the threat of being excluded heightens people's interest in shoring up their bonds with other people. Consistent with this logic, research with hunter-gatherers suggests that people living under ancestral conditions are especially inclined to form alliances with others who cooperate with them, and to exclude those who do not cooperate (Apicella, Marlowe, Fowler, & Christakis, 2012).
Ancestral humans frequently confronted threats from members of other groups (Baer & McEachron, 1982), and through competition over status and material resources, from ingroup members (Daly & Wilson, 1988). Decades of research in social psychology has demonstrated that people often behave aggressively as a means of countering perceived threats (Berkowitz, 1993). People with a defensive attributional style—a tendency to perceive others as threatening—are generally more likely to react aggressively toward others (Dodge, Price, Bachorowski, & Newman, 1990).
Besides increasing aggressive behavior, threats also enhance affiliative motivation in adaptive ways (Mikulincer, Florian, & Hirschberger, 2003; Taylor et al., 2000). For example, thoughts about death lead to increased affiliative tendencies, suggesting that the tendency to affiliate under threat is based in adaptive design: People are safer in groups, and threats lead people to seek the security of a crowd (Wisman & Koole, 2003). Physiological and behavioral evidence suggest that females' primary response to stress is “tend and befriend” rather than “fight or flight.” Fighting or running would have increased risks to dependent offspring (Taylor et al., 2000). Propensities to tend and befriend with group members under threat were also adaptive for males (Geary & Flinn, 2002). As we discuss below, different social consequences of threat-induced affiliative motivation, more than other affiliative goals, may lead to the formation of larger groups (Kenrick et al., 2003).
Self-protective goals lead people to selectively process signals of potential physical threat, especially in males and members of outgroups. People are very quick to detect angry expressions, especially on a man's face (Becker, Kenrick, Neuberg, Blackwell, & Smith, 2007). They are also likely to misperceive men, rather than women, as angry when they are not (Neel, Becker, Neuberg, & Kenrick, 2012). Self-protective goals also elicit cognitive vigilance toward outgroup members. For example, self-protective goals can lead people to see anger in the faces of outgroup members, even when those faces are perceived as neutral in other contexts (Maner et al., 2005). Furthermore, ambient darkness (a context ancestrally associated with increased vulnerability to physical harm) increases people's reports of threat-related prejudices against groups who are heuristically associated with physical danger (Schaller, Park, & Mueller, 2003). Such responses may have evolved to protect humans in particularly precarious contexts.
Exposure to disease-causing pathogens has been a major cause of illness and death throughout human history (Anderson & May, 1991). Consequently, pressures to avoid disease facilitated the evolution of processes that promote avoidance of sources of contagion (Schaller & Park, 2011). Research in evolutionary social psychology suggests that people possess two interdependent systems that help protect them against disease. In addition to a physiological immune system, which is designed to neutralize pathogens that have entered the body, people also possess a behavioral immune system, a psychological-behavioral system that helps people avoid coming into contact with pathogens in the first place (Neuberg, Kenrick, & Schaller, 2011).
A growing literature suggests that when they are concerned about disease, people become highly vigilant to cues in the environment signaling the possible presence of disease (S. L. Miller & Maner, 2012; Mortensen, Becker, Ackerman, Neuberg, & Kenrick, 2010). Some cues, such as rashes and lesions, represent actual physical manifestations of illness. However, the behavioral immune system errs on the side of caution and categorizes as disease cues even some characteristics that are not reliably associated with disease (e.g., old age, obesity). Old age, for example, is associated with a weakened immune system, and thus a higher likelihood of carrying diseases, but the relationship is a weak one, and many older people are quite healthy. When people are concerned about disease, however, they attend carefully to such cues and are biased toward seeing them even when they are not reliably connected with disease (e.g., S. L. Miller & Maner, 2011).
In women, the threat of disease becomes particularly pronounced during the luteal phase of the menstrual cycle (which occurs immediately following ovulation) and during pregnancy. During these periods a woman's body downregulates her natural immune response so as to not treat a growing fetus as an intruding pathogen. To compensate for this temporary immune suppression, women display a host of adaptive responses, such as increased disgust sensitivity, avoidance of potentially dangerous foods, and even avoidance of ethnic outgroup members (Navarrete & Fessler, 2006) who could carry foreign pathogens (see Fincher & Thornhill, 2012).
Social status is a ubiquitous regulator of social interaction (Barkow, 1989; Eibl-Eibesfeldt, 1989). Even in face-to-face interactions between complete strangers, relative status differences emerge quickly and spontaneously, often on the basis of very limited social information (Fisek & Ofshe, 1970). Around the world, “dominant” versus “submissive” is one of the two primary dimensions with which people categorize members of their groups (G. M. White, 1980; Wiggins & Broughton, 1985). Many people are highly motivated to achieve and hold on to positions of high status over others (Maner & Mead, 2010).
Social psychological perspectives have focused on direct consequences of a person's status. Indeed, possessing high status is associated with a variety of important cognitive and emotional outcomes (Keltner, Gruenfeld, & Anderson, 2003). For example, high-status individuals tend to have relatively high self-esteem (Steele, 1988; Tesser, 1988) and be particularly persuasive and successful at influencing the behavior of others (A. G. Miller, Collins, & Brief, 1995). They tend to be desired as friends (Nakao, 1987) and enjoy a relatively great degree of respect and attention, as well as other social benefits and material resources (Cummins, 1998). And, high-status individuals tend to be happier, pay more attention to potential social rewards than punishments, and engage more in automatic cognitive processing (Keltner et al., 2003).
From an evolutionary perspective, males gain an additional set of benefits from status-striving. This follows because females, whose high parental investment predisposes more selectivity about mates, are more likely to use male status as a cue for mate selection (Gould & Gould, 1989; Li, Bailey, Kenrick, & Linsenmeier, 2002; Sadalla, Kenrick, & Vershure, 1987). Dominant men are able to offer their mates relatively greater protection and access to resources, both of which were useful in caring for offspring. Consequently, males are, compared with females, more motivated to seek high levels of social dominance (Hill & Hurtado, 1996) and more likely to pay attention to possible loss of status relative to neighbors (Daly & Wilson, 1988; Gutierres, Kenrick, & Partch, 1999).
Eagly and Wood (1999) argued that differences in status-striving may stem from the male social role's emphasis on power and status, versus the female role's relative emphasis on nurturance. These authors believe that these role assignments for men and women differ across societies because of fundamental evolved differences: Women carry and nurse the offspring, and men are physically larger. Thus, they posit an interaction between evolved mechanisms and the development of cultural norms and are in this way squarely in line with modern evolutionary psychological models of gender role norms (Kenrick, 1987; Kenrick, Trost, & Sundie, 2004; Kenrick & Luce, 2000). However, where they differ from evolutionary models is their claim that evolutionary processes responsible for sex differences ended with the physical differences between men and women. Thus, we believe that Eagly and Wood's biosocial model is too limited and domain general in positing a simple causal link between parental role assignment and various sex differences in social behavior (Kenrick & Li, 2000). It does not, for example, take into account comparative research, including studies suggesting that testosterone is linked to dominance and competitiveness in humans as well as other species (Mazur & Booth, 1998).
Given that differential reproduction is central to natural selection, decisions about mating have significant adaptive consequences. Such decisions can be broadly categorized into two main areas: relationship selection and mate selection.
Individuals in all societies engage in long-term bonding (Daly & Wilson, 1983) and, in many cultures, uncommitted sexual relations (Marshall & Suggs, 1971). Decisions about which type of relationship to enter depend on a person's sociosexual orientation, or tendency to engage in uncommitted sex (Gangestad & Simpson, 2000; Simpson & Gangestad, 1991). Men are more inclined to be sociosexually unrestricted, requiring less commitment, whereas women tend to be restricted, requiring more commitment. Although social psychologists cite differences in social norms for men and women (Eagly & Wood, 1999), evolutionary theorists attribute this difference to differences in minimum obligatory parental investment (Trivers, 1972). Women, like other female mammals, are biologically required to make a much larger investment of time and resources than men if a pregnancy occurs. Thus, the cost-to-benefit ratio of casual sex is higher for women than men, who can invest as little as a teaspoon of seminal fluid. As such, men may have evolved to be more eager than women for casual sex opportunities (Buss & Schmitt, 1993; Kenrick, Groth, Trost, & Sadalla, 1993; Kenrick, Sadalla, Groth, & Trost, 1990; Li & Kenrick, 2006).
However, not all men seek short-term relationships, and not all women seek long-term relationships. There is significant intrasexual variation in sociosexuality, which depends on numerous factors, including one's developmental environment and mate value. For instance, socially dominant (Sadalla et al., 1987) and bilaterally symmetrical men tend to be sexually attractive to women (Thornhill & Gangestad, 1994). Such men start having sex earlier and have more casual sexual partners (Gangestad & Thornhill, 1997; Thornhill & Gangestad, 1994). Women's sociosexuality varies not with their attractiveness but with factors such as their masculinity (Mikach & Bailey, 1999) and father absence (Ellis, 2004), which are both associated with more unrestricted orientations.
A more complete account of sociosexuality should take into consideration how individuals interact with each other (Gangestad & Simpson, 2000). We constructed a dynamical model simulating a network of individuals and found that relatively small sex differences in sociosexual orientation (i.e., decision rule criteria for whether to be restricted or unrestricted) were sufficient to account for observed real-world patterns, where the large majority of people enter long-term relationships, and smaller pockets of people engage in unrestricted relationships. As we discuss later, dynamical models can help psychologists understand how diverse social norms can emerge from small variations in (evolved) decision rules at the individual level (Kenrick et al., 2003).
For marriage partners, women prefer status and resources more than men do, and men prefer physical attractiveness and youth more than women do (Buss, 1989; Kenrick & Keefe, 1992: Sprecher, Sullivan, & Hatfield, 1994). To explain these preferences, social psychologists have invoked sociocultural (Eagly & Wood, 1999) and social exchange models (Hatfield, Utne, & Traupmann, 1979; Homans, 1961). However, a number of features of mate preferences violate simple cultural normative accounts (Kenrick, Gabrielidis, Keefe, & Cornelius, 1996; Kenrick & Gomez-Jacinto, 2014). A more ultimate explanation is that men and women evolved psychological mechanisms to solve different adaptive problems that each sex faced in the ancestral past (Buss, 1989; Symons, 1979). Because female fertility drops off rapidly after 30, men looking for long-term partners may be especially drawn to physical markers of sexual maturity and youth, including full lips, smooth skin (Symons, 1979), and a low waist-to-hip ratio (Singh, 1993). Consistent with this reasoning, teenage boys prefer relatively older females, whereas older men prefer relatively younger females (Kenrick & Keefe, 1992; Kenrick et al., 1996). Male long-term reproductive value, on the other hand, is constrained more by the ability to provide resources (Symons, 1979). Thus, women may be especially attentive to cues relating to a man's status and ability to provide resources (Buss, 1989; Sadalla et al., 1987). Activating mating motives leads men to become more competitive, place higher value on economic gains, and publicly display their creativity and wealth (Griskevicius, Cialdini, & Kenrick, 2006; Griskevicius et al., 2009; Li, Kenrick, Griskevicius, & Neuberg, 2012; Sundie et al., 2011).
Evolved psychological mechanisms for mate selection, however, are more complex than simple preferences for physical attractiveness and status. Because maintaining a long-term relationship and raising offspring are complex tasks, a wide variety of personal qualities are key, including kindness, creativity, energy level, and so on. Nevertheless, in the evolutionary past, infertile females or destitute males, if chosen as partners, would have been reproductive dead ends. To solve this adaptive problem, men may have evolved to prioritize having a moderate level of physical attractiveness (over other traits) in a long-term mate, and women may have evolved to prioritize having a moderate level of social status (Li et al., 2002; Li, Valentine, & Patel, 2011). Once these “necessities” are acquired, then other traits—“luxuries”—are sought after. Indeed, the sex-differentiated prioritization is reflected in people's preferences when considering potential long-term mates (Li et al., 2002) and thinking about themselves as potential long-term mates (Li, 2007). Furthermore, in live-interactive mate-selection contexts, men (more than women) reject partners who are low on physical attractiveness, whereas women (more than men) reject those who are low on social status (Li et al., 2013).
As the intended duration of a relationship decreases, a man's resources become less relevant and the risk of father absence increases. As such, offspring survival, in the ancestral past, became increasingly dependent on a man's genetic quality (Gangestad & Simpson, 2000). Thus, for short-term relationships, women may have evolved to prioritize having moderate physical attractiveness to indicate genetic quality (Li & Kenrick, 2006). For men, women's fertility is even more singular in importance in a short-term context. Thus, men may have evolved to prioritize physical attractiveness even more in short-term mates than long-term mates.
Consistent with this reasoning, numerous studies have found physical attractiveness to be valued more greatly in short-term relationships than long-term ones (Buunk, Dijkstra, Fetchenhauer, & Kenrick, 2002; Regan & Joshi, 2003). Furthermore, both sexes prioritize having physical attractiveness over other traits not only when considering hypothetical potential short-term mates (Li & Kenrick, 2006) but also when encountering and selecting actual ones in live-interactive contexts, where unattractive individuals are shunned by both sexes (Li et al., 2013).
Because human infants are helpless and slow to develop, the continued inputs of both parents are often essential to ensure the offspring's survival (Geary, 1998; Hrdy, 1999). Hence, a key adaptive problem for both sexes is to maintain mating bonds with desirable partners (Buss, 1999; Hazan & Diamond, 2000).
Decisions to maintain or end a relationship involve a dynamic consideration of new information as it becomes available. Relevant information includes the existence of offspring, availability of resources to each parent within and outside the relationship, presence and quality of same-sex interlopers, and the sex ratio of the mating pool (Dijkstra & Buunk, 1998; Guttentag & Secord, 1983; Kenrick & Trost, 1987). Psychological mechanisms may exist to process such inputs and to adaptively weigh the costs and benefits of staying in a relationship. For example, having offspring raises the threshold for leaving a relationship (Essock-Vitale & McGuire, 1985; Rasmussen, 1981). Local availability of desirable alternatives lowers the decision threshold for both sexes (Guttentag & Secord, 1983; Kenrick, Neuberg, Zierk, & Krones, 1994). Also, a partner's breach of fidelity may greatly reduce the benefit-to-cost ratio of staying in the relationship for both sexes (Buss, Larsen, Westen, & Semmelroth, 1992).
People are often highly motivated to guard their long-term partner from same-sex romantic competitors (Buss et al., 1992; Maner, Miller, Rouby, & Galliot, 2009). The threat of intrasexual rivalry evokes strong feelings of jealousy, which is more likely to occur when one perceives one is putting more effort into a relationship than one's partner (G. L. White, 1981) or is singularly dependent on the relationship for happiness (Bringle & Buunk, 1986).
Some evidence suggests sex differences in jealousy such that men are more prone to jealousy when their partner appears to be sexually attracted to others, whereas women are relatively more sensitive to emotional infidelity (Clanton & Smith, 1977). This difference has been attributed to societal norms of men sexually controlling women and women being more attuned to the emotional quality of relationships (G. L. White, 1981).
On the other hand, evolutionary theorists (Buss, 2000; Buss et al., 1992) attribute these differences to factors such as internal gestation and different types of resources contributed to offspring (females contribute bodily resources, males contribute indirect resources such as food and shelter). Because of internal gestation, only females can be certain that offspring carry their genes—males with unfaithful partners risk investing in offspring that are not their own. On the other hand, females confronted with reproductive competitors risk losing all or part of their partners' resources. Thus, jealousy may have evolved to solve each sex's adaptive problem of ensuring their partners' key reproductive resources are not diverted to others (Buss et al., 1992). Though jealous reactions occur in response to actual third-party interference with one's relationship, they also can occur in the absence of infidelity. The traditional view of seemingly unsubstantiated reactions is that they are self-delusional and pathological, but evidence indicates that they often accurately reflect covert infidelity or eventual affairs (Buss, 2000). Thus, jealousy may serve not only to deter ongoing infidelity, but also to preemptively warn of a possible infidelity. Although social psychologists have raised various methodological issues regarding these sex differences, a recent meta-analysis suggests that the sex difference is robust (Sagarin et al., 2012).
There may also be mechanisms to prevent oneself from wandering. Such mechanisms might involve having a positive bias toward one's partner and a negative bias against potential mates. For instance, people who are committed to their partners underrate the desirability of alternative mates, especially when the alternatives are made salient (Johnson & Rusbult, 1989). Compared with people who are not in relationships, those currently in relationships tend to consider viable alternatives as less physically and sexually desirable (Maner, Gailliot, & Miller, 2009; Simpson, Gangestad, & Lerma, 1990). Indeed, evidence shows that those who are committed to and well invested in a relationship are much less attentive to potential alternatives than those who are not committed to and invested in a relationship (R. S. Miller, 1997).
The desire to stay in a relationship is probably not purely altruistic, but ultimately was likely to serve our ancestors' reproductive success. First, couples in the ancestral past who did not stay together very long may not have produced any offspring, or at least ones that survived (Geary, 2000). Second, there is more uncertainty associated with new partners than with existing ones. Thus, from an evolutionary perspective, biases account for the time needed to produce and raise offspring and offset risks associated with starting new relationships.
As noted earlier, parental care has been critical to the survival of human offspring (Geary, 2000; Hrdy, 1999). Yet, motivation to nurture offspring can vary considerably. Evolutionary theorists have posited that decisions about caring for any particular offspring depend on various factors that affect the payoffs for their parental investment (Alexander, 1979; Daly & Wilson, 1980; Hrdy, 1999). In this section, we summarize the underlying evolutionary logic and some research findings associated with the following factors: perceived genetic relatedness to the parent, ability of parental investment to be converted to reproductive success, and opportunity costs.
Because a given offspring shares 50% of each parent's genes, and offspring themselves can someday reproduce, evolution has selected for behaviors that promote the survival and reproductive success of one's offspring. However, consider the following: Fathers tend to invest less in their offspring than mothers do. Maternal grandparents tend to invest more than paternal grandparents do (Laham, Gonsalkorale, & von Hippel, 2005). Compared with biological parents, stepparents invest less in their children and are 40 times more likely to abuse and up to several hundred times more likely to kill them (Daly & Wilson, 1988).
From an evolutionary perspective, these differences in investment reflect differences in genetic relatedness: Women are 100% sure which offspring are theirs, but men face a degree of uncertainty. Thus, investments made by a mother and through the maternal line are more likely to be channeled to genetically related individuals than those made by a father and through the paternal line. And, because investing in other men's offspring is unlikely to be selected for, we can expect the behavior of stepparents toward stepchildren not to be on par with that of biological parents towards their own children.
Parents also prefer investing in offspring who are more likely to channel resources into reproductive success (Alexander, 1979). Compared with normally developing children, those with serious congenital disease have a much lesser likelihood of finding a mate and reproducing. Accordingly, many such children who have Down's syndrome or other serious impediments to mainstream life are sent off to institutions and effectively abandoned (Buss, 2000) or are subjected to high rates of abuse at home (Daly & Wilson, 1980).
Parental investment in male offspring may be riskier than investment in female offspring (Daly & Wilson, 1988; Trivers & Willard, 1973). While there is rarely a shortage of males willing to mate with a female, a male typically needs to compete successfully against other males to get mates. Also, whereas females are physically limited to having children at a relatively slow rate across a shorter reproductive lifespan, males are not constrained by internal gestation and menopause. Rather, male reproductive success differs greatly, ranging from those at the bottom of a status hierarchy with no mates to those at the top, who have been known to sire up to several hundred children (e.g., Betzig, 1992; Daly & Wilson, 1988).
Because of this difference in risk and return, it may pay for a family with abundant resources to invest in sons, but for poor families to allocate what they have to daughters (Trivers & Willard, 1973). In support of this reasoning, a study of families in North America (Gaulin & Robbins, 1991) found that low-income mothers were significantly more likely to breastfeed their daughters than their sons, whereas the opposite pattern was true for mothers of affluent families. Low-income mothers also had another child sooner if the first was a son, whereas high-income mothers had another child sooner if the first was a daughter. In a similar vein, other research suggests that wealthier families leave more of their wealth to their sons, whereas poorer families leave proportionally more to their daughters (Smith, Kish, & Crawford, 1987).
Finally, parental investment makes sense when alternative uses of such time and resources are not more lucrative. For example, because men are not physiologically constrained to childbearing and nursing, extrapair mating is a more viable option to them than it is for women. Indeed, among the Aka pygmies of Africa, high-status men have more wives and spend less time on parenting than low-status men do (Hewlett, 1991). People may also be more willing to pull out of any given investment when their investment time horizon is long versus short. Infanticide records show that women are more likely to kill their infants when the women are younger and when they are unwed with no men acknowledging fatherhood (Daly & Wilson, 1988).
Evolutionary theories, most notably Life History Theory (e.g., Ellis, 2004), suggest that an individual's reproductive timing often is adaptively calibrated to help that individual produce the largest possible number of surviving offspring. At any point in time, a person faces a trade-off between investing in current reproduction (faster life history strategy) versus future reproduction (slower life history strategy). Many factors determine how a person navigates this trade-off. One key factor identified by recent research is a person's childhood socioeconomic status (SES; Griskevicius, Delton, Robertson & Tybur, 2011; Simpson, Griskevicius, Kuo, Sung, & Collins, 2012).
When confronted with circumstances that signal threats to their reproductive potential (e.g., mortality cues or signs of economic uncertainty or instability), people high versus low in childhood SES respond in very different ways. Because their childhood environment signaled that payoffs associated with long-term investments are uncertain and unpredictable, people from low SES backgrounds are sensitized at an early age to the possibility of poor long-term reproductive prospects. Consequently, people from low SES backgrounds tend to respond to forms of threat (e.g., economic threat, stress, or uncertainty) by focusing on short-term gains associated with reproducing quickly (Griskevicius et al., 2011). Individuals from wealthier backgrounds, in contrast, are sensitized to the presence of strong long-term reproductive prospects and learn that those prospects are relatively positive and predictable—that they can afford to invest in building somatic resources now so that they can attract a desirable long-term partner and care for offspring more effectively in the future. People from high SES backgrounds, thus, tend to respond to environmental threat by “weathering the storm”—by focusing more on long-term investments likely to enhance the reproductive quality and welfare of their offspring over the lifespan. Thus, the overall pattern emerging from the literature suggests that salient signs of environmental unpredictability or uncertainty lead adults with a low childhood SES background to speed up their reproductive timing, whereas those same signs of unpredictability or uncertainty lead those with a high childhood SES background to slow down their reproductive timing.
McDougall's evolution-based Social Psychology was actually not the only book with that title published in 1908. In the same year, E. A. Ross (1908) published a text with the same name. But Ross adopted a very different theoretical approach. Ross was a sociologist who saw the wellsprings of social behavior as residing not in the individual but in the social group. He argued that people were carried along on “social currents,” such as “the spread of a lynching spirit through a crowd…[or] an epidemic of religious emotion” (Ross, 1908, pp. 1–2). Ross looked at the group as a whole rather than at the psyche of the individual group member. He viewed crazes and fads as products of “mob mind…that irrational unanimity of interest, feeling, opinion, or deed in a body of communicating individuals, which results from suggestion and imitation” (Ross, 1908, p. 65).
Since 1908, there has been a basic disagreement between sociologically oriented social psychologists who focus on groups as determinants of social behavior and psychologically oriented social psychologists who focus on determinants within the individual. In recent years, advances in theory and research on complex dynamical systems have offered the promise of bridging these formerly discrepant views of the social world (Latané, 1996; Vallacher, Read, & Nowak, 2002). Dynamic models have begun to unravel the formerly mysterious processes by which patterned societal norms emerge from seemingly random interactions between individuals, each acting on the basis of simple and proximately focused decision rules.
We have argued elsewhere that a truly comprehensive model of behavior needs to incorporate insights from evolutionary psychology along with the insights of dynamical systems theory (Kenrick, Li, & Butner, 2003; Kenrick et al., 2002). Consistent with work in evolutionary psychology, the dynamical evolutionary model assumes that human psychological mechanisms can be conceived as a set of conditional decision rules designed to serve fundamental motivations associated with key problem areas confronted by human ancestors. It assumes further, in line with our arguments above, that qualitatively different decision rules are associated with different problem domains, as summarized in Table 39.1. The model further assumes that individuals differ in decision rules as a function of adaptive design and random variation. Men and women, for example, have different rules for choosing mates, and individual men and women may differ from the “average” members of their sex as a function of genetic variation, developmental experiences, or local ecological factors (sex ratios, for example).
Table 39.1 Domains of Adaptive Problems Associated With Social Life in Human Groups, Fundamental Goals, and Possible Examples of Evolved Decision Constraints Associated With Each Domain
| Domain of Social Behavior | Fundamental Goal | Some Evolved Decision Constraints Associated With This Domain |
| Coalition Formation | To form and maintain cooperative alliances. | Cooperation is more likely to the extent that others (a) are close relatives or (b) have shared resources in past. |
| Self-Protection | To protect oneself and alliance members against threats to survival or reproduction. | Male outgroup members are heuristically associated with threat; males are more involved in intra- and intergroup exchanges of threats. |
| Disease Avoidance | To reduce the chances of contracting communicable illnesses. | People will be cautious about unnecessary interactions with (a) people who manifest overt symptoms (coughing, sores) or (b) people from exotic, faraway places. Females will especially avoid strangers during the first trimester of pregnancy, when teratogenic effects of disease are highest. |
| Status | To gain or maintain respect from, and power over, other group members. | The cost-benefit ratio of striving for status is more favorable for males because females emphasize male status in choosing mates. |
| Mate Choice | To obtain a partner or partners who will enhance one's own fitness. | Trade-offs for long vs. short-term relationships differ for men and women and depend on sex ratios; males and females emphasize somewhat different features in long-term mates. |
| Mate Retention | To maintain a mating bond with a desirable partner. | Males will be inclined to break a bond if a partner is sexually unfaithful or if there are physically attractive alternatives available. Females will be inclined to break a bond if a partner compromises resources or if a high-status alternative is available. |
| Parental Care | To promote survival and reproduction of individuals carrying one's genes. | More care is invested in others who share one's genes and who have relatively high reproductive value. |
Consistent with research and theory on dynamical systems, our model assumes that decision mechanisms within given individuals unfold in dynamic interplay with decision mechanisms of others in the social network. As we noted earlier, an individual man's decision rule regarding sexually unrestricted versus restricted behavior interacts with those of other men and women in the vicinity to result in a group-level norm that becomes self-maintaining. The different decision rules operating in each domain lead to different sociospatial social geometries associated with different social goals—hierarchical for status, expansive for self-protection, narrower for cooperation between friends, still narrower for mate choice, and lopsided for parent–child interactions (see Kenrick et al., 2003).
These are exciting times for social psychology. The insights of modern evolution-inspired theory and research have opened up connections with other subdisciplines of psychology and with the other social and life sciences (Kenrick, 2011). The insights added by a dynamical perspective have opened up the possibility of integrating the psychology of the selfish individual and the sociology of the collective self. Although we now have a much better road map for progress, most of the empirical findings that will serve as landmarks on the new intellectual terrain are still awaiting discovery.