Lorne Campbell and Timothy J. Loving
In every known culture, formal marriage arrangements between men and women exist (Brown, 1991; Buss, 1985; Daly & Wilson, 1983). An analysis of 166 societies by Jankowiak and Fischer (1992) concluded that romantic love is found worldwide, and over 90% of people in the world will marry at least once (Buss, 1985). Whereas a large proportion of cultures permit polygyny (i.e., having more than one wife; van den Berghe, 1979), the majority of men in these cultures tend to pair with one partner at a time (Lancaster & Kaplan, 1994). Less than 1% of cultures permit a woman to take more than one husband at a time (i.e., polyandry), and this practice is extremely rare (van den Berghe, 1979). It appears, therefore, that the existence of close affectional bonds in romantic relationships, typically, but not always, involving two people is a universal feature of human existence.
Drawing on the power of evolutionary theory to explain behavior across cultures and species, particularly in the domains of sex and reproduction, evolutionary psychology has emerged as a major perspective in the study of intimate relationships (Fletcher, Simpson, Campbell, & Overall, 2013, 2015). At the same time, intimate relationships have been studied in depth by disciplines not traditionally guided by evolutionary theory (e.g., social psychology, communications, sociology). One common theme across these different literatures is that feelings of love function as a commitment device that joins together intimates in long-term pair bonds. Thus, there is much overlap between the ideas presented across these distinct, albeit related, disciplines. With this overlap in mind, we first discuss social psychological approaches to the study of love and then transition to evolutionary psychological approaches that build on this research by emphasizing possible functions for the existence and experience of love. We end by suggesting other topics of relationship functioning that have been much investigated by traditional psychological approaches but have not been systematically explored through the lens of evolutionary perspectives. It is our belief that there is significant value in exposing mainstream evolutionary psychologists to relevant research in the relationship-science domain more generally, and vice versa, as it is this type of cross-talk that will be most advantageous for spurring mutually beneficial collaborations (Loving & Huston, 2011).
Prior to the 1970s most research on interpersonal relationships focused on identifying factors that increase interpersonal attraction (Berscheid & Walster, 1969). An assumption inherent in this research was that the development of strong feelings for another person and the eventual establishment of a serious romantic relationship begins with initial liking and grows from there. To understand love, therefore, it was believed that we first needed to understand why we begin to like other people. Challenging this assumption, Rubin (1970) conceptualized romantic love as a set of positive thoughts and feelings directed toward opposite-sex peers that could potentially lead to marriage. Liking, in contrast, was conceptualized as having a healthy respect for another person and finding the company of that person very rewarding. Indeed, self-reports of liking and loving on Rubin's newly developed scales designed to tap these different sentiments proved to be only moderately correlated. Liking is, therefore, a part of loving, but Rubin exposed the need for research on the distinctive experience of romantic love.
Over 40 years of research following Rubin's conceptualization has increased our knowledge of love, although the bulk of this research was not guided by the goal of identifying possible (adaptive) functions for love in relationships; rather, this research almost exclusively described the different types of love that may exist between intimates. The culmination of this research identified two types of love that are important for understanding pair bonding behavior in humans: passionate love and companionate love. Passionate love tends to focus the attention and sexual desires of intimates, particularly early in relationships. Companionate love, on the other hand, is presumed to take time to develop and represents strong emotional bonds between intimates.
Passionate love is best described as a state of intense longing for union with another, a feeling that is aroused particularly early in a romantic relationship. When falling in love—a relationship transition characterized if not defined by increasing passion—there is generally a heightened sense of excitement associated with experiencing new activities with a partner. There is also an air of uncertainty in new relationships, of not knowing what the future holds. Obsessively thinking of partners, of when you will be together next, and hoping that the relationship continues indefinitely are all hallmarks of passionate love.
Individuals who experience passionate love, and who spend a lot of time thinking of their partners each day, also have significantly higher circulating levels of hormones and other biological compounds, including cortisol, a stress hormone (Emanuele et al., 2005; Marazziti & Canale, 2004). For example, in a laboratory experiment in which women experiencing passionate love were asked to think of their partners and relationships in detail (e.g., to recall when they met their partners and how they fell in love), individuals exhibited a spike in cortisol that was not observed when participants were asked to think of an opposite sex friend (Loving, Crockett, & Paxson, 2009). Through the process known as gluconeogenesis, cortisol subsequently increases blood sugar which presumably provides the body with energy to confront the instigating stressor (e.g., “fight” or “flight”). Does this mean, therefore, that falling in love is a highly stressful experience that can be detrimental to our health? Not necessarily. In the field of stress and health research “starting a love relationship” and “begin to date” are believed to represent positive forms of stress (Reich & Zautra, 1981). In fact, both positive and negative life events can generate a similar physiological response generally recognized as a “stress” response (e.g., elevated cortisol levels; Rietveld & van Beest, 2007), but the effects of these events on an individual's health outcomes largely depends on the subjective interpretation of those events. Even though falling in love can seem to be experienced as stressful physiologically, it is subjectively perceived as a positive life event that tends to be associated with favourable health outcomes (Brand, Luethi, von Planta, Hatzinger, & Holsboer-Trachsler, 2007). In support of this interpretation, in a recent study both women and men showed acute increases in blood glucose levels following partner-reflection; increases in blood glucose in the partner-reflection condition were associated with concomitant increases in positive affect (Stanton, Campbell, & Loving, 2014). It is perhaps because of the influence of romantic attachments on individuals that even those who have been through a romantic rejection (i.e., unrequited love) continue to experience brain activation in neural reward centers similar to what is seen in those with more successful attachment experiences (Fisher, Brown, Aron, Strong, & Mashek, 2010).
In contrast to passionate love, companionate love is experienced less intensely. It combines feelings of intimacy, commitment, and deep attachment toward others, romantic or otherwise, that occupy an important part of our lives (Walster & Walster, 1978). If you ask individuals to list all the types of love that come to mind, companionate types of love will dominate the list (Fehr & Russell, 1991). Companionate, or friendship-based love, develops across a wide spectrum of important relationships in our lives and is rooted in trust, caring, mutual affection, supportiveness, and friendship, among other things (Fehr, 1988).
Reis and Shaver's (1988) interpersonal process model of intimacy focuses on the role of self-disclosure, or sharing personal information with another person, and how interaction partners respond to such self-disclosures, in the development and maintenance of intimacy. According to this perspective, early relationship self-disclosure alone is not sufficient for intimacy to grow. An additional process crucial to building intimacy is the perception that the relationship partner (e.g., friend, sibling, romantic partner) reacts to the self-disclosure with a warm and sympathetic response that indicates a positive evaluation of the content of the disclosure. This response, in turn, should make the discloser feel validated, understood, and cared for, setting the stage for increasing levels of connectedness and intimacy to develop within the relationship. Feeling close and intimate with someone is based at least in part, then, on how close and intimate you perceive that person feels toward you (see also Reis, 2007). Laurenceau, Feldman Barrett, and Pietromonaco (1998) asked individuals to report on interactions lasting more than 10 minutes they had with others each day for a 1- or 2-week period to see how intimacy develops over short periods of time. Consistent with Reis and Shaver's model, participants felt closer and more intimate with interaction partners when their interactions involved more self- and partner-disclosure and when the participants felt that their interaction partners responded positively to their self-disclosures.
Romantic relationships often contain a mix of both passionate and companionate love, but the absence of companionate love in particular can spell trouble for the stability of relationships. John Gottman has been studying the predictors of marital success and failure for many years, and his observations led him to conclude that a solid friendship between spouses is the strongest possible foundation for successful marriages (1999). For example, in samples of both older married couples and dating couples recruited from a university population, Grote and Frieze (1994) observed that overall relationship satisfaction in both samples was more strongly related to perceptions of greater companionate compared to passionate love. Therefore, even though sexuality is an integral part of most romantic relationships, and societal norms emphasize marriage as the main dyadic relationship within which sex occurs (Sprecher, Christopher, & Cate, 2006), developing a strong friendship with romantic partners may ultimately be more important for the long-term success of the relationship than is hot sex. Evidence of this assertion comes from additional fMRI work on couples whose long-term relationships continue to be characterized by relatively intense levels of passionate love (which may describe upward of 40% of long-term married couples; O'Leary, Acevedo, Aron, Huddy, & Mashek, 2012). Acevedo, Aron, Fisher, and Brown (2012) asked 17 individuals who were married on average 21 years to undergo the same general procedure utilized in the fMRI passionate love studies of new relationships. As with those experiencing the rush that accompanies falling in love, those in long-term marriages also showed activation in dopamine-rich reward centers of the brain when viewing pictures of their spouses. But, interestingly, this long-term married sample also showed activation in oxytocin- and vasopressin-rich areas of the brain as well, the very same regions identified as linked to long-term pair bonds in other species, as well as activation in the globus pallidus, a brain area the authors note is implicated in general liking of reward sources.
Romantic love contains both passionate and companionate types of love. Whereas passionate love may be a force directing attention to a particular partner, particularly in the early stages of relationship formation, companionate love may play a critical role in keeping partners together over relatively long periods of time. Although the body of research reviewed earlier was not guided by an evolutionary perspective, and did not assume particular functions for love in relationships, it does suggest love as being largely responsible for drawing partners together initially and keeping them together over time (Le, Dove, Agnew, Korn, & Mutso, 2010).
Consistent with these social psychological approaches to the study of love, some evolutionary perspectives explicitly view the role of love as drawing partners together and keeping them together over time (i.e., pair bonding). For example, many theorists have posited that romantic love evolved as a commitment device to maintain relational bonds between mothers and fathers and to facilitate mutual investment in offspring (e.g., Frank, 1988; Kirkpatrick, 1998; Mellen, 1981). Over evolutionary time, increased infant dependency placed greater burdens on human mothers and increased the value of paternal support in feeding and protecting young. Given that men have a genetic interest in the survival of their offspring, fathers were able to benefit reproductively by forming committed, investing relationships that would have reliably increased the probability of offspring survival (e.g., Barash, 1977; Fisher, 1998; Kenrick & Trost, 1997).
The formation of pair bonds, therefore, should translate into fitness, and an excellent review of the literature on paternal investment by Geary (2000) provides a great deal of evidence in support of this claim (but see Sear & Mace, 2008). For instance, paternal investment in the form of pair bonds has been linked in pre-industrial times with increased infant health and decreased infant mortality (e.g., Hed, 1987), not only because a working father allowed a mother to spend more time with a young infant that required breastfeeding (Reid, 1997) but also because a couple with a working father enjoyed a relatively higher socioeconomic status (SES) and thus was able to provide better food and shelter for his offspring (Schulz, 1991). Paternal investment is also related to improved social competitiveness for children, such as higher SES in adulthood (e.g., Kaplan, Lancaster, & Anderson, 1998) and increased educational achievement for adolescents (e.g., Amato & Keith, 1991). Children born and raised within pair bonds have, therefore, been more likely to survive to reproductive age and to be more socially competitive later in life when they are attempting to attract mates (Geary, 2000).
Among the biological substrates linked to pair formation and paternal investment, testosterone has been implicated as a key regulator of both processes. Just as men show an increase in testosterone in competitive settings (e.g., Van der Meij, Buunk, Almela, & Salvador, 2010), men's sexual interest in women is positively associated with testosterone levels (but only after habituation to sexual stimuli; Rupp & Wallen, 2007). These findings replicate conceptually in more naturalistic settings. For example, in a study by van der Meij, Buunk, van de Sande, and Salvador (2008) heterosexual male's testosterone levels increased significantly within 5 minutes of interacting with a woman (but not after interacting with a man). Other work demonstrates similar effects (Roney, Lukaszewski, & Simmons, 2007) and indicates that women rate men as more outgoing to the extent that the men's testosterone increases. Roney and colleagues suggest such testosterone increases in these male-female interaction paradigms may motivate behaviors in males that promote attracting a mate, particularly a mate with more feminine features (e.g., Welling et al., 2008).
Whereas high testosterone appears to motivate men to seek and attract mates (and compete with others males for said mates), such efforts and competitive orientations could presumably undermine relationship stability (e.g., via increased extrapair copulation attempts) and eventual demands of the fatherhood context (i.e., nurturance versus dominance striving). Indeed, a number of studies report lower levels of testosterone in paired (i.e., those in a committed, romantic relationship) versus single men (e.g., Burnham et al., 2003; van Anders & Watson, 2007). Higher testosterone is also associated with a lower likelihood of partnering or marriage (Booth & Dabbs, 1993; van Anders & Watson, 2006), as well as decreases in relationship quality and increases in divorce (Booth & Dabbs, 1993).
More recent work suggests that sociosexuality and sexual frequency moderate the link between relationship status and testosterone in men (Edelstein, Chopik, & Kean, 2011; Maestripieri, Klimczuk, Traficonte, & Wilson, 2014; see also van Anders & Goldey, 2010). Importantly, such effects extend across the transition to parenthood. Fathers typically have lower levels of testosterone relative to non-fathers, and this pattern is especially strong for men that demonstrate stronger commitments to their partners or to parenting more generally (e.g., Gray, Kahlenberg, Barrett, Lipson, & Ellison, 2002; Jasienska, Jasienski, & Ellison, 2012; Kuzawa, Gettler, Muller, McDade, & Feranil, 2009; but see Mazur, 2014). Collectively, these lines of work indicate that elevated testosterone (within and between subjects) promotes pair formation attempts, whereas lower testosterone, typically observed in long-term pair bond relationships, promotes relationship stability and investment in offspring, at least until offspring reach reproductive age (although the exact causal direction of this link is unclear).
The prevalence of sexually transmitted diseases (STDs) may have created another selection pressure for the formation of long-term pair bonds. At least 50 STDs have been documented, ranging from viruses, bacteria, fungi, protozoa, and ectoparasites, and it is estimated that nearly 20 million new sexually transmitted infections occur each year in the United States, particularly within the 15- to 24- year-old age group (see Centers for Disease Control and Prevention, 2002). Although many of these STDs have been recently introduced to humans (e.g., AIDS), Mackey and Immerman (2000) suggest that humans have been vulnerable to these types of diseases over evolutionary history. The fertility of women in particular is severely compromised when they contract an STD, and oftentimes the disease can spread to the fetus, or to the infant as he or she passes through the birth canal. For example, women with syphilis have a heightened risk of miscarriage, premature delivery, stillbirth, and infant death, and, if left untreated, the chances that the fetus will contract the disease are almost 100% (e.g., Schulz, Murphy, Patamasucon, & Meheus, 1990). The strongest predictor of contracting STDs is the number of sexual partners (e.g., Moore & Cates, 1990); therefore, the best way to limit the risk of contracting a disease that could have lethal effects on reproductive success is to limit one's number of lifetime sexual partners. Because women are much more susceptible than men are to contracting STDs (e.g., Glynn et al., 2001; Moore & Cates, 1990), ancestral women would have differentially benefited from a more restrictive attitude toward uncommitted sex. If more inhibited women contracted less STDs and experienced greater reproductive success, they would have been more attractive as long-term mates, thus selecting for the proclivity to desire fewer sexual partners. In short, STDs may have been an important factor in the development of pair bonds over evolutionary history by enhancing the benefits associated with sexual exclusivity and increasing the reproductive success of both men and women (Mackey & Immerman, 2000).
In light of the clear evolutionary benefits conferred by long-term pair bonding, one would expect that humans have developed specific psychological and biological characteristics that promote the development and maintenance of pair bonds. Fisher posits that mating behaviors are guided by three distinct emotion systems—lust, attraction, and attachment—and that behaviors related to each set of emotions are governed by a unique set of neural activities (Fisher, 1998, 2000). The lust system is proposed to motivate individuals to locate sexual opportunities and is mainly associated with estrogens and androgens in the brain. The attraction system directs individuals' attention toward specific mates, makes people crave emotional union with these targets, and is associated with high levels of dopamine and norepinephrine and low levels of serotonin in the brain (i.e., passionate love). The attachment system is distinguished by the maintenance of close proximity, feelings of comfort and security, and feelings of emotional dependency (i.e., companionate love), and is associated with oxytocin and vasopressin (Carter, 1998; Insel, Winslow, Wang, & Young, 1998). Additionally, when both men and women who are deeply in love are asked to think of their partners while their brain is being scanned, regions of the brain that are associated with reward become activated, whereas they do not become activated when thinking of an acquaintance (Bartels & Zeki, 2000).
Interestingly, Fisher's attraction and attachment systems are conceptually similar to features of Bowlby's attachment theory (1969). Bowlby proposed that the process of evolution by natural selection equipped infants with a repertoire of behaviors that serve to facilitate proximity to caregivers, particularly in situations when support is required, and that these behaviors are essential for survival. Bowlby believed that the bond forged between mother and infant in childhood provides a cognitive and affective foundation for later attachments, and that as adults the attachment system serves an affect-regulation function similar to what is seen in infancy. Zeifman and Hazan (1997; see also Shaver, Hazan, & Bradshaw, 1988) have proposed that attachment is one of the psychological mechanisms that evolved to solve the adaptive problem of keeping parents together to raise offspring. The secure feelings that partners experience in each other's presence, the lonely feelings experienced while they are apart, and the desire to be together after separations are all emotional hallmarks of the attachment system and serve to keep people together in committed relationships. The hormone oxytocin plays a central role in the formation of attachment bonds between mother and infant (see Hrdy, 1999, for a review), as well as between romantic partners (Carter, 1992; Behnia et al., 2014), suggesting a mechanism that functions to promote attachments at all stages of life.
Gonzaga, Keltner, Londahl, and Smith (2001) also argued that feelings of love promote commitment to one's partner. Specifically, they proposed two psychological mechanisms that link love to commitment. First, the experience of love motivates individuals to approach their intimate partners and thus to move away from tempting alternative partners (e.g., in response to activation of the brain's reward centers). Second, the expression of love in various forms (e.g., telling your partner you love him or her, providing help and support for your partner, or gazing into your partner's eyes and smiling), communicates that you are committed to your partner and the relationship you have formed together, which serves to further strengthen the bond between individuals. Testing these ideas, the researchers asked both partners of a number of couples to answer some questions about their relationships and to engage in some videotaped laboratory interactions. Individuals reporting more love for their partners also reported desiring their partners more, were relatively happier with their relationships, spent more time in the physical presence of their partners, and engaged in a number of unique behaviors while interacting with their partners. Interestingly, these individuals were also observed to be particularly likely to nod their heads in agreement while talking to their partners and to exhibit Duchenne smiles (i.e., a type of smile that uses the orbicularis and zygomatic major muscles and is associated with positive emotional states and is almost impossible to fake). Both of these nonverbal displays reflect spontaneous behaviors linked with positive interpersonal interaction and convey important information to partners and others. In fact, when an independent group of raters was asked to watch the soundless videotaped interactions between couple members, the raters were able to accurately determine which individuals felt more love for their partners simply by observing the expression of nonverbal displays of love (i.e., head nods and Duchenne smiles). It appears, therefore, that intimates can decipher the amount of love felt for each other by observing each other's nonverbal behavior while they interact. Moreover, these findings suggest that humans are capable of tuning in to such displays even when they have no vested interest in the status and or outcome of specific relationships, providing preliminary evidence that identification of love relationships, so to speak, is a natural (and presumably advantageous) aspect of social nature.
Love is correlated with self-reports and interpersonal behaviors associated with high levels of commitment, but as already discussed there are likely biological markers of love and commitment. Addressing this possibility further, Gonzaga, Turner, Keltner, Campos, and Altemus (2006) measured the amount of oxytocin in the blood of a number of women after they had recounted positive emotional experiences regarding love or infatuation (study 2). Oxytocin is linked to commitment and long-term pair bonding; when released, it evokes feelings of contentment, reductions in anxiety, and feelings of calmness and security around a mate. The women in this study were also videotaped while reliving their positive emotional experiences, allowing the researchers to measure the degree to which women spontaneously displayed nonverbal signs of love and affiliation (i.e., head nods and Duchenne smiles) when thinking of their partners. Consistent with prior research, women reporting more love for their partners were observed to display more head nods and Duchenne smiles. The expression of these behaviors was significantly associated with higher levels of oxytocin in the blood. Peripheral oxytocin was not associated with displays of sexual cues (e.g., lip licks) or self-reports of love. The former finding is particularly interesting as it is in line with Fisher's distinction between the lust and attachment systems. This is the first research with humans demonstrating a link between behavioral and biological cues of love and commitment.
Perhaps the biggest threat to the love and commitment people feel toward their current partners is the presence of desirable alternative partners. In modern society, individuals are exposed to myriad attractive potential partners on a daily basis, including television, magazines, the Internet, and of course in person. Does this exposure to attractive alternatives to a current relationship partner undermine feelings of love for that partner? Or, do feelings of love for a partner somehow protect individuals from the lure of attractive alternatives?
Testing these competing possibilities, Kenrick, Gutierres, and Goldberg (1989) asked a sample of men to view a series of Playboy centerfolds (physically attractive nude women), and a sample of women to view a series of Playgirl centerfolds (physically attractive nude men). After viewing the nude photos of beautiful women, men reported being less attracted to their current partners; however, a parallel effect did not emerge for women viewing Playgirl centerfolds. Additional research by Kenrick, Neuberg, Zierk, and Krones (1994) that had romantically involved men and women view profiles of opposite sex others that varied on physical attractiveness and social dominance continued to demonstrate that men rated their current relationships less favorably after being exposed to physically attractive, but not socially dominant, profiles. Women, on the other hand, reported less positive feelings toward their partners after viewing a number of profiles of men described as being high in social dominance. Overall, people tend to feel less positively, including less love, for their current partners after exposure to attractive alternative partners.
A logical question to ask, therefore, is how do individuals in satisfying and committed relationships maintain their affections for one another in a world filled with attractive alternatives? Love may direct our attention to the target of our affections, but does it direct our attention away from potentially appealing others as suggested by Gonzaga and colleagues (2001)?
Researchers have identified at least two psychological processes that serve to buffer established relationships from the lure of attractive alternatives. First, individuals in established relationships, compared to their less committed or single compatriots, tend to perceive attractive opposite sex individuals as less appealing. For example, in a series of studies, Johnson and Rusbult (1989) reported that individuals more committed to their current partners and relationships were more likely to derogate (i.e., put down or devalue) potential alternative partners on a number of traits (e.g., intelligence, sense of humor, faithfulness, dependability). People feeling less committed to their current partners and relationships, however, not only failed to derogate potential alternative partners, but they were also less likely to remain in their relationships over time. Simpson, Gangestad, and Lerma (1990) had samples of dating and single individuals review a number of magazine advertisements that ostensibly were being considered for use in a marketing campaign. Of the 16 advertisements presented to participants, only 6 of them contained pictures of attractive opposite sex models. After viewing each advertisement, participants were asked to rate the physical and sexual attractiveness of each model. Both men and women involved in dating relationships rated the physical attractiveness of the opposite sex individuals in the advertisements much less positively than did single participants. In other research testing individual's motivation to derogate the attractiveness of potential alternative partners, participants in committed relationships were led to believe that an attractive opposite sex individual was attracted to them, thus providing the participants realistic alternatives to their current partners (Lydon, Meana, Sepinwall, Richards, & Mayman, 1999). Those in committed relationships, however, subsequently rated the potential alternative partner as less attractive, presumably to defuse the threat posed by having a realistic alternative to approach. Perceiving potential opposite sex alternative partners as less appealing, therefore, helps individuals maintain the love and commitment they feel toward their current partners.
Second, people in established relationships do not always have to go through the process of devaluing the attractiveness of opposite sex alternatives. Instead, they can simply not pay attention to attractive opposite sex individuals in the first place. For example, Miller (1997) asked dating participants to inspect an array of photographs presented on a screen via a slide projector. Included in the series of slides were pictures of physically attractive members of the opposite sex. Participants controlled the amount of time they spent viewing each picture with a remote control, and the viewing time for each photo was privately recorded by the experimenter. Miller found that individuals that reported being more satisfied and committed to their partners reported spending less time viewing the photos of attractive opposite sex pictures, and, indeed, they were observed to click through the pictures of attractive others more quickly than other photos. Interestingly, spending less time viewing the attractive opposite sex photos was also linked with a lower likelihood of the relationship ending at a 2-month follow-up. Similarly, research by Gonzaga, Haselton, Smurda, Davies, and Poore (2008) established that relatively strong feelings of love for a partner can assist individuals in suppressing thoughts and feelings they may have for attractive alternative partners. In their study, participants were asked to select the picture of an opposite sex individual out of an array of pictures that they found most appealing, and then to write a short essay on (a) why the person in the picture was attractive, and (b) the ideal way to be introduced to this person. Participants were then randomly assigned to write an essay about their love for their current partner, their sexual desire for their current partner, or their current stream of consciousness. Only participants induced to feel love for their partners reported fewer thoughts of the attractive alternative partner during a subsequent task (again providing support for Fisher's three-system model). Feelings of love for a partner, therefore, can reduce temptation to view images of attractive alternative partners as well as ruminate or daydream about the allure of alternative mates.
Does being inattentive to potential alternative partners have to be a conscious choice made by people in loving, committed relationships? Or can these “decisions” be made automatically, thus requiring little if any deliberation? To address this question, Maner, Gailliot, and Miller (2009) used what is known as an implicit measure of (in)attention to alternatives. Maner and colleagues studied participants who were either single or currently in a relationship. They were asked to view a computer screen, and, unbeknownst to them, a word (i.e., prime) either directly related to mating goals (e.g., kiss), or relatively neutral in nature (e.g., floor), was presented under conscious awareness. A picture of an attractive opposite sex individual would then appear in one quadrant of the computer screen for a brief period of time in the trials of interests. After the picture was removed from the screen, a categorization object (a circle or square) would appear on the opposite side of the screen (called the “attention shift trials”). The participants then needed to press a button as fast as possible to categorize the picture as a circle or square. As predicted, participants in committed relationships were able to complete attention shift trials much faster than single individuals, but only when those in relationships were primed with mating-relevant words. The authors suggested that being exposed to mating-relevant words primed the importance of the relationship to the dating participants, thus making them pay less attention to the attractive photo and enabling them to shift their attention away from the photo very quickly during the categorization task. The same pattern of results, using a different method of priming thoughts of romantic love, was also reported by Maner, Rouby, and Gonzaga (2008).
The importance of these studies is that they convincingly demonstrate that individuals can reduce their attention to attractive alternatives at a very early stage of visual processing. That is, inattention to attractive alternatives can be an automatic process for individuals in established relationships, helping them maintain feelings of love and commitment for their partners. In fact, the process may be so automatic that disrupting the process, unbeknownst to subjects, results in greater attention to alternatives. DeWall, Maner, Deckman, and Rouby (2011) demonstrated this paradoxical effect in three experiments in which they interfered with participants' abilities to freely limit their attention to alternatives. Over a series of trials, participants were presented with two faces simultaneously, one more attractive than the other, outside of conscious awareness. Participants were subsequently asked to type a letter that appeared on the screen where one of the two faces had appeared. In one condition, participants were consistently asked to type a letter that appeared where the less attractive face had been whereas participants in the other condition typed a letter that appeared randomly across trials over one of the two faces. Afterwards, participants indicated their degree of satisfaction and commitment toward their relationships as well as their general attitudes about infidelity. Participants in the former condition, what the research team referred to as “attention limiting,” subsequently reported lower relationship satisfaction and commitment and more positive attitudes toward infidelity (study 1) as well as better memory for the attractive faces (study 2). In other words, implicitly limiting the attention individuals can give to alternatives actually causes them to attend to and remember those very same attractive alternatives better and undermines their own relationship evaluations. Interestingly, when attention is left unabated and individuals are given time to closely attend to attractive others, attached women remember an encountered attractive face, but remember the face as being less attractive than it actually was (Karremans, Dotsch, & Corneille, 2011). Collectively these studies suggest that we have evolved complex cognitive processes designed to maintain and promote romantic connections in the face of possible alternatives.
For successful mate retention to occur, however, individuals must avoid tempting alternatives and engage in mate-retention strategies in contexts that actually provide attractive alternatives to their partners. For example, Buss and Shackelford (1997) suggested that situations more closely aligned with the relationship goals of men and women should be related to their mate-retention behaviors. For instance, men and women rely on different qualities of their partners to aid in their own reproductive success. Men are capable of producing sperm from puberty until well into old age, whereas women are born with a limited number of ovum that can be fertilized only during a circumscribed period of time, with fertility peaking in the mid-20s and decreasing significantly over time to essentially zero in the later 40s. Younger women are, therefore, more reproductively valuable relative to older women. Also, physical features related to increased fertility (e.g., low waist-to-hip ratio, Singh 1993) are rated as universally attractive to men (Buss, 1989; Symons, 1979), making physical attractiveness—in addition to age—another component of women's mate value. Younger, more physically attractive women are more desirable mates because of their increased fertility, but are also more attractive to potential “mate poachers” who may attempt to woo them into extrapair copulations, or to leave their partner. Men married to women higher in mate value (i.e., younger, more physically attractive women), should, therefore, devote more time to mate-retention behaviors.
Men's mate value as long-term partners, on the other hand, rests largely on their ability and willingness to provide external resources to the partner and relationship (Buss, 1989). Men that possess many resources, or have the ability to acquire resources, and are more willing to share these resources (Graziano, Jensen-Campbell, Todd, & Finch, 1997), should be more desirable as mates, and may be the target of “mate poaching” tactics of other women. Women married to men with more resources should, therefore, devote more time to mate-retention behaviors.
Buss and Shackelford (1997) tested these hypotheses with a sample 107 married couples. Participants completed Buss' (1988) scale of mate retention that contains 19 different mate-retention acts, as well as various other measures associated with the perceived mate value of partners, and satisfaction with their marital relationship. In general, men, relative to women, reported using resource display as a mate-retention tactic; women, on the other hand, reported using appearance enhancement more as a mate-retention tactic than did their husbands. Importantly, men's use of mate-retention tactics was strongly related to the youth and perceived physical attractiveness of their partners, whereas women's mate-retention behaviors were weakly related to their husband's age and perceived physical attractiveness. However, women's mate-retention behaviors were positively correlated with their husbands' income and husbands' reported status striving behaviors, whereas men's mate-retention behaviors were not related to their wives' income or status striving behaviors. Confirming predictions, men and women appeared more motivated to maintain their relationships, and thus prevent the interference of their relationship goals, when they had partners that possessed the qualities most closely aligned with the success of their relationship goals.
The experience of jealousy is a negative emotional experience resulting from the potential loss of valued relationships to real or imagined rivals (Salovey, 1991) that is closely related to the enactment of mate-retention behaviors. Buss (2000) suggests that, over evolutionary history, individuals that were vigilant to interlopers experienced greater reproductive success compared to those who were less concerned about rivals. If jealousy has played an important role in the evolution of human relationships, it should be a universal human emotion. Indeed, that appears to be the case (Buss et al., 1999). Additionally, men and women do not differ in the frequency or intensity of their jealousy (e.g., Buss, 2000; Buunk, 1995; Shackelford, LeBlanc, & Drass, 2000), suggesting that jealousy has played (and continues to play) an important role in the retention of partners and relationships for both sexes.
There are differences, however, between men and women in their experiences of jealousy, and these differences neatly overlap with their different goals in relationships. Whereas women can be confident that they are in fact the mothers of their children, men cannot be certain that they are the father. Paternity uncertainty should make men more sensitive to cues of sexual infidelity of their partners, and wary of rivals that are friendly or flirtatious with their partners (Symons, 1979). Natural selection may have even favored men who have a low threshold to cues of sexual infidelity, as the benefits of being cautious outweigh the costs of not being cautious enough (e.g., Haselton & Buss, 2000). Although maternity uncertainty has not been an issue for women, securing the resources necessary to raise highly dependent offspring was a challenge for ancestral women. The ability to raise offspring to reproductive age would be severely compromised if paternal investment were to be directed elsewhere, and, therefore, women should be sensitive to cues indicating emotional infidelity of their partners. If a man “falls in love” with another woman and subsequently leaves the relationship to form another, his resources will be largely directed away from the abandoned woman. Natural selection may have, therefore, favored women who underestimate the amount of commitment men have to relationships, and are particularly sensitive to signals that their partners are forming emotional bonds with other women (Haselton & Buss, 2000).
To date, a great deal of research supports the notion that men's jealousy is particularly responsive to cues of sexual infidelity, whereas women's jealousy is principally related to cues of emotional infidelity (for a review, see Buss, 2000; but see DeSteno, Bartlett, Braverman, & Salovey, 2002, and Harris, 2003, for challenges to these data). For example, Buss, Larsen, Westen, and Semmelroth (1992) asked men and women to imagine a close romantic relationship, and then to imagine the partner becoming involved with someone else. When asked what sort of involvement would bother them the most, men selected imagining their partner enjoying passionate sexual intercourse with another person, whereas women selected imagining their partner forming a deep emotional attachment to another person. This basic pattern of effects was replicated with physiological data showing that men displayed greater electrodermal activity (EDA) and increased pulse, as well as greater muscular tensions measured by EMG activity of the corrugator supercilii muscle (a muscle associated with “furrowing” of the brow, and expressing negative emotion), when imagining a partner's sexual relative to emotional infidelity, whereas the pattern was reversed for women. Men also report more difficulty in forgiving a sexual infidelity than did women; men also report a greater likelihood of ending a relationship following a partner's sexual rather than emotional infidelity that did women (Shackelford, Buss, & Bennett, 2002). Further, evidence from cross-cultural research suggests that husbands are more likely to divorce wives who have engaged in sexual infidelities, whereas wives are less likely to divorce husbands who have engaged in similar behaviors (Betzig, 1989). This general pattern of results is not surprising given that men's relationship goals center on directing resources to their own, and not somebody else's, children, whereas women's relationship goals center on retaining resources to direct to offspring.
Overall, there seems to be a great deal of empirical evidence—spanning cognitions, behaviors, and physiology—for the argument that love is a commitment device that brings intimates together and helps keep them together for relatively long periods of time (see Fletcher et al., 2013, 2015). Interestingly, this evidence is consistent regardless of the theoretical perspective guiding the research (e.g., social psychological theories focusing on proximate causation, or evolutionary perspectives focusing on ultimate causation, of behavior). Whereas the social psychological research reviewed has done an excellent job of describing different types of love and discovering the importance of love in the development of romantic relationships, the evolutionary psychology research reviewed has played a critical role in understanding the etiology and functions of love and pair bonding in humans. Considered collectively, a more holistic view of love in human romantic relationships is achieved.
Where do we go from here? A large number of topics of investigation in the field of relationship science (e.g., relationship maintenance, conflict resolution, sexual intimacy, dissolution, attributions, interdependence) are the focus of relatively less research guided by evolutionary theory (but see Meltzer, McNulty, Jackson, & Karney, 2014, for recent research on marital satisfaction over time and wives' physical attractiveness). As one example, research on links between passionate and companionate love do not make predictions regarding specific types of “shared novel activities” that should heighten passion. It is quite possible that novel activities that have served important adaptive functions are particularly profound inducers of passion. Additionally, research on conflict in romantic relationships typically focuses on how to best manage conflict and on ways to “fight fairly,” but does not always make specific predictions regarding issues that are likely to be the target of difficult conflicts. Further, we still need to do more work on identifying exactly what specific hormone changes that accompany relationship formation and maintenance actually mean and whether such changes involve neuroendocrine systems implicated in older versus newer neurological structures. These are areas of research in which evolutionary perspectives can assist in making more fine-grained hypotheses and provide more nuanced interpretations of empirical findings. Given the large body of existing research on these topics, there are, therefore, many opportunities for relationship research to view these topics through an evolutionary lens, as was done with the topics of love and commitment.