David M.Buss
The study of human mating strategies must surely count as one of the first empirical success stories in evolutionary psychology. The conceptual foundations of human mating can be traced to Darwin's monumentally important theory of sexual selection, which identified intrasexual competition and preferential mate choice as key processes in the evolution of mating adaptations (Darwin, 1871). Although largely ignored by biologists for many decades, sexual selection theory was given new life by Robert Trivers a century later with his seminal 1972 paper, “Parental Investment and Sexual Selection,” in which he identified relative parental investment as a driving force behind the two components of the process of sexual selection.
The next critical watershed in the study of human mating strategies was the publication in 1979 of Donald Symons's trenchant classic, The Evolution of Human Sexuality. Many of the foundations of human mating strategies described in this section owe a great debt to Donald Symons. He was the first to articulate the theoretical foundations of a fully adaptationist view of male and female mating minds, arguing that they should be no less dimorphic than male and female bodies. Symons was the first social scientist to take the writings of George C. Williams (1966) to heart, applying rigorous standards for invoking the onerous concept adaptation. Indeed, although evolutionary psychologists are often accused of being “hyperadaptationist,” Symons argued forcefully that certain aspects of human sexuality failed to meet the criteria needed to invoke adaptation, and were therefore likely to be by-products. Symons's 1979 book is regarded, by some, as the first major treatise on evolutionary psychology proper, highlighting the centrality of psychological mechanisms as adaptations, and using human sexuality as a detailed vehicle for this more general argument. Hence, it is a great intellectual treat to have an original essay by Symons on adaptationism and human mating psychology, with an illustration using the fascinating phenomenon of “mating anxiety.”
David Schmitt (Chapter 11) furnishes a broad and insightful overview of the foundations of human mating strategies. He considers the large menu of evolved human mating strategies, and outlines the evolutionary processes of sexual selection by which they evolved. He then proceeds to review the ways in which human mating strategies are highly sex differentiated and exquisitely sensitive to context, in particular the temporal dimension of short-term and long-term mating, as proposed by Sexual Strategies Theory (Buss & Schmitt, 1993). He then discusses individual differences in mating strategies within sex. Finally, based on his own massive cross-cultural project and the prior work of others, he discusses the ways in which culture and ecology predictably affect the activation of human mating strategies from the universal menu.
Lawrence Sugiyama (Chapter 12) provides a comprehensive, up-to-date, and penetrating update of his original chapter on the evolutionary psychology of attractiveness. Conceptually, he locates the study of attractiveness within a broader framework of relationship value, including mate value, coalition value, and kin value. He provides the most compelling arguments to date for why attractiveness is important in all social relationships, not merely mating relationships. Some elements, such as cues to health, are important components of social value across relationship types. Others are specific to mate value, and some of these differ for males and females. Sugiyama summarizes the voluminous empirical evidence on specific attributes that contribute to our standards of attractiveness, including skin condition, hair, symmetry, waist-to-hip ratio, and many others.
David Puts (Chapter 13) provides an entirely new chapter on a key topic that fills an important gap in the first Handbook: contest competition. In logical fashion, Puts reviews the empirical evidence for special design in humans for contest competition. This includes the design of the human body (e.g., size, strength, nature of muscle fibers, and visual and acoustic signals) and the human mind (e.g., behavioral and psychological) that all point to a deep evolutionary history of human contest competition, both dyadic and coalitional. This chapter fills a critical gap in sexual selection theory applied to humans, which has historically focused primarily on the preferential mate choice component. Both components of sexual selection are clearly important.
Steven Gangestad, Randy Thornhill, and Christine Garver-Apgar (Chapter 14) provide a chapter on adaptations to ovulation—a long ignored, but now burgeoning, area of theoretical and empirical analysis. They place the study of adaptations to ovulation within the broader theoretical context of sexually antagonistic coevolution. Gangestad and his coauthors then discuss the theories and empirical evidence for the evolution of relatively concealed ovulation and extended female sexual receptivity across the menstrual cycle. This establishes the groundwork for conflicts of interest, the evolution of female infidelity, and cyclic changes in female mate preferences and sexual interests. This chapter, highlighting the long ignored importance of the female ovulation cycle, heralds a sea of change in the way scientists think about the evolution of human mating strategies. Simultaneously, it offers an example par excellence of the heuristic value of evolutionary hypotheses, guiding researchers to discover phenomena that otherwise would have remained entirely unexamined without an evolutionary psychological framework. Finally, it offers a serious challenge to mainstream nonevolutionary psychologists, whose theories currently cannot explain, even in principle, why males and females both would show such well-designed adaptations to ovulation.
Todd Shackelford, Aaron Goetz, Craig LaMunyan, Michael Pham, and Nicholas Pound (Chapter 15), discuss the evolutionary psychology of sperm competition, a form of postcopulatory sexual selection. Starting with a brief review of the nonhuman literature on sperm competition, they assemble compelling evidence that sperm competition has been a recurrent phenomenon for humans. They discuss physiological, anatomical, and psychological evidence for sperm competition adaptations in men. Then they turn to hypothesized sperm competition adaptations in women, including precopulatory female choice and the timing of the female orgasm. They conclude by suggesting that sperm competition has been an important, and relatively neglected, arena for sexually selected adaptations in humans. This excellent chapter highlights the heuristic value of evolutionary thinking in discovering phenomena entirely missed by psychological theories that ignore evolutionary processes.
A new, exciting chapter by Debra Lieberman (Chapter 16) focuses on a retrospectively obvious, but strangely neglected domain—that of adaptations for inbreeding avoidance. She highlights two key selective forces leading to these adaptations—avoiding disease-causing organisms and preventing defects through making deleterious recessive genes homozygous. She reviews arguments and evidence that inbreeding avoidance adaptations are not invariant, but rather are sensitive to fertility status, mate value, and opportunity costs. Lieberman provides novel insights into the information processing architecture of inbreeding avoidance adaptations and explores the fascinating issue of why third parties should object to inbreeding among others.
Mark Huppin and Neil Malamuth (Chapter 17) provide an excellent chapter on another region of conflict between the sexes: sexual coercion by men. They furnish a judicious analysis of competing hypotheses about rape—whether it is caused by adaptations specifically designed for forced sex, or instead is a by-product of more general adaptations to use force to achieve a variety of ends (e.g., stealing resources). They then focus on one potential candidate design feature of a rape adaptation—men's sexual arousal to forcing women into unwanted sex. In particular, they discuss individual differences among men in sexual arousal to force, and attempt to identify the variables that lead some men, and not others, to adopt force in the context of sex. Strong conclusions about the conceptual status of rape are not possible at this point, but these authors provide a nuanced description of the possible psychological mechanisms involved and an up-to-date description of the relevant empirical evidence.
Lorne Campbell and Tim Loving (Chapter 18) conclude the mating section with a stimulating chapter on love, commitment, and mate retention. They highlight the different adaptive benefits men and women would accrue from forming long-term pair bonds, and delve into the underlying motivational and emotional mechanisms underlying such relationships. They nicely interweave theory and research emanating from mainstream (not explicitly evolutionary) researchers with more functional analyses of long-term mating. Whereas they propose that an underlying psychological system captured by “love” motivates relationship formation, they suggest that anger and upset are motivational mechanisms designed to monitor signals of “strategic interference” with the relationship. The Campbell and Loving chapter nicely illuminates the complexity of the evolutionary psychology of long-term mating, relationships formed and maintained by emotions ranging from love to rage.
Although these chapters take stock of the current status of the science of mating, it is worthwhile to step back and see how far the field has come. In the mid-1980s, the field of mating was barely visible on the scientific map. Social psychologists had discovered a few things about attraction, but theories of mating were woefully simplistic. Most invoked single variables responsible for the selection of mates, such as similarity, proximity, or equity. Most theorists implicitly assumed that all mating was exclusively for the long term. Short-term mating was largely ignored. Little was known about the processes of mate selection or mate attraction. Concepts such as mate value, mate retention, sexual conflict, adaptations to ovulation, sexually antagonistic coevolution, contest competition, mate poaching, and many others were entirely absent.
Beginning in the mid- to late 1980s, the first raft of empirical studies on human mating appeared. In the 1990s, work on the evolutionary psychology of human mating mushroomed to become the most studied domain of evolutionary psychology. Although much scientific evidence has now cumulated supporting many hypothesized human mating adaptations, the area continues to yield new discoveries. Because mating is so close to the reproductive engine of evolution, it follows that selection has fashioned a rich array of psychological adaptations to deal with the complex and recurrent adaptive problems that mating poses. The chapters in this section take stock of what we now know about human mating and point to fertile fields of mating adaptations yet to be discovered.
Donald Symons
An orchid, Trichoceros Antennifer, that I tend on my back porch is gravid with lessons for students of human mating psychology. When a naïve house guest first encounters a T. antennifer, usually there is a brief moment of confusion followed by a burst of delighted laughter, as the guest realizes what he or she is seeing. While most orchids attract pollinators by offering them a food reward, T. antennifer is pollinated by the males of a certain type of Ecuadorian fly as they attempt to copulate with the orchid's flower. The males do this because the T. antennifer is an astonishingly realistic mimic of a female fly. (And if the flower is realistic to the human eye, how much more realistic is it likely to be to the eye of the male fly that it was designed by natural selection to bamboozle?)
The first lesson that I draw from this orchid's sex life is that we really should not be astonished by the complexity and precision of its flower's mimicry; or, rather, we should not be more astonished than we are by the complexity and precision of biological adaptations in general. What makes T. antennifer's mimicry seem so uncannily superb is that it is one of the rare cases in which we have immediately available in our mind's eye an image of optimal design (in this case a fly), and thus we can instantly and intuitively compare the actual adaptation (the orchid's flower) to this standard. For the vast majority of biological adaptations, however, we do not have an image of optimal design in our mind's eye and thus cannot quickly or intuitively assess how closely most adaptations approximate optimality.
Human psychological mating adaptations, though buried deep between our ears rather than worn on our sleeves, were designed by the same evolutionary processes as was T. antennifer's flower, and there is no reason to expect these human adaptations to be less exquisitely adapted for their purposes than T. antennifer's flower is for its purpose. This adaptationist view of life informs the scientific imagination of Darwinian students of human mating psychology. The result—as represented in the following chapters—is a body of research that very likely would never have been conceived or conducted absent an explicit, conscious Darwinism. Researchers innocent of Darwinism can palaver about learning, culture, gene-environment interaction, levels of analysis, and how complicated everything is until the cows come home, but they're unlikely to ask such a simple research question as the following: Do our brains contain species typical devices whose functions are to (unconsciously) detect deviations from bilateral symmetry in the faces we observe and to cause us to prefer individuals with more symmetrical faces as mates (all else equal)? The 20th-century histories of psychology and the social sciences do not encourage the belief that such a question ever would have been asked had evolutionary psychology not come along.
A nutshell summary of modern Darwinism is this: An organism is an integrated collection of problem solving devices—that is, adaptations—that were shaped by natural selection over evolutionary time to promote, in some specific way, the survival of the genes that directed their construction. The specific way that an adaptation was designed to promote gene survival is that adaptation's function (or goal, or purpose). The function of the heart is to pump blood, the function of pancreatic beta cells is to secrete insulin, and so forth. Unlike nonliving matter, living matter is not just complexly organized, it is functionally organized. The specific aspects of the environment to which an adaptation is adapted, and upon which its normal functioning and development depend, are sometimes called its “environment of evolutionary adaptedness,” or EEA.
The second lesson that I draw from T. antennifer's sex life is that it is logically impossible to describe an adaptation without (at least implicitly) describing the adaptation's EEA. Without the EEA there is no science of adaptation. Any scientifically useful description of T. antennifer's flower will necessarily include a description of the morphology of certain female flies and the mating psychology of male flies found in T. antennifer's natural habitat, the high-altitude cloud forests of Ecuador. Moreover, my brief description of T. antennifer's flower would be intelligible only to those readers who possessed a basic understanding of the nature of flowers and their evolved relationships with environmental vectors, such as insects.
The EEAs of the vast majority of human adaptations still exist today and usually are too obvious to merit explicit mention. For example, a neurophysiologist describing the function of a certain component of the human visual system probably will simply assume that her colleagues know (a) a great deal about the nature of electromagnetic radiation, and (b) that the (natural) light falling on human retinas today is essentially identical to the light that fell on our ancestors' retinas during the evolution of our visual system. But human environments, especially those of modern industrialized societies, have changed in many ways in the brief period since the origin of agriculture 10,000 years ago, and some of these changes potentially affect the functioning of human mating adaptations. Darwinian students of human mating psychology thus have another advantage over other researchers: The Darwinian is alert to potentially significant differences between current and ancient environments, and this EEA mindedness can inform hypothesis formation. In some cases, it can even lead the Darwinian to posit the existence of adaptation where others perceive pathology or folly.
Here is an example of the sort of thing I have in mind. A striking feature of human courtship—in its broadest sense—is the powerful effect that fear of rejection seems to have on behavior. Sexual/romantic rejection hurts; the memory of being rejected hurts; the thought of being rejected hurts; hence, it is not surprising that the possibility of being rejected affects most people's mating behavior. Yet on the face of it, fear of rejection seems to be astonishingly dysfunctional. The potential benefits of propositioning an attractive member of the other sex, which include everything from a sexual fling to a lifetime mateship, would appear to vastly outweigh the potential costs, which seem to consist mainly of a small amount of wasted time.
The potent effect that fear of rejection has on human courtship should inspire students of human mating psychology to consider whether this fear might have been adaptive during the vast majority of human evolution, even if it is not adaptive in many current environments. In other words, sexual/romantic rejection might have entailed real and significant costs in the human evolutionary past that it does not usually entail today. I propose the following hypothesis. During most of human evolutionary history our ancestors lived in relatively small face-to-face groups wherein sexual/romantic rejections were very likely to become common knowledge. When Ann the gatherer rejected Andy the hunter's proposition, everyone in their community probably found out about it before long (assuming that our ancestors were no less interested in other people's sex lives, and no less prone to gossip, than we are). The information that Ann had rejected Andy could diminish his perceived mate value in the eyes of others, including other potential mates (Ann may have rejected Andy because she had acquired mate-value-relevant information about him that others were not privy to). On a modern university campus, with thousands of students and enormous scope for anonymity, Bob's anxiety at the prospect of hitting on Bobbi is, perhaps, irrational in the sense that he has little to fear but fear itself; but the underlying motivational system may have been shaped by selection to function in an environment in which rejection had real and substantial costs.
Even if the historical, ethnographic, and archeological records did not unanimously indicate that humans evolved in, and are adapted to, life in much smaller groups than most of us encounter today, many aspects of our psychology, including fear of rejection, might allow us to infer the existence of such an ancestral world—just as Darwin correctly inferred that the orchid Angraecum sesquipedale, whose nectar producing organ lies 30 cm inside it, must be pollinated by a then unknown insect with a proboscis at least 30 cm long.
In conclusion, although Darwinism does not confer on its practitioners some sort of magical pipeline into human mating psychology, a conscious, explicit adaptationism does give the Darwinian at least two advantages in generating scientifically productive hypotheses. First, Darwinians expect the human brain to contain many complex, exquisitely engineered devices that were shaped by selection to solve the specific mating problems that our ancestors reliably encountered during the course of human evolutionary history. Second, Darwinians are ever mindful that these devices, whatever they may be, are adapted to a world that, in some respects, no longer exists. These are no mean advantages.