Evolution psychology

CHAPTER 1 Evolutionary Psychology RUSSIL DURRANT AND BRUCE J. ELLIS 1 LEVELS OF EXPLANATION IN EVOLUTIONARY PSYCHOLOGY 2 THE METATHEORY LEVEL OF ANALYSIS 3 METATHEORETICAL ASSUMPTIONS THAT ARE CONSENSUALLY HELD BY EVOLUTIONARY SCIENTISTS 3 Natural Selection 4 Adaptation 4 Sexual Selection 6 Inclusive Fitness Theory 7 SPECIAL METATHEORETICAL ASSUMPTIONS OF EVOLUTIONARY PSYCHOLOGY 7 Psychological Mechanisms as the Main Unit of Analysis 8 Domain Specificity of Psychological Mechanisms 9 The Environment of Evolutionary Adaptedness 10 THE MIDDLE-LEVEL THEORY LEVEL OF ANALYSIS 11 Parental Investment Theory 12 Good Genes Sexual Selection Theory 14 THE HYPOTHESES LEVEL OF ANALYSIS 17 Good Genes Sexual Selection Theory: Hypotheses 17 THE PREDICTION LEVEL OF ANALYSIS 19 Good Genes Sexual Selection Theory: Predictions 20 THE FUTURE OF EVOLUTIONARY PSYCHOLOGY 22 The Impact of Evolutionary Psychology 24 Future Directions 26 REFERENCES 28 Evolutionary psychology is the application of the principles and knowledge of evolutionary biology to psychological theory and research. Its central assumption is that the human brain is comprised of a large number of specialized mechanisms that were shaped by natural selection over vast periods of time to solve the recurrent information-processing problems faced by our ancestors (Symons, 1995). These prob- lems include such things as choosing which foods to eat, negotiating social hierarchies, dividing investment among offspring, and selecting mates. The field of evolutionary psy- chology focuses on identifying these information-processing problems, developing models of the brain-mind mechanisms that may have evolved to solve them, and testing these models in research (Buss, 1995; Tooby & Cosmides, 1992). The field of evolutionary psychology has emerged dra- matically over the last 15 years, as indicated by exponential growth in the number of empirical and theoretical articles in the area (Table 1.1). These articles extend into all branches of psychology—from cognitive psychology (e.g., Cosmides, 1989; Shepard, 1992) to developmental psychology (e.g., Ellis, McFadyen-Ketchum, Dodge, Pettit, & Bates, 1999; Weisfeld, 1999), abnormal psychology (e.g., Mealey, 1995; Price, Sloman, Gardner, Gilbert, & Rhode, 1994), social psychology (e.g., Daly & Wilson, 1988; Simpson & Kenrick, 1997), personality psychology (e.g., Buss, 1991; Sulloway, 1996), motivation-emotion (e.g., Nesse & Berridge, 1997; Johnston, 1999), and industrial-organizational psychology (e.g., Colarelli, 1998; Studd, 1996). The first undergraduate textbook on evolutionary psychology was published in 1999 (Buss, 1999), and since then at least three other undergradu- ate textbooks have been published in the area (Barrett, Dunbar, & Lycett, 2002; Cartwright, 2000; Gaulin & McBurney, 2000). In this chapter we provide an introduction to the field of evolutionary psychology. We describe the methodology that evolutionary psychologists use to explain human cognition and behavior. This description begins at the broadest level with a review of the basic, guiding assumptions that are em- ployed by evolutionary psychologists. We then show how evolutionary psychologists apply these assumptions to de- velop more specific theoretical models that are tested in re- search. We use examples of sex and mating to demonstrate how evolutionary psychological theories are developed and tested. gall_ch01.qxd 9/13/02 2:01 PM Page 1 2 Evolutionary Psychology LEVELS OF EXPLANATION IN EVOLUTIONARY PSYCHOLOGY Why do siblings fight with each other for parental atten- tion? Why are men more likely than women to kill sexual rivals? Why are women most likely to have extramarital sex when they are ovulating? To address such questions, evolu- tionary psychologists employ multiple levels of explanation ranging from broad metatheoretical assumptions, to more specific middle-level theories, to actual hypotheses and predictions that are tested in research (Buss, 1995; Ketelaar & Ellis, 2000). These levels of explanation are ordered in a hierarchy (see Figure 1.1) and constitute the methodology that evolutionary psychologists use to address questions about human nature. At the top of the hierarchy are the basic metatheoretical assumptions of modern evolutionary theory. This set of guid- ing assumptions, which together are referred to as evolution- ary metatheory, provide the foundation that evolutionary scientists use to build more specific theoretical models. We begin by describing (a) the primary set of metatheoretical assumptions that are consensually held by evolutionary sci- entists and (b) the special set of metatheoretical assumptions that distinguish evolutionary psychology. We use the term evolutionary psychological metatheory to refer inclusively to this primary and special set of assumptions together. As shown in Figure 1.1, at the next level down in the hier- archy, just below evolutionary psychological metatheory, are middle-level evolutionary theories. These theories elaborate the basic metatheoretical assumptions into a particular psy- chological domain such as mating or cooperation. In this chapter we consider two related middle-level evolutionary theories—parental investment theory and good genes sexual TABLE 1.1 Growth of Publications in the Area of Evolutionary Psychology, as Indexed by the PsycINFO Database Years of Publication Number of Publicationsa 1985–1988 4 1989–1992 25 1993–1996 100 1997–2000 231 aNumber of articles, books, and dissertations in the PsycINFO database that include either the phrase evolutionary psychology or evolutionary psycho- logical in the title, in the abstract, or as a keyword. All articles from the Journal of Evolutionary Psychology, which is a psychoanalytic journal, were excluded. Evolutionary Psychological Metatheory Middle-Level Theories Hypotheses Specific Predictions Basic metatheoretical assumptions of modern evolutionary theory. Special metatheoretical assumptions of evolutionary psychology. Attachment theory (Bowlby, 1969) Parental investment theory (Trivers, 1972) Good genes sexual selection theory Individuals who more fully display traits indicative of high genetic quality should be healthier and in better condition than should conspecifics who display these traits less fully. The frequency and timing of female orgasm should vary in a manner that selectively favors the sperm of males who display indicators of high genetic quality. Males who display indicators of high genetic quality should have more sexual partners and more offspring. More symmetrical individuals should have better mental and physical health, better immune system functioning, and lower parasite loads than should less symmetrical individuals. The timing and frequency of orgasms by women should be patterned to selectively retain the sperm of more symmetrical men. More symmetrical men should have more lifetime sexual partners and more extrapair sexual partners than should less symmetrical men. Figure 1.1 The hierarchical structure of evolutionary psychological explanations (adapted from Buss, 1995). gall_ch01.qxd 9/13/02 2:01 PM Page 2 Metatheoretical Assumptions That Are Consensually Held by Evolutionary Scientists 3 selection theory—each of which applies the assumptions of evolutionary psychological metatheory to the question of reproductive strategies. In different ways these middle-level theories attempt to explain differences between the sexes as well as variation within each sex in physical and psychologi- cal adaptations for mating and parenting. At the next level down are the actual hypotheses and pre- dictions that are drawn from middle-level evolutionary theo- ries (Figure 1.1). A hypothesis is a general statement about the state of the world that one would expect to observe if the theory from which it was generated were in fact true. Predic- tions are explicit, testable instantiations of hypotheses. We conclude this chapter with an evaluation of hypotheses and specific predictions about sexual behavior that have been de- rived from good genes sexual selection theory. Special atten- tion is paid to comparison of human and nonhuman animal literatures. THE METATHEORY LEVEL OF ANALYSIS Scientists typically rely on basic (although usually implicit) metatheoretical assumptions when they construct and evalu- ate theories. Evolutionary psychologists have often called on behavioral scientists to make explicit their basic assumptions about the origins and structure of the mind (see Gigerenzer, 1998). Metatheoretical assumptions shape how scientists generate, develop, and test middle-level theories and their de- rivative hypotheses and predictions (Ketelaar & Ellis, 2000). These basic assumptions are often not directly tested after they have been empirically established. Instead they are used as a starting point for further theory and research. Newton’s laws of motion form the metatheory for classical mechanics, the principles of gradualism and plate tectonics provide a metatheory for geology, and the principles of adaptation through natural selection provide a metatheory for biology. Several scholars (e.g., Bjorklund, 1997; Richters, 1997) have argued that the greatest impediment to psychology’s develop- ment as a science is the absence of a coherent, agreed-upon metatheory. A metatheory operates like a map of a challenging con- ceptual terrain. It specifies both the landmarks and the bound- aries of that terrain, suggesting which features are consistent and which are inconsistent with the core logic of the meta- theory. In this way a metatheory provides a set of powerful methodological heuristics: “Some tell us what paths to avoid (negative heuristic), and others what paths to pursue (positive heuristic)” (Lakatos, 1970, p. 47). In the hands of a skilled re- searcher, a metatheory “provides a guide and prevents certain kinds of errors, raises suspicions of certain explanations or observations, suggests lines of research to be followed, and provides a sound criterion for recognizing significant ob- servations on natural phenomena” (Lloyd, 1979, p. 18). The ultimate contribution of a metatheory is that it synthesizes middle-level theories, allowing the empirical results of a variety of different theory-driven research programs to be explicated within a broader metatheoretical framework. This facilitates systematic cumulation of knowledge and progres- sion toward a coherent big picture, so to speak, of the subject matter (Ketelaar & Ellis, 2000). METATHEORETICAL ASSUMPTIONS THAT ARE CONSENSUALLY HELD BY EVOLUTIONARY SCIENTISTS When asked what his study of the natural world had revealed about the nature of God, biologist J. B. S. Haldane is reported to have made this reply: “That he has an inordinate fondness for beetles.” Haldane’s retort refers to the extraordinary di- versity of beetle species found throughout the world—some 290,000 species have so far been discovered (E. O. Wilson, 1992). Beetles, moreover, come in a bewildering variety of shapes and sizes, from tiny glittering scarab beetles barely visible to the naked eye to ponderous stag beetles with mas- sive mandibles half the size of their bodies. Some beetles make a living foraging on lichen and fungi; others subsist on a diet of beetles themselves. The richness and diversity of beetle species are mirrored throughout the biological world. Biologists estimate that anywhere from 10 to 100 million different species currently inhabit the Earth (E. O. Wilson, 1992), each one in some respect different from all others. How are we to explain this extraordinary richness of life? Why are there so many species and why do they have the particular characteristics that they do? The general principles of genetical evolution drawn from modern evolutionary theory, as outlined by W. D. Hamilton (1964) and instantiated in more contemporary so-called self- ish gene theories of genetic evolution via natural and sexual selection, provide a set of core metatheoretical assumptions for answering these questions. Inclusive fitness theory con- ceptualizes genes or individuals as the units of selection (see Dawkins, 1976; Hamilton, 1964; Williams, 1966). In con- trast, “multilevel selection theory” is based on the premise that natural selection is a hierarchical process that can oper- ate at many levels, including genes, individuals, groups within species, or even multi-species ecosystems. Thus, mul- tilevel selection theory is conceptualized as an elaboration of inclusive fitness theory (adding the concept of group-level adaptation) rather than an alternative to it (D. S. Wilson & gall_ch01.qxd 9/13/02 2:01 PM Page 3 4 Evolutionary Psychology Sober, 1994). Whereas inclusive fitness theory is consensu- ally accepted among evolutionary scientists, multilevel selec- tion theory is not. Thus, this review of basic metatheoretical assumptions only focuses on inclusive fitness theory. Natural Selection During his journey around the coastline of South America aboard the HMS Beagle, Charles Darwin was intrigued by the sheer diversity of animal and plant species found in the tropics, by the way that similar species were grouped together geo- graphically, and by their apparent fit to local ecological condi- tions. Although the idea of biological evolution had been around for some time, what had been missing was an explana- tion of how evolution occurred—that is, what had been miss- ing was an account of the mechanisms responsible for evolutionary change. Darwin’s mechanism, which he labeled natural selection, served to explain many of the puzzling facts about the biological world: Why were there so many species? Why are current species so apparently similar in many respects both to each other and to extinct species? Why do organisms have the specific characteristics that they do? The idea of natural selection is both elegant and simple, and can be neatly encapsulated as the result of the operation of three general principles: (a) phenotypic variation, (b) dif- ferential fitness, and (c) heritability. As is readily apparent when we look around the biological world, organisms of the same species vary in the characteris- tics that they possess; that is, they have slightly different phenotypes. A whole branch of psychology—personality and individual differences—is devoted to documenting and un- derstanding the nature of these kinds of differences in our own species. Some of these differences found among mem- bers of a given species will result in differences in fitness— that is, some members of the species will be more likely to survive and reproduce than will others as a result of the spe- cific characteristics that they possess. For evolution to occur, however, these individual differences must be heritable— that is, they must be reliably passed on (via shared genes) from parents to their offspring. Over time, the characteristics of a population of organisms will change as heritable traits that enhance fitness will become more prevalent at the expense of less favorable variations. For example, consider the evolution of bipedalism in humans. Paleoanthropological evidence suggests that upright walking (at least some of the time) was a feature of early ho- minids from about 3.5 million years ago (Lovejoy, 1988). Pre- sume that there was considerable variation in the propensity to walk upright in the ancestors of this early hominid species as the result of differences in skeletal structures, relevant neural programs, and behavioral proclivities. Some hominids did and some did not. Also presume that walking on two feet much of the time conferred some advantage in terms of survival and re- productive success. Perhaps, by freeing the hands, bipedalism allowed objects such as meat to be carried long distances (e.g., Lovejoy, 1981). Perhaps it also served to cool the body by re- ducing the amount of surface area exposed to the harsh tropi- cal sun, enabling foraging throughout the hottest parts of the day (e.g., Wheeler, 1991). Finally, presume that these differ- ences in the propensity for upright walking were heritable in nature—they were the result of specific genes that were reli- ably passed on from parents to offspring. The individuals who tended to walk upright would be, on average, more likely to survive (and hence, to reproduce) than would those who did not. Over time the genes responsible for bipedalism would be- come more prevalent in the population as the individuals who possessed them were more reproductively successful than were those who did not, and bipedalism itself would become pervasive in the population. Several points are important to note here. First, natural selection shapes not only the physical characteristics of organisms, but also their behavioral and cognitive traits. The shift to bipedalism was not simply a matter of changes in the anatomy of early hominids; it was also the result of changes in behavioral proclivities and in the complex neural programs dedicated to the balance and coordination required for upright walking. Second, although the idea of natural selection is sometimes encapsulated in the slogan the sur- vival of the fittest, ultimately it is reproductive fitness that counts. It doesn’t matter how well an organism is able to survive. If it fails to pass on its genes, then it is an evolution- ary dead end, and the traits responsible for its enhanced survival abilities will not be represented in subsequent gener- ations. This point is somewhat gruesomely illustrated by many spider species in which the male serves as both meal and mate to the female—often at the same time. Ultimately, although one must survive to reproduce, reproductive goals take precedence. Adaptation Natural selection is the primary process which is responsi- ble for evolutionary change over times as more favorable variants are retained and less favorable ones are rejected (Darwin, 1859). Through this filtering process, natural selec- tion produces small incremental modifications in existing phenotypes, leading to an accumulation of characteristics that are organized to enhance survival and reproductive suc- cess. These characteristics that are produced by natural selec- tion are termed adaptations. Adaptations are inherited and gall_ch01.qxd 9/13/02 2:01 PM Page 4 Metatheoretical Assumptions That Are Consensually Held by Evolutionary Scientists 5 reliably developing characteristics of species that have been selected for because of their causal role in enhancing the survival and reproductive success of the individuals that possess them (see Buss, Haselton, Shackelford, Bleske, & Wakefield, 1998; Dawkins, 1986; Sterelny & Griffiths, 1999; Williams, 1966, 1992, for definitions of adaptation). Adaptations have biological functions. The immune sys- tem functions to protect organisms from microbial invasion, the heart functions as a blood pump, and the cryptic coloring of many insects has the function of preventing their detection by predators. The core idea of evolutionary psychology is that many psychological characteristics are adaptations—just as many physical characteristics are—and that the principles of evolutionary biology that are used to explain our bodies are equally applicable to our minds. Thus, various evolution- ary psychological research programs have investigated psy- chological mechanisms—for mate selection, fear of snakes, face recognition, natural language, sexual jealousy, and so on—as biological adaptations that were selected for because of the role they played in promoting reproductive success in ancestral environments. It is worth noting, however, that natural selection is not the only causal process responsible for evolutionary change (e.g., Gould & Lewontin, 1979). Traits may also become fixated in a population by the process of genetic drift, whereby neutral or even deleterious characteristics become more prev