Research TOC

BEHAVIORAL ECOLOGY, “SOCIOBIOLOGY,” AND HUMAN BEHAVIOR
Bobbi S. Low

W

hen Juliet was twelve, her father, without consulting her, betrothed her to a man more than twice her age. She, being in love with Romeo, complained. Her father’s answer was (Act III, Scene V):
An you will not wed, I’ll pardon you! Graze where you will, you shall not house with me; … An you be mine, I’ll give you to my friend; An you be not, hang, beg, starve, die in the streets, For, by my soul, I’ll ne’er acknowledge thee, Nor what is mine shall never do thee good.

Today, in the United States, Juliet would probably sue her father for child abuse. And she would be likely to win. What is common, approved, and thought ethical varies widely across human cultures in time and space: whether one may marry more than one person at a time; who chooses marriage partners; whether abortion and infanticide are approved or forbidden; whether one may eat all meats, some meats, or none; what kinds of killings are forbidden or encouraged. How are we to make sense of all this variety? Human behavior has traditionally been the province of anthropology, sociology, and psychology. Within each of these fields there exist diverse approaches. Recently, behavioral ecology, an evolutionary approach to why we behave as we do, has joined other fields in trying to explain some of the diversity in human behavior. With its roots in Charles Darwin’s work 1 on natural selection, it examines how environmental conditions influence living things. Human behavioral ecology is one of a variety of quite similar evolutionary approaches to human behavior, with a variety of names: evolutionary ecology, biosocial science, human ethology, sociobiology, socioecology, evolutionary biological anthropology, and others.2 Depending on who is reporting, these may vary subtly, or be different names for the same thing. When E. O. Wilson first published his book Sociobiology he defined the subject as “the systematic study of the biological causes of behavior.”3 His definition may have seemed relatively straightforward at the time; however, much controversy has arisen since then, perhaps because Wilson did not define what he meant by “biology.” Today some writers use (or accuse others of using) the term to mean “genetic determination of traits, independent of environmental variation.” In fact, no one really uses such a definition.4 When we want to study why we act as we do, in an evolutionary framework, we begin by acknowledging that environments, genes, and history interact to influence behavior. Human behavioral ecology focuses on how much of human behavior we can understand and predict, by using the same logic 3

4

RESEARCH FRONTIERS

and principles we use to analyze behavior in other species. Of course we don’t expect to be able to predict all of human behavior. Human behavioral ecology begins, at least for the purposes of generating hypotheses, by using the central paradigm in biology: Humans, like other living organisms, have evolved to maximize their genetic contribution to future generations through producing offspring and assisting nondescendant relatives; the particular strategies accomplishing such maximization will differ in specific ways in different environments, and, just as for other mammals, these strategies will typically differ between the sexes. Behavior is assumed to be the product of the interaction of genes and environment—not the result purely of genetics or environment. Behavioral ecology asks when and how environmental conditions (including social conditions) change individuals’ genetic costs and benefits, and therefore predict the kinds of behavior we are likely to see. The propositions underlying this paradigm include the following:
1. Organisms are generally well suited to the environments in which they live; they achieve success in any environment by getting resources that help them survive and reproduce (get genes into the next generation). 2. Variation exists in how well organisms do this. Only heritable (correlated between parents and children) variation is appropriately considered, for only this accumulates over time. a. An individual organism can help its genes to spread by reproducing directly, by assisting individual organisms carrying copies “identical by descent” (IBD) of its genes (kin selection), or, under certain circumstances, by helping individual organisms that do not carry IBD copies of its genes, if such assistance is returned in genetically effective ways (reciprocity). b. The genes of individual organisms that help reproductive competitors without any reciprocation will tend to disappear from existing lineages. 3. In the evolutionary history of all species, there have been important proximate correlates of reproductive success, including resource control (food, territory), rank (status, power, or wealth), and, in highly social species such as humans, social “reputation.” However, no organisms, including humans, have evolved to perceive or assess directly the spread of genes. Instead, organisms behave as though these proximate correlates were their goals. Because environments change, species may find themselves in novel evolutionary environments, and their behavior may be currently maladaptive.

HUMAN BEHAVIORAL ECOLOGY
4. Humans are not qualitatively different from other living organisms. Like other living things, they evolved to get and use resources to survive and enhance the spread of their genes. To behavioral ecologists, this is parallel to arguing that humans, for example, while they can make airplanes fly, are still subject to the laws of gravity.

5

I will explore examples both of the controversies concerning this novel approach and of how such an approach can be used to complement other knowledge. Controversy arises over several issues. Some of the propositions taken to be true here are simply not accepted by everyone; for example, not everyone thinks that humans are sufficiently like other species to allow us to generalize from propositions well understood in other species to human behavior, even to allow us to test hypotheses. Some feel that we are psychologically unique, and we should not analyze certain aspects of ourselves. These concerns seem to me more a problem of communication than of science. The tests are relatively straightforward; one can make the assumptions that generate the predictions, and then see whether they are upheld or falsified.5 But there are more interesting controversies surrounding an evolutionary approach. So much heat was generated by the early fights over E. O. Wilson’s work that real misunderstandings remain. Two of the most common are (1) if genetic representation is the important criterion, anything less than maximum physiologically possible reproduction must represent an individual sacrifice or an evolutionary mistake; and (2) explanations that focus on the particular stimulus for a behavior (proximate cause) are in conflict with explanations that focus on predicting the evolutionary conditions under which such behavior will be genetically profitable (ultimate cause). Neither of these conceptions is true.6 These misunderstandings are so widespread, and so unnecessary, that I will explore them in more detail later. Finally, there are controversies within the field generated by poor work and poor communication. For example, a recent study proposed that there are racial differences in law-abidingness, sexuality, and intelligence.7 Careful examination of the work shows that the traits considered (such as “law-abidingness,” defined as arrest rates) are meaningless because they are closely correlated with other factors (socioeconomic status, racial prejudice). Some of the “racial” groups are defined in unique ways; the results would be different if different definitions were used. Not only the results but also the definitions of race and the traits studied are controversial. Still, the obvious solution is the usual one in scientific debates:

6

RESEARCH FRONTIERS

challenge and test. Good science should drive out bad science, even on highly-charged topics.

WHAT CAN A BEHAVIORAL ECOLOGICAL PERSPECTIVE ADD?
Several new perspectives mark behavioral ecological approaches. First is the importance of using repeatable observations in ways similar to other sciences: primarily, optimality modeling (seeking to model and test optimality of behavior—for example, in a given environment, which strategy is most effective?), and the comparative method (using existing variation as a “natural experiment” to test hypotheses).8 Most novel, when we think of human behavior, is the idea that genes themselves are a sort of currency to be maximized, and that by understanding the impact of particular behaviors in specific environments on gene frequencies (or on their proximate correlates—things that vary with them—like status or health) we can make new, testable, and sometimes unexpected predictions. It follows from this approach that individual reproductive interests can lead to group-wide patterns—in stark contrast to the older idea that we do things for the good of the group. This approach can often make sense of previously puzzling patterns. Behavioral ecology straddles traditional subfields in anthropology; it looks at behavior, like cultural anthropology, but does so using principles derived from biology, like biological anthropology. It bridges fields as diverse as animal behavior and the decision sciences, like the rational-actor approach in economics. It incorporates quantitative methods and techniques from many fields to analyze complex social behavior. Behavioral ecology has a strong tradition of scientific method, phrasing questions as testable hypotheses in order to examine them with repeatable observations. Perhaps most exciting, behavioral ecology leads us to ask new questions about human behavior that might not have occurred to us before. Behavioral ecology’s linking of behavior to environment—for all species—might surprise you, if you imagine that nonhuman species are too simple and stereotypic to be good models for human behavior. But other species’ behavior is often more complex than we might suspect, and can show us something of how genes and environment interact. For example, optimal foraging theory postulates that foraging efficiency increases relative reproduc-

HUMAN BEHAVIORAL ECOLOGY

7

tive fitness. In ground squirrels, optimal foragers survive better and have more babies than nonoptimal foragers. Optimality in foraging is heritable: Babies forage more like their parents than like others in the population. However, heritability is only about 60 percent genetic; the other 40 percent of the parent-offspring correlation in optimality comes from babies foraging near their mother and learning what to eat.9 Thus, there is intergenerational information transfer in ground squirrels. The functionally important facts are that heritable variation exists; that one can predict, in a specific environment, which strategies (learned as well as genetically transmitted) ought to result in increased reproductive fitness for their possessors and in a larger proportion of the possessors in the population; and that one can test and falsify these predictions. Learning, then, is a form of intergenerational transmission (cultural heritability) that we humans have elevated to a high art, but it is not unique to us. Learning and/or cultural transmission are important, for example, in chimpanzees and other primates, ground squirrels, birds, amphibians, fish, honeybees (in which different individuals, depending on their experience, learn to specialize on various flower species, thus becoming more efficient) and sweat bees (who learn to identify their kin in order to guard their nests). Learning appears to be important in complex and unpredictable environments. The evolutionary environment of humans had many uncertainties, so it may come as no surprise that we evolved to be premier learners and transmitters of learned behaviors.

ASKING BEHAVIORAL QUESTIONS
Consider ravens: They show a curious behavior. A small group of ravens, feasting on a dead moose, may give a distinctive, loud, high-pitched “yell.” Other ravens are attracted to the yelling; the result is more ravens at the kill, and less food for the yellers. Why doesn’t the raven who first finds a kill just keep quiet? The ravens seem to be sharing in a truly nonprofitable way. Yet behavioral ecologists find true genetic altruism to be so rare as to be a fluke. The first question, of course, is whether the “called” birds are related to the callers—since sharing with individuals who carry at least some of your genes can help spread your genes. After much work, Bernd Heinrich, the behavioral ecologist who found this behavior, eliminated that possibility.10 What else could be possible? Beginning with the hypothesis that ravens are likely to act in

8

RESEARCH FRONTIERS

self-interested ways, Heinrich made a series of careful observations, comparing the behavior of different ravens under various circumstances (similar to the comparative method of cross-cultural research). He found that adult ravens were territorial, controlling any carcasses in their territory and driving off any juveniles. Territorial adults never yelled when they found a carcass. However, when a juvenile found a carcass, it was likely to yell, attracting other juveniles; when enough juveniles were present, the resident territorial adults could not drive them off. Heinrich did not simply create a “just-so story”; he used careful hypothesis-making, observation, and testing to discover what is, in fact, the most likely functional reason for the ravens’ behavior. If our goal is to understand, and (we hope) predict how human behavior might vary in different environments, we could profitably use similar careful approaches.

NATURAL SELECTION, KIN SELECTION, AND RECIPROCITY
At the core of behavioral ecology rests the notion of the selfish gene: Genes that get themselves copied into more and more individuals will be the genes that prevail and persist through time.11 Genes, of course, travel about in interactive groups (the genotype) inside individuals, and individual strategies for survival and reproduction are all-important. It may be complicated to analyze; in fact, we seldom know the actual genetics of behavior in any species. We make some simplifying assumptions: that there has been enough genetic variation in the past for natural selection to operate, and that natural selection does operate to mediate possible conflicting forces. Grafen calls this the phenotypic gambit.12 The selfish gene is a modern version of the simple logic first employed explicitly by Darwin, and given in the four propositions stated earlier. The organisms we see today are the descendants of those that successfully survived and reproduced in past environments. Genetically selfish behaviors, those that enhance an individual’s genetic representation, are always favored. Seems simple—but complex, unexpected, and profound effects follow. For example, having the maximum number of babies is often not the route to having the maximum number of grandchildren—this is a wide-

HUMAN BEHAVIORAL ECOLOGY

9

spread misunderstanding. What matters is the effectiveness of parental care in making offspring successful. If parents can protect their offspring from dying (which is expensive and likely to lower fertility), successful parents may be those that produce fewer offspring but raise more of them to adulthood. This is the crux of reproductive strategies in other species, and crucial for humans.13 Behavioral ecology focuses on predicting behaviors that succeed in particular environments. The currency is genes. Heritable variation is our focus; but in complex social animals like ourselves, not only genetic but cultural heritability—learning—can be significant, and the interaction between cultural and genetic transmission complicates analysis (discussed later in “Caveats and Complexities: Novelty and Learning”).

THREE COMMON PROBLEMS OF INTERPRETATION
Oddly, some of the simplest explanations of behavior are proposed for human behavior—surely a paradox, since our behavior must be at the complicated end of any behavioral scale. Many misunderstandings arise because of one of three problems: confusing proximate triggers for behavior with their functional evolutionary causes; measuring reproductive costs and benefits incorrectly or sloppily, leading to confusion over whose costs and benefits matter; and failing to understand the impact evolutionary novelty can have on behavior. Let’s examine each of these problem areas in more detail.

Proximate versus Ultimate “Causes”
To understand why humans act as they do in particular circumstances we’d better understand what we mean by why, because we use it to mean different things, resulting in unnecessary disagreements. Why questions have two principal forms in biology: questions about proximate triggers and questions about ultimate selective causes. These two approaches are not alternative hypotheses (if one were true, the other couldn’t be), but complement each other. The ultimate cause of a behavior’s existence, in evolutionary terms, is always its effectiveness in getting one’s genes copied. Proximate triggers, the mechanisms or stimuli that cue or release

10

RESEARCH FRONTIERS

behaviors, are sometimes also called causes; they tell us what kinds of environmental factors are important. Again, let us start with a simple nonhuman example. If we ask why a bird migrates, one answer might be “changing daylength causes hormonal changes, triggering migration.” Both changing daylength and changing hormones are proximate triggers. If we could interview birds as we do ourselves, we might have another set of proximate causes, the equivalent of our reasons: “I really hate the cold,” “it makes me feel good,” “that way I get to see my relatives.” All of these are proximate cues. They do not explain why individuals in this species migrate (as opposed to others who do not), why not all individuals migrate, or why daylength (as opposed to temperature, some other cue, or a combination of cues) has become the trigger. The ultimate cause of migration is a seasonal better-versusworse geographic shift in foraging and nesting areas; individuals who seek the better areas, shifting seasonally, leave more descendants than those who remain in one area. (This is why insect-eating birds of the northern hemisphere are likelier to migrate south in the winter than seed-eaters; insects are relatively unavailable in northern regions during the winter). When daylength is the most reliable predictor of these seasonal shifts, individual birds who use it as a cue will fare better than those who use some other proximate cue or who fail to migrate. The benefits and costs of migration in terms of survival and reproduction may differ substantially for older, prime-age birds, compared with yearlings; in such cases, different categories of individuals are more or less likely to migrate. As we try to understand human behavior, it is crucial to differentiate proximate causes from ultimate causes, for they tell us very different things. Ultimate arguments are based on the relative reproductive costs and benefits for individuals. Elucidating proximate mechanisms can enrich our understanding, especially when our primary concern is intervention (e.g., as in medicine or family planning). When we wish to understand the functional evolutionary response, the evolutionary study of trait-environment correlations takes precedence. Answers to both kinds of why questions are important; however, no proximate cause will be maintained if it does not create an ultimate selective advantage. As one of my colleagues is fond of pointing out, only a handful of the Shakers, who imposed celibacy on their members, remain alive. That is why biologists frequently focus on the ultimate causal relationships.

HUMAN BEHAVIORAL ECOLOGY

11

Who Helps and Cooperates? The Levels of Selection Problem
The evolutionary function of some behaviors is immediately clear: behavior that makes food-getting efficient, for example. But costly behavior that helps someone else is often difficult to explain at first, and this topic is troublesome to many. Whose benefit drives the success of any strategy? The solutions reduce to the following:
1. Inclusive fitness maximization, or kin selection. Hamilton noted that not only one’s children but also other relatives carry copies of one’s genes; thus, helping relatives, even when it might reduce one’s own fertility, can be genetically profitable.14 This important insight solves many apparent problems when we look closely at behavior. For example, Darwin worried about the fact that honeybees had many nonreproductive workers, who raised the offspring of others in the hive. Hamilton showed that, because of their peculiar reproductive system, female bees shared more genes with their sisters than with their offspring. Thus, it could pay a worker bee (always female) genetically to give nepotistic care rather than reproducing herself. We expect organisms, including humans, to engage in activities that benefit relatives; the extent to which this is true will depend on the degree of relatedness, and the cost of the behavior. There is a story (probably apocryphal) that someone once asked the geneticist Haldane (who shared Hamilton’s insight) whether he would give his life for his brother. “No,” Haldane is supposed to have answered, “but I would for more than two brothers or eight cousins.” 2. Reciprocity. Sometimes we help individuals who are clearly unrelated. When help is never reciprocated, we lose—but mutual cooperation can be a highly effective competitive strategy.15 Such cooperation, called reciprocity, occurs just where you would predict it—only in long-lived social species in which individuals recognize each other and are likely to interact repeatedly. Organisms in long-lived social species, including humans, do things that help potential reciprocators without immediate payback, if there is some probability that there will be future interactions between them. If individuals interact only rarely or occasionally, such indirect reciprocity is extremely vulnerable to cheating; when this is true, individuals will mirror the behavior of others in a “tit-for-tat”16 manner (I’ll start by cooperating, but if you default I will, too). When risks are high and individuals mobile, it is not surprising that helping behaviors occur primarily among kin.

12

RESEARCH FRONTIERS
3. Group selection. The above helping strategies are clearly based on genetic costs and benefits. In addition, there are four quite different arguments about helping behavior that are sometimes referred to as “group selection.” Generally, these attempt to explain how individually-detrimental genes (like a gene for total altruism) could spread in a population. Although they are all called by the same name, the arguments and logic differ greatly. Wynne Edwards first suggested that animals might act against their own interests in order to help the group, directly opposing the theory of natural selection.17 Three other arguments that are also called group selection are logically consistent with ordinary natural selection.

Wynne Edwards argued that groups have traits such as dominance hierarchies that are not a simple statistical sum of individual traits, such that in any conflict situation the good of the group would prevail. Thus, he expected individuals routinely to pay reproductive costs for the good of the group. He argued that animals limit their individual reproduction for the good of the group, to keep numbers sustainable. Interestingly, Wynne Edwards argued that all species except humans were group-selected, because human populations seemed not to be regulated. He proposed appropriate tests: For example, he suggested that not only should female pronghorn antelope in bad years try to avoid matings, but that even strong territorial males (who usually monopolize matings) should not bother to mate. No such observation has been found, highlighting the problem that highly fertile “cheaters” would win if there is no individual disadvantage to high fertility. In fact, natural populations are not particularly stable. Wynne Edwards’ theory has been invoked to explain why so few children are born in societies like the !Kung. However, we must ask whether low fertility means low fitness. Careful empirical work (see “Optimizing Maternal Effort: Spacing Children”) shows that women with longer interbirth intervals, and thus rather low fertility, actually have more surviving children. They are more, not less, fit. Thus what looked like an individual disadvantage in fact turns out to be an individual advantage in that environment. Sometimes the group may seem to benefit as a result of the cumulative selection on individuals even though natural selection has acted on the level of the individual. It is important to test, rather than to assume, what the costs and benefits are. Although some anthropologists have uncritically accepted Wynne-Edwards’ model, Hawkes and Charnov18 have clearly shown its fallacies for humans.

HUMAN BEHAVIORAL ECOLOGY

13

The three other arguments often called group selection all consider genes (like natural selection), and all assume natural selection is working. Coalition group selection is really a special case of reciprocity, but I put it here because it is sometimes called group selection.19 An example is laws by which withingroup coalitions of some individuals in the group constrain the behavior of other individuals in order to seek their own interests. Interdemic group selection, simply stated, predicts that if certain conditions are met, the structure and composition of groups (demes are small groups) can influence the frequency of genes.20 Within each group, altruists lose to selfish-gene lineages. The conditions under which deleterious genes could spread are very restricted: There must be a number of small, viscous (inbred) groups close to each other, and occasional mixing of groups followed by restricted interchange. Because differential success occurs at birth and death (and the group equivalents of these), and because the turnover of individuals within groups is so much more rapid than the turnovers of groups, a bias exists for selection to be more powerful at the level of individuals. Thus, this model is logical and consistent, but unlikely to be widely powerful. Culture-gene interplay theory says that because the rules for cultural transmission and genetic transmission may differ, even under the same selective pressures, different genetic equilibria could result from cultural versus genetic selection.21 Boyd and Richerson note that this model of group selection, when applied to altruistic behavior “clearly is not verified by the data concerning ethnic cooperation among humans.” 22 This model, like interdemic selection, is logical but unlikely to be widely powerful. All explanations for the question of why individuals help others, except Wynne Edwards’s, acknowledge that ordinary natural selection operates. The bottom line is that only behaviors that enhance the success of a genetic lineage (such as behaviors that are selfish, parental, reciprocal, or help relatives and therefore enhance inclusive fitness) can evolve by natural selection. Cultural invention and cultural transmission can certainly give rise to behaviors that would not be favored by natural selection. But, because of natural selection, these behaviors are not likely to become common, or to replace genetically profitable behaviors. The general consensus is that ordinary individual selection will be relatively much stronger than such interdemic selection under all but the most restrictive conditions.

14

RESEARCH FRONTIERS

Caveats and Complexities: Novelty and Learning
We humans change our environment more, and more rapidly, than any other species—and when the environment changes, previously adaptive behaviors can lose their advantage. This creates enormous complications for analyzing our behavior. Imagine a behavior (eating sweet fruits) that has enhanced fitness (better survivorship) for a long time. Fruits are full of fiber and vitamins—and they are sweet. Now we invent refined sugar and create a new condition: We have super-sweet, super-delicious foods that are actually full of empty calories and bad, not good, for us. Yet, like other organisms, we did not evolve to be aware of ultimate selective relationships—just the proximate cues, correlated things that we can sense (e.g., pain from burns, pleasure from sex). That is why, in general, we like things that are good for us. But when conditions change, a behavior may continue to be driven by proximate cues (pleasure in delicious taste), even though these are now unhinged from the past functional advantage. Today we retain a preference for sweet taste that can be counteradaptive; the results include health risks and obesity. Humans create novel environments, and so it is likely that we can identify numerous behaviors, particularly in modern societies, that do not have current reproductive benefit.

UNEXPECTED PREDICTIONS FROM A BEHAVIORAL ECOLOGICAL APPROACH
Were We Ever Really Conservationists?
Behavioral ecology presents a challenge to our conventional wisdom about the way humans used resources in the past, compared to today. For example, what about the idea that people in traditional societies, being more directly and immediately dependent on the ecology of the natural systems around them, were more conserving and respectful of those resources? This is the Ecological Noble Savage concept.23 Yet if human resource use has followed

HUMAN BEHAVIORAL ECOLOGY

15

behavioral ecological rules, people are unlikely to give up shortterm individual or familial benefits for long-term societal or global gains, and this is likely to have been true throughout our history as a species, rather than a new phenomenon associated with new technology. Behavioral ecology generates alternative testable hypotheses about conservation:24
1. Deliberate exploitation in traditional societies is predicted when (a) it yields individual genetic profit (and/or its proximate cues, status or wealth enhancement), and (b) technology is sufficient to accomplish exploitation. Resources are most conservatively used when there is rapid and clear feedback regarding the impact on family and individual welfare (when overexploitation carries clear individual and familial costs). If too-heavy hunting means lean times soon, conservation is likely. 2. High density population and/or very efficient technology may lead to overexploitation and environmental degradation if there is profit. For example, Great Lakes Indian societies had quite sufficient technology to wipe out resident beaver populations, but until the Hudson’s Bay Company arrived to provide an insatiable market, there wasn’t much one could do with the thirtieth beaver pelt (given a market, one could turn it into fishhooks or tobacco). The impact of new, more efficient technologies will vary, depending on whether their use results in greater (short-term) individual and familial benefit.

Cross-culturally, expressions of conservation (e.g., reasons to leave some resource untouched) are very rare; only about 5 percent of societies have such rules.25 Resource-use patterns have very strong ecological correlates, but expressions of conservation do not. And finally, a conservation ethic does not, in fact, appear to result in broad protection of the resource base. In detailed studies of individual cultures, many follow the predictions of optimal foraging theory (maximizing individual and familial returns for effort), but so far no analyzed culture has shown any evidence of long-term conservation (e.g., avoiding killing a rare species if it is encountered, or avoiding killing pregnant females).26 These findings, predicted from behavioral ecological theory, suggest that while we may have evolved to be efficient resource-getters, and perhaps good short-term environmental managers, we have no history of far-sighted global conservation. Misunderstandings arose because low consumption was taken to imply a conservation ethic. Three things interact to cause some level of consumption: population level, level of available technology,

16

RESEARCH FRONTIERS

and profit. When there are few people, particularly when there is inefficient technology and low profit, consumption will be low— not because of a conservation ethic, but because a small group of people with low technology cannot overexploit.

The Ecology of Mating Systems
Plural marriage (polygamy) is common, but there is a very strong bias. Polygyny (one man married to more than one woman) is common ( allowed in about 93 percent of societies), but polyandry (one woman married to more than one man) is very rare. Perhaps because polygyny is so common, we rarely ask why, though cultural anthropologists know much about the patterns, and the correlates, of polygamy.27 But behavioral ecology makes some startling and unforeseen predictions about the ecology of mating systems. For example, the kind of polygyny—whether men are polygynous by controlling mates (harems) or resources (resource-based polygyny)—has ecological correlates (e.g., are resources like cattle defensible?), just as in other species. And there is a strong relationship predicted between serious pathogens and parasites (things like sleeping sickness), and polygyny, which holds even within geographic area and inside the tropics (two possible complicating factors).28 Why does behavioral ecological theory make such unexpected predictions? Pathogens, because they are living, evolving, biological entities, pose greater uncertainties than those of simple physical unpredictability.29 Why would polygyny be favored? If women can raise children more successfully with the help of men, and if pathogens are serious, a woman may be better off to become a healthy man’s second wife than the first wife of an infected man (in other species, this is called the polygyny threshold). 30 From a man’s point of view, because pathogens are an uncertainty, getting genetically more variable children would be reproductively profitable when pathogens are serious. However, we must be careful: A successfully polygynous man has more children, as well as more variable children, than a monogamous man. Fortunately, there is another test. Cross-culturally, some societies have a preference for marrying sisters (sororal polygyny). Since children of sisters will be genetically more alike than children of nonsisters, we would expect that when pathogen stress is high, nonsororal polygyny would be preferred—and this is exactly what we find.

HUMAN BEHAVIORAL ECOLOGY

17

Monogamy is less common than polygyny, for when some men can profit from polygyny, we predict that they will. There seem to be two kinds of monogamy. One occurs in relatively lowproductivity environments, in which men appear to gain more from investing in the children of one woman than from multiple matings—perhaps it is simply too expensive, and resources are too limited, to be polygynous successfully. The second kind, sociallyimposed monogamy, is characteristic of larger societies with relatively little class and caste distinction. In these cases, it is likely that powerful men are forced to give up some reproductive advantages for the sake of coalitions with other men. Even when monogamy is preferred, those few men who can marry more than one wife do so, both concurrently and sequentially.31 Polyandry is rare in mammals, including humans, for important cost-benefit reasons: Men can have more children by having more than one mate, but women cannot. Thus, the conditions under which polyandry is reproductively profitable are very rare. Polyandry seems to occur in two ecological conditions. In quite marginal environments, as for the Lepcha of northeastern India, brothers (kinship again) marry a single woman and work together to support her children, all of whom are considered socially to belong to the elder brother. In other situations, polyandry appears to concentrate the wealth of a lineage in a smaller number of descendants, effectively trading more children or grandchildren for fewer, extremely well-invested, ones.32

Optimizing Maternal Effort: Spacing Children
Earlier, I noted that behavioral ecologists suggest that having fewer-than-possible children is typically an optimization strategy, rather than a sacrifice (as Wynne-Edwards would have it). How could this be so? A mother’s investment in one child comes at the expense of investment in others, and closely spaced pregnancies, when nutrition or other factors are limiting, may result in lowered lifetime reproduction. Consider !Kung women, who have interbirth intervals of about four years. This seems, at first, to represent far lower fertility than is possible, and this is why group-selection arguments were made for the !Kung. But because predators are prevalent, !Kung women who depend on bush foods may carry their children at least occasionally for up to six years. Women with longer interbirth intervals had more (rather than fewer) surviving

18

RESEARCH FRONTIERS

children. A woman’s “backload” (weight of child plus foraged material) predicted the inter-birth intervals for each environment.33 !Kung women living in compounds, not dependent on bush foods, showed shorter interbirth intervals. Bush-living women maximized successful descendants not by maximizing rate of births, but by responding to the conflict between production of a new child versus the cost of such production on the survivorship of other children. (Refer back to “Who Helps and Cooperates? The Levels of Selection Problem.”) Nursing women can forage less than others.34 In some societies, these costs are partially defrayed by peer or sibling child care, and the availability of peer or sibling caretakers can have an impact on a mother’s lifetime fertility. On the island of Ifaluk, for example, a woman’s lifetime fertility is correlated with the sex of her first two children; women whose first two children were girls have greater lifetime fertility than others, because the (older) daughters could be asked to help with child care.35 Even grandmothers continue to contribute to the welfare of their grandchildren, working and gathering more than nursing mothers.36 In some societies, having enough money to hire wet nurses defrayed these costs. In the best-analyzed case the richest women had very short interbirth intervals, very high fertility, and low infant mortality; these richest women, who could afford the best wet nurses, fared best.37 Among the bourgeois, complexities created more variation in pattern. Poor women had longer interbirth intervals, lower fertility, and high infant mortality; and the wet nurses fared worst of all, with very long interbirth intervals, very low fertility, and very high infant mortality.

Abortion, Infanticide, and Abandonment
In other species, it is typically not parents but reproductive competitors (e.g., males taking over a harem) who commit infanticide, and this makes selective sense.38 Among primates, the overwhelming majority of infanticides are committed by immigrant males, or males who do not belong to the victim’s social group. In humans, also, stepparents are more likely to abuse or neglect children than genetic parents. Even when socioeconomic factors are taken into consideration, the risk for being killed is seventy times as great if the child lives with a stepparent as well as its natural parent.39 Among the Tikopia and the Yanomamö, a man may demand the

HUMAN BEHAVIORAL ECOLOGY

19

death of his new wife’s prior children.40 Such cases dramatically reflect the conflict of genetic interests between the parent and the nonparent who may be called upon to invest in the children. Yet parents can commit infanticide and abortion, and they can abandon their children. This seems counterselective, but because each infant requires great care, investment biases, even to the extent of infanticide, can sometimes be reproductively profitable.41 Selective reasons for killing or abandoning a child include a mother’s ability to invest, a mother’s access to additional resources (family, mate), a child’s ability to succeed, and the economic and reproductive value of other existing or future possible children. Cross-culturally, deformed or seriously ill newborns, those who have least chance of succeeding, are at greater risk for infanticide.42 Similarly, when circumstances reduce a mother’s chance of successful care (e.g., too-close births, twins, lack of an investing male), infanticide or neglect is more likely. Abortion, too, is more common when the birth of an additional child is likely to reduce the mother’s lifetime reproductive success. As women age, and their reproductive value declines, termination of investment is less likely. Even in our society, attitudes toward abortion are related to the proportion of women in any group who are “at risk” of unwanted pregnancy.43 Historical studies of child abandonment also reflect such selective considerations as a mother’s ability to invest in the child (including own health, familial resources, economic conditions), and the child’s health, legitimacy, and sex. Child abandonment in historical France, Spain, and Russia was related to economic factors, child’s condition, and mother’s abilities.44 In one study of child abandonment, 77 percent of cases were clearly related to maternal ability to invest and offspring quality, despite crude data and great variation in time, country, and other circumstances.45

IDEAS FOR EXPLORATION
Precisely because human behavior is so diverse, it is useful to look at it from a number of perspectives, with different levels of focus. The behavioral-evolutionary ecological perspective leads us to ask questions we might never ask otherwise, and can make sense of patterns that are otherwise puzzling. Behavioral ecologists study adaptive behavior in the organism, without worrying too much

20

RESEARCH FRONTIERS

about whether it is genetic, learned, or contextual—what is important is that heritable variation exists, not exactly how inheritance is mediated (though that is another interesting question). We typically assume Grafen’s 46 phenotypic gambit and simply study whether a particular trait or behavior enhances or reduces fitness in a particular environment. This new perspective suggests that there may be a continuum from other species to our own, despite our cultural complexity and variability, that we can use to analyze behavior. Success in foraging, wealth-seeking, and other forms of striving increase reproductive success, especially for men. Sex differences in behavior may make some ecological sense, though we must not commit the “naturalistic fallacy” (if it’s there, it should be so). We need to be careful and rigorous in our framing of hypotheses. It is also important to remember that we need not be bound by our evolutionary past, and that we live in environments that are novel, compared to our evolutionary past. The disagreements between behavioral ecology and more traditional approaches are fewer than they seem at first sight. Some, like Wynne Edwards’, simply arise from a misunderstanding of definitions (i.e., producing fewer than the maximum physiologically possible number of offspring is not necessarily a genetic sacrifice). Others are not disagreements at all: Fields like cultural anthropology look at the more proximate correlates of a behavior, while behavioral ecology makes predictions from the ultimate selective point of view, and both fields could profit from sharing perspectives. Thus anthropologists note that men have a higher death rate than women in most polygynous societies, and this results in fewer men being available as mates (a strong proximate correlate of polygyny).47 Behavioral ecology makes a reproductive cost-benefit argument; it notes that unless males can gain more through parental care than through many matings (with all the risks mating struggles incur) they will spend their reproductive effort in getting mates (a high-risk, high-gain strategy) rather than in caring for young. Thus, the conditions favoring polygyny also favor risky behavior in males, human or otherwise, and this means that males, in order to succeed in getting mates, must do many things that seem counter to ordinary selection—wait until they have grown large in order to win fights over rivals (thus delaying reproduction), fight for status, power, or territory, and often risk death.48 Thus, behavioral ecologists and anthropologists generate complementary explanations at different levels for the same phenomenon. Finally, behavioral ecology sometimes makes a priori predic-

HUMAN BEHAVIORAL ECOLOGY

21

tions, unexpected from any other paradigm, about behavior. Here I have concentrated on some examples of cross-cultural comparisons and optimization models, and on more ecological rather than psychological approaches to human behavior. Some very interesting new work focuses on other problems, including: the optimality of foraging; why and how boys and girls are taught differently; the ecology of warfare; moral systems as the societal outcomes of individual conflicts of interest; why men and women are so different; and psychological mechanisms (e.g., why it is so much easier for us to solve problems when they are set in evolutionarily-relevant contexts).49 Exploring these new areas can prove exciting as well as useful.

NOTES
1. Charles Darwin, On the Origin of Species (London: Murray, 1859); The Descent of Man and Selection in Relation to Sex (London: Murray, 1871); and The Expression of Emotions in Man and Animals (London: Murray, 1872). 2. Lee Cronk, “Human Behavioral Ecology,” Annual Review of Anthropology 20 (1991): 25–53. 3. Edward O. Wilson, Sociobiology: The New Synthesis (Cambridge, MA: Harvard University Press, 1975). 4. A typical criticism of the adaptationist program is that of Steven J. Gould and Richard Lewontin, “The Spandrels of San Marco and the Panglossian Paradigm,” Proceedings of the Royal Society of London B 205 (1979): 581–598. Critical discussions and responses include Richard Dawkins, The Blind Watchmaker (Essex: Longman, 1986); and Helena Cronin, The Ant and the Peacock (Cambridge: Cambridge University Press, 1991), pp. 81–110. 5. Good discussions of hypothesis testing in the study of behavior include: John Alcock, Animal Behavior, 5th ed. (Sunderland, MA: Sinauer, 1993), chapters 1 and 17; Eric A. Smith and Bruce Winterhalder, eds., Evolutionary Ecology and Human Behavior (New York: Aldine De Gruyter, 1993), pp. 5–23; and Richard D. Alexander, “Evolutionary Approaches to Human Behavior: What Does the Future Hold?” in Laura Betzig, Monique Borgerhoff Mulder, and Paul Turke, eds., Human Reproductive Behaviour: A Darwinian Perspective (Cambridge: Cambridge University Press, 1988), pp. 317–341. 6. Ibid. 7. J. P. Rushton, “Race Differences in Behaviour: A Review and

22

RESEARCH FRONTIERS
Evolutionary Analysis,” Personality and Individual Differences 9 (1988): 1009–1024. Optimization is discussed well in Smith and Winterhalder, Evolutionary Ecology and Human Behavior, pp. 50–60. Comparative method is discussed by Richard D. Alexander, Darwinism and Human Affairs (Seattle: University of Washington Press, 1979). Mark Ritchie, “Optimal Foraging and Fitness in Columbian Ground Squirrels,” Oecologia 82 (1990): 56–67. Bernd Heinrich, Ravens in Winter (New York: Summit, 1989). Richard Dawkins, The Selfish Gene, 2nd ed. (Oxford: Oxford University Press, 1989).

8.

9. 10. 11.

12. Alan Grafen, “Natural Selection, Kin Selection, and Group Selection,” in J. R. Krebs and N. B. Davies, eds., Behavioural Ecology: An Evolutionary Approach, 2nd ed. (Oxford: Blackwell Scientific, 1984), pp 62–84. 13. Bobbi S. Low, “Ecological Demography: A Synthetic Focus in Evolutionary Anthropology,” Evolutionary Anthropology 1, no. 5 (1993): 106–112. 14. William D. Hamilton, “The Genetical Evolution of Social Behavior,” Journal of Theoretical Biology 7 (1964): 1–16, 17–52. 15. Robert L. Trivers, “The Evolution of Reciprocal Altruism,” Quarterly Review of Biology 46 (1971): 35–57; Richard D. Alexander, The Biology of Moral Systems (Hawthorne, NY: Aldine De Gruyter, 1987). 16. Robert Axelrod, The Evolution of Cooperation (New York: Basic Books, 1984). 17. V. C. Wynne Edwards, Animal Dispersion in Relation to Social Behavior (Edinburgh: Oliver and Boyd, 1962). 18. Kristen Hawkes and Eric Charnov, “Human Fertility: Individual or Group Benefit?” Current Anthropology 20 (1988): 469–471. 19. Alexander, The Biology of Moral Systems; W. Irons, “How Did Morality Evolve?” Zygon 26 (1991): 49–89. 20. Sewall Wright, “Tempo and Mode in Evolution: A Critical Review,” Ecology 26 (1945): 415–419; William D. Hamilton, “Innate Social Aptitudes in Man: An Approach from Evolutionary Genetics,” in Robin Fox, ed., Biosocial Anthropology (New York: Wiley, 1975); D. S. Wilson, Natural Selection of Populations and Communities (Menlo Park, CA: Benjamin Cummings, 1979); Wilson, Sociobiology: The New Synthesis. 21. Robert Boyd and Peter Richerson, Culture and the Evolutionary Process (Chicago: University of Chicago Press, 1985); William Durham, Coevolution: Genes, Culture, and Human Diversity (Stanford: Stanford University Press, 1991); C. J. Lumsden and

HUMAN BEHAVIORAL ECOLOGY

23

22. 23. 24.

25. 26. 27.

28.

29.

Edward O. Wilson, Genes, Mind, and Culture (Cambridge, MA: Harvard University Press, 1981). Boyd and Richerson, Culture and the Evolutionary Process, p. 240. Kent Redford, “The Ecologically Noble Savage,” Orion 9 (1991): 137–154. Bobbi S. Low and Joel Heinen, “Population, Resources, and Environment,” Population and Environment 15, no. 1 (1993): 7–41; Raymond Hames, “Time, Efficiency, and Fitness in the Amazonian Protein Quest,” Research in Economic Anthropology 11 (1989): 43–85; Raymond Hames, “Wildlife Conservation in Tribal Societies,” in Margery Oldfield and Janis Alcorn, eds., Culture, Conservation, and Ecodevelopment (Boulder, CO: Westview Press, 1991); Michael Alvard, “Testing the ‘Ecologically Noble Savage’ Hypothesis: Interspecific Prey Choice by Piro Hunters of Amazonian Peru,” Human Ecology 21 (1993): 355–387. Bobbi S. Low, “Behavioral Ecology of Conservation in Traditional Societies,” Human Nature, in press. Alvard, “Testing the ‘Ecologically Noble Savage’ Hypothesis,” pp. 355–387. See, for example, George Peter Murdock, Social Systems (New York: Free Press, 1949); Carol R. Ember and Melvin Ember, Anthropology, 7th ed. (Englewood Cliffs, NJ: Prentice Hall, 1993); Melvin Ember and Carol R. Ember, Marriage, Family, and Kinship (New Haven, CT: HRAF Press, 1983); and Mark V. Flinn and Bobbi S. Low, “Resource Distribution, Social Competition, and Mating Systems in Human Societies,” in Richard Wrangham and David Rubenstein, eds., Ecological Aspects of Social Systems (Princeton: Princeton University Press, 1986), pp. 217–243. For the general arguments, see Cronk, “Human Behavioral Ecology,” pp. 25–53; and Monique Borgerhoff Mulder, “Reproductive Decisions,” in Eric A. Smith and Bruce Winterhalder, eds., Evolutionary Ecology and Human Behavior (Chicago: Aldine, 1992), pp. 339–374. For the parasite correlations, see Bobbi S. Low, “Marriage Systems and Pathogen Stress in Human Societies,” American Zoologist 30 (1990): 325–339; and Bobbi S. Low, “Pathogens and Polygyny in Humans,” in Laura Betzig, Monique Borgerhoff Mulder, and Paul Turke, eds., Human Reproductive Behaviour: A Darwinian Perspective (Cambridge: Cambridge University Press, 1988), pp. 115–127. Bobbi S. Low, “Human Responses to Environmental Extremeness and Uncertainty: A Cross-Cultural Perspective,” in Elizabeth Cashdan, ed., Risk and Uncertainty in Tribal and Peasant Economies (Boulder, CO: Westview Press, 1989), pp. 229–255; Low, “Marriage Systems and Pathogen Stress in Human Societies,”pp. 325–339.

24

RESEARCH FRONTIERS

30. The original description of polygyny threshold was that of Gordon Orians, “On the Evolution of Mating Systems in Birds and Mammals,” American Naturalist 103 (1969): 589–603. It is applied to human systems in Mulder, “Reproductive Decisions,” pp. 339–374. 31. Richard D. Alexander, John L. Hoogland, Richard D. Howard, Katharine M. Noonan, and Paul W. Sherman, “Sexual Dimorphism and Breeding Systems in Pinnipeds, Ungulates, Primates, and Humans,” in Napoleon A. Chagnon and William Irons, eds., Evolutionary Biology and Human Social Behavior: An Anthropological Perspective (North Scituate, MA: Duxbury Press, 1979); Bobbi S. Low, “Measures of Polygyny in Humans,” Current Anthropology 29, no. 1 (1988): 189–194. 32. William Durham, Coevolution: Genes, Culture, and Human Diversity (Stanford: Stanford University Press, 1991); M. Goldstein, “Pahari and Tibetan Polyandry Revisited,” Ethnology 17 (1978): 325–337; N. E. Levine, The Dynamics of Polyandry: Kinship, Domesticity, and Population on the Tibetan Border (Chicago: University of Chicago Press, 1988). 33. Nicholas G. Blurton Jones, “Bushman Birth Spacing: A Test for Optimal Interbirth Intervals,” Ethology and Sociobiology 7 (1986): 91–105; N. G. Blurton Jones, “Bushman Birth Spacing: Direct Tests of Some Simple Predictions,” Ethology and Sociobiology 8 (1987): 183–203. 34. A. Magdelana Hurtado and Kim Hill, “Tradeoffs between Female Food Acquisition and Childcare among Hiwi and Ache Foragers,” Human Nature 3 (1992): 185–216. 35. Paul W. Turke, “Helpers at the Nest: Childcare Networks on Ifaluk,” in Laura Betzig, Monique Borgerhoff Mulder, and Paul Turke, eds., Human Reproductive Behaviour: A Darwinian Perspective (Cambridge: Cambridge University Press, 1988), pp. 173–188. 36. Kristen Hawkes, J. O’Connell, and Nicholas Blurton Jones, Hardworking Hadza Grandmothers, in V. Standen and R. Foley, eds., The Behavioural Ecology of Mammals and Man (London: Blackwell Scientific, 1989), pp. 341–366. 37. Sarah Blaffer Hrdy, “Fitness Tradeoffs in the History and Evolution of Delegated Mothering, with Special Reference to WetNursing, Abandonment, and Infanticide,” Ethology and Sociobiology 13 (1992): 409–442. 38. Sarah Blaffer Hrdy and Glen Hausfater, eds., Infanticide: Comparative and Evolutionary Perspectives (New York: Aldine de Gruyter, 1984); Martin Daly and Margo Wilson , “Child Abuse and Other Risks of Not Living with Both Parents,” Ethology and Sociobiology 6 (1985): 197–210; Martin Daly and Margo Wilson,

HUMAN BEHAVIORAL ECOLOGY

25

39. 40. 41. 42. 43.

44.

45.

46.

47. 48.

“Children as Homicide Victims,” In R. J. Gelles and Jane B. Lancaster, eds., Child Abuse and Neglect: Biosocial Dimensions (New York: Aldine, 1987), pp. 201–214. Martin Daly and Margo Wilson, Homicide (Hawthorn NY: Aldine de Gruyter, 1988). Daly and Wilson, Homicide. See review by Low, “Ecological Demography: A Synthetic Focus in Evolutionary Anthropology,” pp. 106–112. Daly and Wilson, Homicide. Elizabeth Hill and Bobbi S. Low, “Contemporary Abortion Patterns: A Life History Approach,” Ethology and Sociobiology 13 (1991): 35–48. Laura Betzig and L. Lombardo, “Who’s Pro-Choice and Why,” Ethology and Sociobiology 13 (1991): 49–71. R. Fuchs, Abandoned Children: Foundlings and Child Welfare in Nineteenth-Century France (Albany: SUNY Press, 1984); J. Sherwood, Poverty in Eighteenth-Century Spain: Women and Children of the Inclusa (Toronto: University of Toronto Press, 1988); D. Ransel, Mothers in Misery: Child Abandonment in Russia (Princeton: Princeton University Press, 1988). John Boswell, The Kindness of Strangers: The Abandonment of Children in Western Europe from Late Antiquity to the Renaissance (New York: Vintage Press, 1990); See review by Low, “Ecological Demography: A Synthetic Focus in Evolutionary Anthropology,” pp. 106–112. The phenotypic gambit assumes that the phenotype (what you can see about the organism) reflects its genotype, or genetic composition, and that the organisms we see have nonrandom characteristics arising from particular genes interacting with particular environments. This allows us to ask, “What kinds of traits or characteristics work better in this particular environment?”—even when we cannot specify what particular gene or genes produce those characteristics. The phrase phenotypic gambit was coined by Grafen, “Natural Selection, Kin Selection and Group Selection,” pp. 62–84. Melvin Ember, “Warfare, Sex Ratio, and Polygyny,” Ethnology 13 (1974): 197–206. Bobbi S. Low, “An Evolutionary Perspective on War,” in William Zimmerman and Harold K. Jacobson, eds., Behavior, Culture, and Conflict in World Politics (Ann Arbor: University of Michigan Press, 1993). For the underlying general arguments, see Cronk, “Human Behavioral Ecology,” pp. 25–53; Mulder, “Reproductive Decisions,” pp. 339–374; Low, “Ecological Demography: A Synthetic Focus in Evolutionary Anthropology,” pp. 106-112; and Matt Ridley, The Red Queen: Sex and the Evolution of Human Nature (New York: Viking, 1993).

26

RESEARCH FRONTIERS

49. For general arguments, see Martin Daly and Margo Wilson, Sex, Evolution, and Behavior, 2nd ed. (Boston: Willard Grant, 1982); J. R. Krebs and N. B. Davies, eds., Behavioural Ecology: An Evolutionary Approach, 3rd ed. (Oxford: Blackwell Scientific, 1991); Smith and Winterhalder, Evolutionary Ecology and Human Behavior; and Elizabeth Cashdan, ed., Risk and Uncertainty in Tribal and Peasant Economies (Boulder, CO: Westview Press, 1990). For foraging models see B. Winterhalder and E. A. Smith, eds., Hunter-Gatherer Foraging Strategies: Ethnographic and Archaeological Analyses (Chicago: University of Chicago Press, 1981); For evolutionary psychology arguments, see Jerome Barkow, Leda Cosmides, and John Tooby, eds., The Adapted Mind: Evolutionary Psychology and the Generation of Culture (Oxford: Oxford University Press, 1992).

SUGGESTED READINGS
Alcock, John. Animal Behavior, 5th ed. Sunderland, MA: Sinauer, 1993, chapter 1, pp. 1–21, chapter 17, pp. 541–574. Here, treatment of human behavior in a classic animal behavior text gives insight into the behavioral ecological approach. Betzig, Laura L., Monique Borgerhoff Mulder, and Paul Turke, eds. Human Reproductive Behaviour: A Darwinian Perspective. Cambridge: Cambridge University Press, 1988. Interesting collection of empirical work—how to test hypotheses in the real world. Cronk, Lee. “Human Behavioral Ecology.” Annual Review of Anthropology 20 (1991): 25–53. Probably the best short, easy-to-read overview of the general issues. Daly, Martin, and Margo Wilson. Sex, Evolution, and Behavior, 2nd ed. Boston: Willard Grant, 1982. Easy to read, with many excellent examples—makes the evolutionary continuity between humans and other species clear. Dawkins, Richard. The Selfish Gene, 2nd ed. Oxford: Oxford University Press, 1989. The classic statement of the selfish gene paradigm. Smith, Eric A., and Bruce Winterhalder. Evolutionary Ecology and Human Behavior. New York: Aldine de Gruyter, 1992. An edited collection of optimality approaches to the behavior of humans (and related primates).

Research TOC