An economic approach to the evolution of male-female exchange.

AN ECONOMIC APPROACH TO THE EVOLUTION OF MALE-FEMALE EXCHANGE William O. Shropshire

Oglethorpe University

Males and females of a number of animal species divide labor and provide jointly for offspring. Males may provide food, for example, while females protect defenseless young. This exchange is unlikely, however, unless a prior partnership has been established in which a female practices fidelity in exchange for a male's provisioning activity. The formation of the trading partnership is itself an exchange, and economic theory can help explain when and why there are mutual gains from trading fidelity for resources. Environmental factors determine the potential gains from trade while evolved psychological mechanisms influence the extent to which gains are realized. KEYWORDS: Certainty of paternity; Exchange theory; Fidelity; Pair bonding; Parental investment

Artifacts appearing to be about two million years old have been found together with animal bones of the same age, suggesting that p r o t o h u m a n s killed (or scavenged) the animals in one place and ate them in another. On the basis of these facts, Isaac (1978, 1983) has speculated that males brought food to a home base to share with females w ho remained at h o m e to protect the young. Males m ay have given food in exchange for defensive services provided by females, but such exchange w oul d require males and females already to have established a trading partnership in which

Received August 31, 2001; accepted pending revision November 20, 2001; revised version received October 30, 2002.

Address all correspondence to William O. Shropshire, 1516 Emory Road, Atlanta, Georgia 30306. E-maih [email protected] Copyright 2003by Walter de Gruyter, Inc., New York Human Nature, Vol. 14, No. 3, pp. 235--266. 235

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they exchange resources for fidelity.1 At least as early as David H u m e ([1739] 1992:570), writers have observed that a male would give food to a female who is protecting his offspring, not the offspring of some other male. Thus, in order understand early exchange of goods and services between males and females, we must investigate the origins of the partnership that would make such exchange feasible (Irons 1983). As an aid to this investigation, I will apply the standard theory of economic exchange to argue that pair bonding itself could have been a case of mutually beneficial exchange. If, as is argued here, pair bonding provided reproductive benefits to both males and females, then such behavior has an evolutionary basis and is not entirely the result of the exercise of uneven power by males. Pair bonding, however, would not have been possible without the evolution of psychological mechanisms that m a d e our ancestors, for the first time, capable of experiencing emotions that we call love, shame, guilt, and jealousy. These mechanisms, along with environmental factors, are discussed in subsequent sections. Although the focus of the paper is on the evolution of h u m a n pair bonding, the theory of economic exchange is generally useful in organizing thinking about the origin of mating structures in other animals as well. As another objective, I will indicate the potential broader applications of the approach taken here.

MALE A N D FEMALE REPRODUCTIVE STRATEGIES A N D G A I N S FROM EXCHANGE

Natural selection can be expected to have produced an efficient allocation of animals' time among all activities. Let us define the marginal product of an activity as the increment to reproductive succ6ss attributable to an increment of time spent on that activity. Animals, then, will achieve efficiency when they equate marginal products of all their activities (Hames 1992). First, consider efficiency in the allocation of time spent on mating activities. In a species without male/female partnerships, males would spend all their mating effort seeking, courting, competing for, and inseminating females, and guarding against copulations with other males. Since efficiency would require the marginal products of all these activities to be equal, we can speak of the marginal product of mating. Next, consider survival effort. When males spend time mating, they forgo nourishment and become more subject to predation, thereby endangering their survival (Roff 1992:chap. 6). Some reduction in life span is consistent with improved reproductive success, but sacrificing survival for reproductive activity will reduce reproductive success if taken too far. Again, maximization of reproductive success would require males to

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equalize the marginal products of each survival activity---seeking new food sources (Charnov 1976), competing for existing sources, and defending against predators. Moreover, the marginal products of these two sets of activities--mating and surviving--will be equal. For simplicity let us assume that the marginal products are equal when males divide the hours of the day equally between mating and surviving so that total time spent mating is twelve hours. Under the right ecological conditions, an additional potential path to reproductive success could arise. A male could provide resources to a selected female and her (and his) offspring (Hamilton 1964; Trivers 1972). For such parental investment to be a good strategy, switching one of his twelve mating hours to an hour of providing must produce a net increase in offspring that survive to reproductive age. That is, to warrant switching, the marginal product of provisioning must exceed the marginal product of mating. There is, however, a consideration in addition to the physical productivity of provisioning effort. The male must also have some certainty that the female's offspring are his, and not those of some other male (Hill and Kaplan 1988). With sufficient physical productivity of provisioning, some level of certainty of paternity will raise the marginal product of provisioning above the marginal product of mating. If so, then the male would gain in reproductive success by shifting an hour of effort from mating to provisioning. Now consider the reproductive strategy of the females of our ancestor species, in which males made no investment in females they impregnated or in the resulting offspring. As with present-day nonhuman primates, males of this species may have been solicitous of a female's existing infants (Clutton-Brock 1991) and may have brought her food, particularly meat, in order to win her sexual favors (H. Fisher 1982), but provisioning would have ended with copulation. Under these circumstances, mating with multiple partners was probably adaptive for our human female ancestors for a number of possible reasons (Pusey 2001). There is evidence that the reproductive variability of female primates, while less than that of males, is considerable for both nonhumans (Hrdy 1981:chap. 6, 1986; Hrdy and Williams 1983) and humans (Brown and Hotra 1988; Einon 1998; Gould 2000; Hewlett 1988). Having multiple partners would have raised the number of pregnancies toward the biological maximum (R. Smith 1984). In addition to increasing the number of pregnancies, females could have increased the probability of being inseminated by the "best" male through having multiple partners, which seems to be the case with other species (Cronin 1991:chap. 8; Hrdy 1997; Petrie et al. 1992; Ridley 1993). A female also could have benefited by having a number of partners bringing meat while competing for her sexual favors. Moreover, some of her many sexual partners may have protected their putative offspring from infanticidal

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attacks by other males (Bercovitch 1995; Buss and Schmitt 1993; H. Fisher 1992"chap. 4; Hrdy 1981:chap. 5; M~ller 1988). The extent to which and the w a y in which female ancestors of humans were able to control which males fertilized their ova is a subject of contemporary theory and research (Gowaty 1997). If, however, a female could exercise some choice, she could potentially gain from a new strategy; she could attract and select a male who would provide resources and protection to her and her offspring during a long period of infant dependency (Trivers 1972). The appearance in the population of such a male and his selection by a female would require new genetic variants in both males and females through recombination or mutation. (These are the ultimate sources of variation. Frequency changes in the population occur through genetic drift or natural selection [Endler 1986].) If such a choice became possible, the female could gain reproductive success if the benefit from a male's provisioning exceeded her cost of giving up some other sexual partners. Although there were probably fundamental differences in the reproductive strategies of our male and female ancestors (Lancaster and Kaplan 1992; R. Smith 1984), benefits to both sexes could have accrued from new strategies if the differences were resolved by trading partnerships. The trade-offs necessary for the evolution of these partnerships can be analyzed with the economic theory of exchange.

Graphic Depiction of Male Strategy Figure 1 depicts the supposed reproductive strategies available to a male h u m a n ancestor. He divides his reproductive effort between Mating and Provisioning (not shown on Figure 1). The horizontal axis of Figure 1 (Hours Mating) measures hours a male spends on various mating activities with females other than his potential trading partner. Based on the arbitrary assumption made above that the marginal product of reproductive activity equals the marginal product of survival activity at 12 hours, Hours Mating = 12 - Hours Provisioning. (Upper case letters will be used w h e n referring to the numbers on the axis of this and the following graphs.) A male would allocate Hours Provisioning to providing food, shelter, transportation, and protection. Again, natural selection should produce an efficient allocation of effort so that the marginal products of all provisioning activities would be equal. Other things remaining unchanged, a male who reduced Hours Mating and increased Hours Provisioning would almost certainly experience a decrease in reproductive success. Hirshleifer (1995) and Hawkes et al. (1995) found that a male is unlikely to gain from a shift from mate guarding (an activity included in Mating) to provisioning because competing males will "steal" the increased reproductive success. A gain is possible,

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however, if a male mates with a female w h o shows signs of rejecting copulations with other males. When the male reduces Hours Mating, he reduces mate guarding along with seeking, competing for, and inseminating other females. But what he loses in certainty of paternity because he guards less can be offset by his gain from the female's behavior. The vertical axis in Figure I is an index of the certainty that a female can give a male that he is providing resources for his own offspring rather than for those of some rival male. The index C (for certainty) ranges from zero to one and measures only the certainty the male obtains from the female. Each point (such as a) on the graph represents a combination of hours spent on the various mating activities (Hours Mating) and the degree of the male's certainty of paternity (C). Associated with each point is a measure of reproductive success. The curved line, called an isoquant, passing through point a shows all the other strategies that will render the same measure of reproductive success as the strategy at a. 2 The isoquant passing through a, for example, also passes through the point x on the horizontal axis. At x, the male spends 12 Hours Mating and 0 Hours Provisioning because female behavior provides him with no certainty of paternity. If his C index rises to .31, he can (as shown at a) spend 5 Hours Mating and 7 H o u r s Provisioning and maintain the same reproductive success as at x. Strategies a and x and all other strategies on the line yield him the same level of reproductive success, which is indicated by the label MRS 2. (The subscript is an ordinal, not a cardinal, measure of reproductive success.) The line intersects the vertical axis at the level of C necessary for the male to attain the same reproductive success as at x, where he spends all his time Mating. (Because of the productivities used to graph the isoquant, this C is 1.00, but it need not be so. Individual differences in productivities depend, for example, on male attractiveness [Buss 2000:chap. 6].) The assumption underlying the male isoquants in Figure 1 is that reproductive success is the sum of functions of Mating and Provisioning: MRS = f(M) + Ch(P). The marginal products (the derivatives, f' and h') are positive and decline with increases in the activities. Declining marginal productivity is a common assumption in the analysis of both h u m a n and nonhuman productive activity (Hill and Kaplan 1988; Ricardo [1817] 1911:chap. 2; Weissburg 1992). The isoquant MRS 2 is negatively sloped because when the male reduces Mating his reproductive success will be less than at x unless he has some certainty that he is providing for his o w n offspring. Therefore, to maintain the same reproductive success, C must rise when Mating falls. The same is true of other points on the isoquant, such as a. 3 The shape of the isoquant--its convexity over some ranges and concavity over others--is the result of the interaction of the levels and rates of change in C and the marginal products of Mating and Provisioning. For any given allocation of time between Provisioning and Mating, a higher level of certainty of paternity (C) will yield the male a higher level


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of reproductive success. As seen in Figure 2, by spending 11 H o u r s Mating and I Hour Provisioning, for example, the male can attain MRS 2 if the level of C is about .06. If, however, he could have a C of .32, his reproductive success would rise to the level represented by MRS 3. Therefore, isoquants such as MRS 3 and MRS 4, which lie above and to the right of MRS2, represent higher levels of reproductive success. As we shall see, males can reach higher isoquants through exchange. These isoquants m a y have a positively sloped segment. 4 For example, with C of .32 and Hours Mating of 11 on isoquant MRS3, the marginal product of Provisioning (h') is very high relative to the marginal product of Mating (f'). Specifically, at Mating = 11, h' = .42 and f' = .08. Thus, a male can reduce Hours Mating and maintain the same reproductive success even with a drop in C. Hence, the isoquant is positively sloped in this range. As reallocation from Mating to Provisioning continues, however, the productivity of Provisioning declines while the productivity of Mating rises, and the male cannot attain the same level of reproductive success without a rise in C. At this point, the slope becomes negative. In general, an isoquant's slope will be negative or positive according to whether the absolute value of (CMPp - MPm) is negative or positive. Natural selection w o u l d not favor males w h o pursued Hours Mating beyond the point at which the isoquants become positive, so we will only consider the negatively sloped sections of the curves in the following analysis.

Graphic Depiction of Female Strategy N o w consider the female's possible strategies, or the trade-offs between attracting a long-term provider and soliciting multiple partners. As with the male, her strategies can be depicted with an isoquant diagram, assuming all sources of reproductive success other than the number of partners (N) and the amount of provisioning (P) remain unchanged. The production function used to draw the isoquants in Figure 2 is FRSi = (2P)Y + w N z, where w is a constant and the exponents y and z are assumed to be less than one, which results in diminishing marginal returns to P and N. The isoquant shows the combinations of N and P that yield the same level of reproductive success for a female. If males control the female in ways that prohibit her from choosing a mate w h o provides (Gowaty 1997), she cannot move to different points on the isoquant. In this case, if a female reduces the number of partners she w o u l d move d o w n w a r d on the vertical axis and reduce her reproductive success. The vertical axis in Figure 2 measures an index of the number of partners and ranges from 0 to 1.00. An index of 1.00 represents the number of partners per estrus cycle if there is no pair bond. (When sexually receptive, female chimpanzees mate about twice per hour for twelve days, and some of these copulations are with the same males [Hrdy 1997]. If our female

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protohuman ancestors were like chimpanzees they w o u l d have mated with all male members of their band several times during a cycle.) An increase in the number of a female's sexual partners corresponds to a decline in the male's certainty of paternity index, C. Hence, C = (1 - N). W-hen her N = 0, she is forgoing all advantages of multiple partners b y refusing all but one male, whose certainty index (C) is 1.00, meaning that he is the father of all her offspring. The horizontal axis measures the number of hours the male will spend providing for the female and her offspring during their infancy (Hours Provisioning). Each point (such as b) on the graph represents a combination of the Hours Provisioning a female receives and the number of partners she accepts. The isoquant passing through b (FRS6) is the locus of all combinations of Hours Provisioning and the number of partners that give the female the same reproductive success as the combination at b. At x, for example, the female seeks success through multiple partners exclusively and does not grant differential sexual access to any sexual partner. Hence, she receives no Provisioning. She could move, however, to point b and attain the same level of reproductive success. She w o u l d sacrifice some advantages of multiple partners b u t she w o u l d offset this loss with an increase in a male's investment of resources in her and her offspring. Other isoquants (FRS7 and FRS8, for example) represent higher levels of female reproductive success. She potentially can attain these higher isoquants through exchange, as explained in the following section. The female's reproductive success isoquants are negatively sloped and convex to the origin because the marginal products of Provisioning (P) and the number of partners (N) are assumed positive but declining, s

Mutual Determination of Strategies Figures 1 and 2 are combined in the Edgeworth-Bowley box diagram shown in Figure 3. 6 This figure is constructed b y rotating Figure 2 to the left by 180 degrees and joining it with Figure 1. The vertical axes range between 0 and 1.00 in Figure 3, with an increase in C corresponding to a decrease in N. (The left axis is read bottom to top and the right axis is read top to bottom.) The horizontal axes range from 0 to 12. (The upper axis is read right to left and the lower axis is read left to right.) A one-hour increase in Hours Mating on the male's horizontal axis results in a one-hour decrease in Hours Provisioning on the female's horizontal axis. In that humans form pairs and other great apes do not, I will assume that pairing began more recently than the family tree's branching (about 5 million years ago) that led to humans, chimpanzees, and bonobos (Wrangham and Peterson 1996:4). That is, the approach taken here is that their common ancestors were promiscuous, with both males and females


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having many sexual partners and with no special, lasting relationships forming between any pair. The evolutionary path I have in m i n d does not lead from an early pair-bonded A d a m and Eve living in pure m o n o g a m y until falling into the sins of infidelity and deceit. Instead, it seems more likely that humans evolved from chimp-like ancestors (de Waal 2001; Wrangham 1993) when ecological circumstances m a d e pair bonding beneficial to both males and females. Hence, I assume that the male and female h u m a n ancestors depicted in Figure 3 do not form a trading partnership. This means they are at point x on the diagram. The female does not offer differential sexual access to the male in exchange for resources. Instead, she attains her greatest reproductive success (FRS6) by seeking the number of partners that will optimize her reproductive success from mating (as opposed to survival) activities. The male spends all his nonsurvival time on mating activities and has a C index of zero. With zero certainty of paternity, this strategy yields his highest reproductive success (isoquant MRS2). The diagram illustrates with hypothetical numbers the possibility of mutually beneficial exchange in this situation. The male could have the same degree of reproductive success (remain on MRS2) by providing 7 hours of resources to offspring that were his with .31 certainty. That is, he could exchange 7 hours for .31 increase in certainty and be equally well off at point a as at point x. How about the female? She could move to point b and remain on the same isoquant if she reduced her N by .31 to .69 and received about one-quarter hour's worth of resources in exchange. So, a female's loss of .31 in N in exchange for less than 7 hours' but more than one-quarter hour's worth of resources would improve the reproductive success of both male and female. Any point in the unshaded area, such as d, represents an improvement in the reproductive success of both parties. At d, a male attains higher isoquant MRS3 and a female attains higher isoquant FRS7. Moving to d from x would mean the male increases his certainty by .31 and spends about 1.5 fewer Hours Mating (1.5 more Hours Provisioning). Providing up to 7 hours worth of resources would have left him equally successful. The female could have given up .31 of N to get only one-quarter hour's worth of resources, but at d she would get 1.5 hours' worth. She, too, would be clearly better off at d than at x. Males and females that engage in this type of exchange become a pair and improve their reproductive success relative to those who do not exchange and remain at x. The offspring of the exchanging pair carry the genetic material that made pairing possible and are also differentially reproductively successful. Thus, the movement from x to a point like d illustrates the economics of the evolution of trading partnerships in those species that practice it. An ethologist observing the behavior depicted at x would call it promiscuity. Exchange between male

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and female, such as that at d, would be reported as m o n o g a m y accompanied by some adulterous behavior by both males and females.

ENVIRONMENTAL CONSTRAINTS O N MATING STRUCTURES The possibilities for mutually advantageous exchanges of certainty for provisioning are constrained by the shapes of the isoquants, which result in turn from the productivity of mating, provisioning, and having multiple partners. Following a discussion of the determinants of these productivities, I will illustrate the general applicability of the model with some examples, including an analysis of the divergent evolutionary paths of humans, chimpanzees, and bonobos. Determinants of Productivities

Provisioning. Visually, the closer the isoquants MRS 2 and FRS6 are to the axes, the larger the unshaded area and the further from x the mutually determined strategies can be. Figure 4 illustrates how isoquants w o u l d appear if a male were more productive at provisioning than assumed in Figure 3. As shown, the possibilities for mutually beneficial exchange are greater to the extent that male provisioning productivity is greater. A species for which provisioning productivity is low relative to mating productivity will have male and female isoquants passing through x that are close together and enclose a small unshaded area. Two aspects of provisioning activity will lead to high productivity of Hours Provisioning and the greater unshaded area of Figure 4: (1) high physical output from the activity and (2) high value of the output to the female and her offspring. Physical output will depend in part on what nature provides in the way of climate and natural resources. In addition, the contribution to reproductive success of an Hour Provisioning will be determined by the extent of the male provider's use of tools, his ability to cooperate with others to achieve the benefits of division of labor and specialization, and the level of competition from other species. The value of a male's provisioning output to a female and her offspring will depend importantly on the female's own provisioning productivity. If it is low, then an hour's worth of provisioning from a male will save her more time for other activities than if her provisioning productivity is high. Therefore, the value to her of provisioning from a male will be higher the lower her own productivity. Further, a male's physical output will contribute to reproductive success to the extent that the value of additional increments of food remains high as the male brings more and more food. The first units of food contribute

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to the survival and health of offspring, but subsequent units will add less and less to the chances that the offspring will have high reproductive success. The rate of decline in the value of additional units of food will depend on such things as the rate of metabolism of mothers and infants and the incidence of multiple births. Multiple births and high metabolism will result in slowly declining marginal value productivity of provisioning activity. On the other hand, another w o r m brought to an already surfeited single chick will add little to reproductive success. In addition, the value of male activity can decline less rapidly if the food provided by the male enables the female to gather less food and defend the young more effectively or come into estrus more quickly (Key and Aiello 2000). Or, if the male can provide defense or transportation instead of food, returns from his efforts can remain high as he spends more hours provisioning. Male carrying is particularly important in species where twinning is typical (Dunbar 2001; Goldizen 1986). In general, the male's productivity of provisioning will diminish less rapidly if he and the female are less specialized.

Mating. Consider a species with males whose mating productivity is high relative to that of the species shown in Figure 3, other things being the same. In this case, the male isoquant passing through x will be steeper than the one shown in Figure 3. A steeper isoquant would reflect, for example, a situation in which females were spatially less widely distributed, allowing males to attract, guard, and monopolize multiple mates with less effort (Clutton-Brock and Harvey 1978). The steeper isoquant would indicate a smaller unshaded area and less opportunity for mutually beneficial exchange.

Multiple partners. The lower the returns from a female's having multiple partners, the steeper the female isoquants and the larger the unshaded area in Figure 3. If females and their infants are isolated and therefore less likely to be exposed to infanticide committed by males, they will gain little from any given level of N. The degree of isolation, in turn, will depend on the dispersion of food supplies, as in the case of gibbons and gorillas. Females of these species rely on a single strong male for protection rather than on the confusion of paternity gained through a high N. The benefit from any level of N is also likely to be low w h e n males do not control resources that females might otherwise obtain through sex. If females with dependent infants can obtain food at a low cost, they gain relatively little reproductive success from having m a n y partners.

Applications of the Analysis of Environmental Constraints Parts of Wrangham and Peterson's (1996) and Wrangham's (2001) account of the different evolutionary paths of bonobos, chimpanzees, and


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humans can be expressed in terms of Figures 3 and 4. The ancestor species could have had a small unshaded area that it never exploited to the mutual reproductive success of males and females. Environmental conditions could have remained the same for chimpanzees but changed to increase the unshaded area for protohumans and to reduce it for bonobos. In the extreme, the environment for bonobos could have changed so much that the female isoquant passing through x was below the male isoquant at all levels of Mating. That is, there was no unshaded area. The logic of Figure 3 suggests that we should look to some changes in the productivities to explain the divergence in mating structures between humans and chimpanzees/bonobos on the one hand and between chimpanzees and bonobos on the other. An increase in male provisioning productivity and declines in the productivity of having multiple partners and in male mating activities would have promoted the human mating system. We should look to environmental events as sources of changes in these productivities. Wrangham and Peterson (1996) and Wrangham (2001) explain how climate changes could have led to bipedalism and cooking in the h u m a n ancestor species (australopiths). Bipedalism made australopiths more vulnerable to predators, which m a y have fostered the evolution of cooperation against cooperative attack (Wrangham 2001). If this new capacity affected males differentially, what originated for defense also could have served to increase hunting productivity and thus male provisioning productivity. When cooking became common, a new role for males app e a r e d - t h a t of guarding roots that females accumulated and cooked (Wrangham 2001). Both these developments from climate changes could have increased the provisioning productivity of males. While it is not clear what impact these environmental changes would have had on the other productivities, if these other effects were neutral, the improved provisioning productivity would have shifted the isoquants as shown in Figure 4. Wrangham and Peterson (1996) trace the evolutionary divergence of bonobos from chimpanzees to a climate change that isolated a group of chimpanzees in an area with no gorillas. Because they did not have to compete with gorillas for food, male and female bonobos could travel and eat together in large groups. Females could form alliances, which enabled them to overcome the male dominance that characterized chimpanzee social life. Becoming co-dominant, females did not depend on a male for resources or for protection from other males that might threaten them and their infants. In terms of Figure 3, the value of resources or protection that a male could offer a female in exchange for fidelity fell. Consequently, the female isoquant passing through x became less steeply sloped and perhaps even fell below the male isoquant at all levels of Provisioning. If this were the case, no mutually beneficial exchanges between males and females were possible and pair bonding was not consistent with the environment.

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The usefulness of Figures 3 and 4 can be illustrated further with Robert Smith's (1984) treatment of what he calls the conflict in reproductive interests of males and females. He says males want exclusive access to a female with minimum expenditure. On the other hand, he argues, selection favors females w h o extract maximum contributions from principal mates while engaging in extrapair copulations. Stated this way, the interests of males and females cannot be depicted in Figure 3, which recognizes trade-offs. Exclusive access (C = 1) is not compatible with minimum expenditure (zero Provisioning). There are no free goods in economics or evolution. Nature does not select maximum benefits or minimum costs; it selects the maximum difference between costs and benefits. When there are differences between costs and benefits of actions for both males and females, as shown in Figure 3, there is not conflict but opportunities for mutually beneficial exchange. As a final, brief, example, H r d y (1999:233) does not really mean that an increase in the susceptibility of infants to death w o u l d cause men to try to "sire as many [children] as possible." More correctly, this environmental change w o u l d cause a change in productivities, steepen the male isoquant in Figure 3, and alter the range of beneficial exchange between males and females. The outcome would depend on the shapes of the isoquants, which w o u l d set the terms of exchange between males and females. In the case of pair bonding (or other instances of coevolution), each party loses some reproductive success because it reduces one activity, but the activity of the other party adds to its reproductive success. If there are net gains to each party, the evolution can occur. The slopes of the isoquants in Figure 3 reflect the losses and gains and the possibility for change. Moreover, the isoquants can be used to analyze the possible outcomes of environmental changes. Such changes will result in a predictable change in the isoquants, but not a predictable outcome, which will be the result of the parties" evolutionary response to the changes in the terms of exchange. For example, the effect on Provisioning of an increase in male provisioning productivity will depend upon the shapes of the isoquants and the behavioral responses discussed in the next section. EVOLUTIONARY BEHAVIORAL ADJUSTMENTS The environment could have produced opportunities for mutually beneficial exchange of fidelity for resources and protection by h u m a n ancestors, but protohumans could not have achieved the gains without the evolution of new behavioral traits. The transition from the promiscuous mating system of our protohuman ancestors to the serial m o n o g a m y with adultery that characterized our forebears in the Pleistocene (H. Fisher 1989, 1992; Miller 2000:187) required a set of novel abilities. To be suited for long-term


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partnerships, males and females would have had to able to (1) advertise their capacity for being a partner, (2) assess alternatives, and (3) persuade each other to live up to their advertised capacities. The following sections deal with the likelihood that such capacities could have evolved.

Capacities Required for Pair Bonding Advertising. Suppose a typical male had no special exchange relationship with a typical female. Thus, the two would have been at x. If a male, because of a mutation, unilaterally gave up some Hours Mating and moved to c, he would have been reproductively unsuccessful and the mutation would not have persisted. A similar fate would have awaited a mutation that caused a female to move to c'. If, however, a male with the ability and inclination to invest could find a potentially faithful female, natural selection could reward the pair with a move to a point such as d. Such a pairing, however, would not have been possible without the capacity to advertise the traits. The h u m a n tendency to recognize hierarchies of status and power D found in traditional hunter-gatherer societies, ancient civilizations, and contemporary industrial and nonindustrial societies--indicates that humans do advertise their access to resources (Buss 1999:chap. 4). As for advertising an inclination to invest resources, American m e n seeking sexual partners tend to imply a willingness to invest by overstating their feelings of commitment (Buss 1994:chap. 7). The prevalence of the same tendencies in other species suggests that these h u m a n tendencies could have a biological rather than purely cultural basis. Male bowerbirds display their capacity to provide resources by building elaborate structures (Cronin 1991:62; Trivers 1985:333). Brush turkeys build compost piles (Ridley 1993:127). Gray shrikes advertise their capacity to provide resources by augmenting their usual store of edible prey and other items and displaying it in anticipation of the arrival of prospective mates at their nesting grounds (Buss 1994:22; Yosef 1991). It is reasonable to suppose that the analogous h u m a n advertising is also an evolved trait. What of female ability to show promise of faithfulness? First, it might seem that a female would not have to advertise faithfulness--that she could enjoy the benefits of multiple mating while receiving provisions from a male who stayed around to help after copulation. Natural selection, however, would favor a male who could detect multiple partners and reduce provision of resources accordingly (Hill and Kaplan 1988; Irons 1983). Moiler (1988) found, for example, that male swallows fed nestlings less when he experimentally increased their mates' extrapair copulations. Human evidence also suggests that our female ancestors would have had to accept a trade-off between resources and having multiple partners. In

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186 societies studied, Betzig (1989) found that adultery was a significantly more common cause for dissolution of marriage than was any other except infertility. Although not confirmed by cross-cultural tests, the American evidence is that males rate sexual loyalty as the most highly valued characteristic in a committed relationship (Buss 1999; Buss and Schmitt 1993). The capacity of females to recognize, measure, and avoid the potential costs of extrapair copulations was probably necessary for pair bonding to evolve. Thus, females apparently would have had to advertise their capacity for faithfulness. What evidence is there that our female ancestors did so? H u m a n females avoid public promiscuity and report on the promiscuity of other females in order to denigrate rivals in the eyes of males seeking long-term relationships (Buss 1999:chap. 10, 2000). Custom and tradition may provide additional ways to advertise faithfulness (Dickemann 1981), but the basic tendency to avoid public promiscuity is probably a biologically based trait. Hrdy (1999:260) ponders the extent to which modesty is the result of early learning (as claimed by H u m e [1992 [1739]:570]) or a genetic trait that evolved in humans because those lacking it died young at the hands of dominant, mate-guarding males. The existence of modesty in other species suggests that it is not entirely the result of learning. Some female birds that form partnerships attempt to hide extrapair copulations (Ridley 1993:213). Female chimpanzees know to hide matings with subordinate males from the alpha male (de Waal 1996:91). The contemporary practice of modesty, which other species share, is probably a trait that our protohuman ancestors also practiced to show that they were worthy recipients of resources and protection provided by potential male partners or consorts.

Assessing.

What were the requirements for a protohuman male who had to assess various reproductive strategies? He had to be able to recognize a female with w h o m he had copulated and to provision that female and not others. To allocate optimally, males had to equate returns from seeking additional mates and returns from provisioning. This, in turn, required that males determine the probability that they were providing for their own offspring and not the offspring of another male. That is, they must have had some capacity for measuring C. Except for the rare sufferers from prosopagnasia, human males can recognize sexual partners and know w h o m to provision. Moreover, there is evidence that they prefer to marry (and provide for) females who will offer reproductive exclusivity. P6russe (1994) hypothesized that w o m e n w h o had a high number of extramarital affairs would run a higher risk of divorce or separation. His hypothesis was supported by French Canadian data showing that, with respect to lifetime acquisition of simultaneous partners, divorced / separated w o m e n behaved the same as never-married


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women, who had significantly higher rates than married w o m e n did. Age data from the same sample suggested that some of these additional, simultaneous acquisitions of partners took place during marriage and not after its dissolution. Essock-Vitale and McGuire (1988) conclude that, for a sample of middle-class American women, the more secure a male is of his paternity, the more likely he is to contribute to the offspring of the w o m a n with w h o m he has mated. Christenfeld and Hill (1995) have given evidence that humans have evolved a means for males to recognize their offspring, but Br6dart and French (1999) question their finding that h u m a n infants resemble their fathers more than they do their mothers. Other species also have these capacities for assessing, which suggests the importance of biological factors. Chimpanzees can recognize previous associates (including sexual partners), and this was probably a protohuman trait, as well. Some male birds follow a strategy of providing for a spouse while engaging in adultery, and they are presumably able to determine how far to pursue each activity. Male swallows can recognize their "spouses" and bring resources to them instead of to other females (Ridley 1993:219). They are also able to detect attention their mates get from other males and reduce provisioning accordingly (Moller 1988). Male dunnocks, apparently by chronicling their access to females, have fairly accurate knowledge of which young they have fathered and bring food to them. They also switch strategies when other males and females alter their behavior, indicating that they can assess the costs and benefits of feeding (Davies 1992). What problems would have confronted protohuman females who, like males, probably had alternative reproductive strategies? First, they had to be able to evaluate and select partners that would provide them the greatest advantage from multiple matings. Similarly, they had to be able to identify a male with a capacity for providing resources, and they must have preferred him as a long-term and more frequent sexual partner. In addition to identifying bearers of different advantages, the female had to choose between alternative strategies. To do so effectively, she must have been able to compare the resources she gained with the advantages she lost when she gave up a mating opportunity. Modern females apparently are able to identify males who bear good genes and are relatively likely to choose them as extrapair sexual partners (Gangstead and Thornhill 1997; Thornhill and Gangstead 1999). (The absence, however, of evidence in n o n h u m a n primates of female choice for good genes [Hrdy 1997] causes one to ponder the evolutionary origin of this tendency in humans.) With regard to permanent partners, American w o m e n rate prospective resources more highly as the seriousness of the relationship increases from dating to marriage (Kenrick et al. 1990). Similarly, Moroccan females seem to rank responsibility more highly than

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physical attractiveness in a prospective permanent mate (Walter 1997). Betzig (1988) is skeptical about the effect on reproductive success of female choice, but she concludes that there is evidence that females choose males for the provisions they offer. The prevalence of the female choice for resources in a permanent partner suggests the behavior has a biological basis. It may be, however, that females' choices are limited and they have always sought males with resources because males have always and everywhere monopolized resources (Hrdy 1997; Smuts 1995). On the other hand, nonhuman evidence supports the case for a biological element in the behavior. Females of several other species are able to compare and choose mates based on the relative advantages they bring (Birkhead and Moiler 1992:chap. 11; Cronin 1991:chaps. 8-10). It is reasonable to expect that these capacities could have evolved in our protohuman ancestors as well.

Persuading. The problems discussed so far pertain to the abilities individual organisms must have in order to advertise capacities and assess alternative strategies. But moving from x to d involves the capacities necessary to insure that both potential trading partners live up to their promise. What would have made a female practice the fidelity that she advertised and a male to provide the resources? In terms of Figure 3, suppose a male was spending 7 Hours Provisioning. H o w could he have insured that a female w h o showed promise of having N of only .69 in Figure 3 did not raise it to 1.00? First, the male is spending 5 H o u r s Mating, and he is probably allocating at least some of this time to mate guarding. Adoption of the n e w provisioning behavior would not have required that he cease the mate guarding that may have been part of his mating strategy. Wilson and Daly (1992) cite research showing that some male birds pursue a variety of anticuckoldry tactics and suggest that modern humans have inherited a psyche that prompts similar guarding activity. Moreover, as suggested by Hill and Kaplan (1988) with regard to the Ache, a male could have reduced the amount of resources provided if females mate with several males. Moller (1988) found this to be the case with a type of male swallow that provided fewer feedings if his mate engaged in extrapair copulations. Absence of support, however, may not have prevented a female protohuman from attempting this strategy, since she w o u l d have been no worse off; she w o u l d simply have remained at x. Another possibility is that males showed a capacity to punish a female for failure to live up to her promise. A male, for example, might have made a convincing threat to abandon or kill offspring if he found that her N was above .69. H u m a n male jealousy has been interpreted by some (Buss 1994:chap. 6, 2000; Daly et al. 1982) but not all (Small 1992) as an emotion that evolved to induce a credible threat of violence that helped to insure that females lived up to their advertised faithfulness.


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H o w could a female protohuman have persuaded a male to spend as many Hours Provisioning as he showed promise of doing during courtship? A threat to retaliate by reneging on her advertised capacity to reject other suitors would have had no effect if she had already borne the male's children. The spiteful behavior of allowing predators to devour their joint offspring would appear to have no adaptive advantage, and such behavior is rare in nature (Miller 2000:294; Trivers 1981:7; Williams 1989:195). An occasional display of spite, however, could have been adaptive if the male recognized it and responded by living up to his promise (Frank 1988; Nesse 1990). Evolution of emotions--fear of justified punishment, remorse, guilt, or love that w o u l d have enabled successful persuasion without reliance upon the use of violence w o u l d have been even more successful.

Evolving Toward Pair Bonding Escape from the Prisoner's Dilemma ? Protohumans w h o were unable to develop the capacities described above w o u l d remain at x in Figure 3. These protohumans would have become extinct or evolved into the ancestors of chimpanzees and bonobos. On the other hand, a move to d w o u l d have rewarded individuals w h o successfully advertised willingness to invest and be faithful, assessed the costs and benefits of new behavior, and persuaded mates to be true to their advertising. Such a movement w o u l d have been analogous to a Prisoner's Dilemma solution, in which a continuing series of cooperative moves gives greater long-term benefits to both parties than either failing to cooperate or taking turns exploiting each other (Axe]rod 1984; Axelrod and Hamilton 1981; Badcock 2000:chap. 3). Moves that improved the reproductive success of both males and females w o u l d have led to an increase in the numbers of the species practicing cooperation. Evidence has been given above that the capacities for cooperation have evolved in other species, indicating that natural selection has solved the Prisoner's Dilemma for these species. Also, I have argued that contemporary human mating behavior has a biological as well as a cultural component. It remains, however, to suggest the process through which pair bonding in humans might have evolved from promiscuous chimpanzeelike ancestors. What could have happened to cause the family tree to split into a branch that became chimpanzees (without pair bonding) and a branch that became humans (with pair bonding)? Given the favorable natural environment described in a previous section, the central new behavioral element w o u l d have been the evolution of the desire to be committed on the one hand and the evolution of a preference for mates w h o had this desire on the other. For both males and females, this means an evolution of a feeling of pleasure or well being that emanates from continuing to show kindness

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toward the same individuals over a period and being recognized and appreciated for doing so. There is evidence that this has occurred in humans. Nearly all males and females from all cultures consider love as an indispensable part of marriage, apparently because it is a reliable cue to commitment (Buss 1999: 121). If both male and female protohumans were attracted to commitment in a mate, its practice would have been sexually selected in both males and females. Or, at least, the appearance of being committed w o u l d have been selected. But given that false advertising of commitment w o u l d have evoked the development of ways to detect cheating, the best w a y of appearing committed would have been to have evolved a sense of pleasure and well being that accompanied acts of commitment. That is, just as evolution produced emotional states that constitute lust, emotions that could be discerned and not easily faked could have evolved to advertise commitment. There is biochemical evidence that certain hormones and neurotransmitters do give rise to pleasurable states in mammals that remain attached over a period of time (H. Fisher 1998; Insel 1992). If this occurred in protohumans that had the capacities discussed in the previous sections, then potentially cooperating males and females could have found each other. The next question, however, is that of h o w these emotions w o u l d have evolved in a society of promiscuous chimp-like ancestors. The evolution could have involved two crucial steps. First, males could have begun providing for females in the absence of certainty of paternity. The results of a Prisoner's Dilemma simulation model of Key and Aiello (2000) suggest that males could provide for females and their offspring when there is a great difference in male and female energetic costs of reproduction. In terms of Figure 3, provision without certainty means that the initial point x is not at the right-hand origin. It w o u l d instead be at a point such as c to the left of the origin. At this point, the male attains his highest reproductive success when he provides some resources without any assurance of paternity from the female. Such nonreciprocal altruism typically w o u l d not occur because the male's costs w o u l d exceed his benefits (Hawkes et al. 1995; Hirshleifer 1995; Olson 1971). Nonreciprocal altruism could pay, however, if the female's reproductive costs are very high relative to the male's, as in the case of callitrichids (Goldizen 1986), which can raise twins only with male help. Suppose, for example, that a group-living male's probability that he is the father of an offspring is p when he does not provide resources and that the total offspring of the group of which he is a member is T. Then the number of offspring he fathers (O) will be the total for the group multiplied b y the probability that he is the father, or O = (p)(T). If the male provides for a female instead of spending time mating and the drop in p is small rela-


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tive to the rise in T, it is possible that the O of the individual male will increase as the result of providing for a female. Other things being unchanged, he could experience an increase in reproductive success by proriding for the female even if she gave no increase in C in exchange. So, if a protohuman male that derived pleasure from helping appeared on the scene, he could have prospered without any promise of receiving fidelity. The second step would have occurred if females showed a preference for males that provided help, as suggested as a plausible hypothesis by Hrdy (1981:54). Showing preference would have involved an emotional commitment that would have been difficult to fake. Although the advantage of providing help may have been weak on its own, the selection by females of this trait would lead to an increase in males having the trait and to an increase in females who selected it. Thus, these two steps--male provisioning and female selection of the trait--form the basis for Fisher's runaway process (R. Fisher 1930:136). Once the process was underway, males and females could have moved progressively away from point x and toward points such as d in Figure 3. By providing resources, males would have benefited from providing for their offspring and from females" differentially selecting them. Females, by showing commitment to preferred males, would have given certainty of paternity and received provisioning help in return. The two steps leading to Fisher's runaway process could have been the evolutionary basis of the hypothetical account of early pair bonding given by Helen Fisher (1982:93-94). The evolution of emotional commitment that permitted free exchange could have been a low-cost supplement and partial replacement of costly mate guarding, coercion, deception, and defense. As pointed out by Buss (2000), if commitment and the risk of abandonment are the problems that give rise to these costly activities, love is the solution. Invasion by defectors? Suppose males and females developed the biological capacities to move from x to d. Would this new species have been vulnerable to invasion by individuals who successfully faked commitment in an environment of trust and thereby enjoyed differential reproductive success? It is possible that individuals with the behavioral characteristics of point x could have persisted amongst pair-bonding protohumans and that their descendents survive in today's population. In the terms of evolutionary game theory, mutual sexual selection could have enabled cooperators to find each other and succeed against defectors playing the strategy characteristic of point x. The success of cooperators, however, need not have eliminated defectors, who could have continued to exist as predators in a state including both cooperators and defectors (Skyrms 1994, 1996). These predators, however, would only experience aboveaverage fitness in a world of trusting cooperators, and their fitness would

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diminish as they increased in numbers relative to cooperators. Under these circumstances of frequency-dependent selection, both genetically based behaviors could exist simultaneously (Bailey 1998; Wilson 1994). THE OUTCOME The size of the unshaded area in Figure 3 indicates the variety of mating structures that can result from the mutual determination of strategies. Can anything be said about what structures are more likely? The range of possibilities can be narrowed somewhat by taking into account an economic efficiency criterion. Any point in the unshaded area is better for both male and female than point x, but only some of them are optimal in the sense that it would be impossible to improve the reproductive success of one without reducing the reproductive success of the other. Point d, for example, is better than point x, but it is not optimal because both male and female can simultaneously move to higher isoquants. Males and females could have continued solving the Prisoner's Dilemma until they reached a point of tangency of male and female isoquants, such as d'. This and other points of tangency are optimal. Let the line passing through d' and connecting d" and d" be locus of all such points. Any point along this line is optimal in the sense described. Where any particular pair would end up would depend upon the relative abilities of each partner to persuade the other to live up to advertised capacities, as described in an earlier section. Or, in the language of conflict, it would depend on the male's manipulation/control and the female's resistance (Gowaty 1992, 1996, 1997). Even if he provides resources, a male m a y manipulate a female in a way that leaves her on FRS6 with the same reproductive success as at x.7 Such an outcome would increase the reproductive success of the manipulating male, but for males in general this practice would lead to an increase only in the variance of male reproductive success, not its level. Only mating structures involving some improvement in the reproductive success of females could produce a species with biologically based pair bonding. It is the argument of this paper that there is a biological basis to human pair bonding and that it has resulted from a mating structure that is exemplified by a point lying somewhere between points d" and d" in Figure 3. (Figure 3 is, of course, just an example. Different assumed productivities would result in differently shaped unshaded areas and different outcomes.) CONCLUDING REMARKS Mating systems can be usefully analyzed as the outcome of mutually beneficial exchange of resources for fidelity. Mating systems can range from


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promiscuity to m o n o g a m y d e p e n d i n g o n (1) e n v i r o n m e n t a l conditions, which d e t e r m i n e the potential gains from trade, a n d (2) b e h a v i o r a l adjustments, which d e t e r m i n e the extent to w h i c h males a n d females exploit the potential gains proffered b y the e n v i r o n m e n t . Cultural b e h a v i o r g o v e r n s some behavioral adjustments, b u t this article e m p h a s i z e s h o w natural selection could h a v e s h a p e d the p s y c h e of p r o t o h u m a n s in w a y s that enabled t h e m to f o r m pairs. 8 Although I h a v e used it to argue for the biological influence on h u m a n pair bonding, the s t a n d a r d m o d e l of economic e x c h a n g e p r o v i d e s a general m e c h a n i s m for analyzing b e h a v i o r that results f r o m the interaction of two parties with the environment. In these cases, changes in the environment alter the productivities of different courses of action. N e w p r o d u c tivities m e a n a c h a n g e in w h a t one p a r t y can offer the other, while k e e p i n g its r e p r o d u c t i v e success the same. The n e w terms of e x c h a n g e alter the costs a n d benefits of actions to the second party, p r o m p t i n g f u r t h e r changes. While these complex interactions can defeat p u r e l y verbal analysis, they yield m o r e readily to the d i a g r a m m a t i c a p p r o a c h suggested here. I am grateful to Christopher Badcock of the London School of Economics for introducing me to many of the ideas in this paper and for his continuing support. In addition, a NEH/NSF Summer Institute under the direction of Roger Masters and Robert Perlman contributed significantly to the development of my thought in this area. Finally, anonymous reviewers made numerous helpful suggestions. William Shropshire (B.A., Washington and Lee University; Ph.D., Duke University) is emeritus Callaway Professor of Economics at Oglethorpe University, Atlanta, Georgia. He is currently interested in the social thought of David Hume and other Scottish moralists and the extent to which it is consistent with the findings of contemporary evolutionary theory.

NOTES 1. Such trading partnerships have been mentioned by Hrdy (1997:25), who refers to a communication with J. Boone; Wrangham and Peterson (1996:242); and Lancaster (1997). These authors do not analyze the economics of the origin of the trade-off of resources for fidelity. 2. Chase (1980) used this device from economics to show combinations of an animal's cooperation and noncooperation that will achieve a given level of parental investment. He assumes a pair already exists, whereas this paper is analyzing the formation of a pair. 3. The function MRS = f(M) + Ch(P) can be converted into a function of M only, since P = 12 - M. Hence MRS = f(M) + Cg(M). Along any isoquant, MRSi, reproductive success is constant, so rearranging terms and solving for C, C = [MRSi - f(M)] / g(M)

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and the isoquant's slope is d C / d M = [ - ( f ' + Cg')] / g(M) where f' and g' are the first derivatives of the two functions of M that constitute the MRS function. The specific functional forms used to d r a w the isoquants are f(M) = (2M)q and g(M)= (24 - 2M) r, where q a n d r are less than 1. Taking the derivatives and substituting for f(M), f', g', and C, w e have d C / d M = [ - 2 q ( 2 M ) q - 1 / g(M)] + {[MRSi - (2M)q] [2r(24 2M) r - 1]/g(M)} / g(M) -

Multiplying through b y g(M), substituting for g(M), rearranging, a n d simplifying, d C / d M will be negative if r[MRS i - (2M)q] < [q(2M)q - 1)] (24 - 2M). Dividing both sides by q(2M)q - 1 yields 24 - 2M > (r/q)[MRS i (2M) 1 - q - 2M] as the condition for a negative derivative. In the case of the isoquants in Figure 1, the productivities of Mating and Provisioning are a s s u m e d to be equal so that r = q. Hence, in this case, an isoquant's slope is negative if MRSi(2M) 1 - q - 24 < 0. A l o n g isoquant MRS 2, reproductive success is the a m o u n t attainable w h e n all effort, 12 hours, is spent on Mating. Given the function f(m) = (2M)q, MRS 2 = 24q. Therefore, substituting into the equation above, w e see that along this isoquant, 24q(2M) 1 - q - 24 < 0 for q < 1 and M less than 12, which is the d o m a i n u n d e r consideration. The analytical proof of the negative slope of MRS 2 for the general case in w h i c h q and r are less than one b u t not equal to each other is s o m e w h a t complicated, b u t trials of a wide range of values for q a n d r p r o v i d e convincing evidence of the result. (The analytical proof has been p r o v i d e d b y Alex Nagel, University of Wisconsin, Madison.) 4. The reproductive success represented on isoquants M R S 3 and MRS 4 is greater than 24q, so MRS i (2M) 1 - q - 24 will be greater or less than zero d e p e n d i n g on the level of Mating, M (see note 3). 5. Given the function FRS i = k(P, N), where the partial derivatives k v a n d k n are assumed to be positive b u t declining, the slope of an isoquant d N / d P = - k R / k n. Following H e n d e r s o n and Q u a n d t (1958), w e take the total derivative of d N / d P to obtain d2N / dP 2 = - 1 / kn3(kppkn 2 - 2k~pl~l% + knnkp2). In the case of the additive function a s s u m e d here, the cross partial derivative (k~p) is zero and the marginal products are positive. The marginal p r o d u c t s are a s s u m e d to be declining, so kp. and k ~ are b o t h negative. Hence the term inside the parentheses is negative, and being m u l t i p h e d b y a negative, the entire expression is positive. Thus, the slope of the isoquants gets greater (less negative) as P increases, a n d the isoquants are convex from below. 6. The authorship of this d i a g r a m m a t i c device is not clear, hence the d o u b l e name (Ekelund a n d H t b e r t 1990:598). 7. In the terminology of Robert Trivets (1983), w e can call m a n i p u l a t i o n w i t h skewed results "subtle cheating." A "gross" male cheater w o u l d attain c', whereas a subtle cheater w o u l d only attain d" in Figure 3. 8. With respect to the exchange of goods, A d a m Smith ([1776] 1981:chap. 2) comes to a different conclusion. H e suggests that the "propensity to truck, barter, 9

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and exchange one thing for another" is not one of those "original principles in human nature" but that it is "the necessary consequence of the faculties of reason and speech."

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