Different Reproductive Strategies in Males and Females Eleanor E. Maeeoby Stanford University MACCOBY, ELEANOR, E. Different Reproductive Strategies in Males and Females. CHILD DEVEL-
OPMENT, 1991, 62, 676-681. The claim for a connection between stressful, unstable childhood environments and early pubertal maturation has only modest empirical support. However, granting the claim for purposes of discussion, and taking an evolutionary perspective, it is argued here that early puberty need not imply a shift from a "quality" toward a "quantity" reproductive strategy. Indeed, for females to make such a shift when they cannot count on secure pair bonding or paternal investment from a male would not serve their inclusive fitness interests; indeed, probably the reverse. Delayed puberty among juveniles with secure, long-continued bonds with the parental generation may serve a different evolutionary function: to minimize inbreeding. Nonevolutionary factors are more than adequate to account for precocious sexuality in individuals with stressful childhood histories.
The Belsky, Steinberg, and Draper paper (1991, in this issue) has a very broad sweep. It points to several familiar links in a causal chain from early childhood to adulthood, adds one less familiar link, and then brings evolutionary theory to bear as an explanatory framework. The familiar links are these: (1) Stress factors impinging on parents tend to diminish the quality of their parenting; they become harsh, unresponsive, and/ or minimally involved (neglectful). (2) Such parenting increases the risks of insecure attachments in their children—an insecurity that may develop with increasing age into a generalized lack of trust in others. (3) Early insecure attachments are associated with a variety of problematic childhood outcomes, particularly in the interpersonal relationships of the children concerned. (4) Among children with elevated rates of "problem" behaviors, there is an increased risk of subsequent "deviance," including early and/or promiscuous sexual behavior.
the discussion below, I will accept them as established. The less familiar link is the alleged one between stressful rearing conditions and early puberty. This link has been pointed to in two earlier papers by Steinberg (1988, 1989), who has argued that we need to consider individual differences in pubertal timing not only as an antecedent factor affecting adolescent social behavior but also as a possible outcome of variations in individuals' social history. Specifically, he suggests that stressful rearing and insecure attachment lead to children's becoming detached, at an earlier than usual age, from the parental system of control and support, and that this psychological "distancing" allows puberty to occur earlier than it otherwise would.
Steinberg's hypothesis does seem counterintuitive on first hearing, but the evidence on subhuman primates is impressive. It appears that sexual maturation is indeed In the present article, these links are delayed if young monkeys and apes are artiquite tightly drawn. The authors give us a ficially kept closer to the parent generation careful review of much of the positive evi- than they would be in the wild, and that disdence for them, and touch only lightly on inhibition occurs quickly once the juveniles any contrary findings, thus painting a more are separated from dominant seniors. Some coherent picture than many knowledgeable human evidence is consistent with these readers will fully accept. But the authors are findings, in that "premature emancipation" careful to state that each link is only a proba- of children has been shown to be associated bilistic one, and that many other factors do with early sexual maturation, at least in girls. affect the developmental sequence. The au- The human findings are, of course, correlathors make a good case for these links, as tional, and so are less conclusive than the others have done, and for the purposes of experimental work with nonhuman pri-
[Child Development, 1991, 62, 676-681. ® 1991 by the Society for Research in Child Development, Inc. All rights reserved. {)009-3920/91/6204-0012$OLOO]
Eleanor E. Maccoby mates. And to date, there is not adequate replication. But the authors have laid out a good groundwork for the pubertal-timing hypothesis. Let us accept, for purposes of^ our discussion, that this link too has been established. We turn, then, to the main theme of the article—its effort to bring evolutionary principles to bear as an explanatory framework. We should note first of all that no such effort is needed to integrate the sequential picture outlined in the first four steps listed above. This sequence is perfectly understandable from the standpoint of social-psychological conceptualizations of socialization processes. (Altemative sequences might be equally defensible!) I will argue that the additional link to pubertal timing is flawed; a change in timing might promote the reproductive fitness of the individuals concerned but may serve a different survival function than the one Belsky et al. propose. Expanding on the earlier paper by Draper and Harpending (1982), the authors argue that sexual activity at an earlier than usual age—and with an increased number of partners—is adaptive for individuals who have grown up in a stressed early environment marked by untrustworthy human relationships. That is, it serves the inclusive fitness of such individuals by increasing the chances that they will produce viable progeny. Like other recent writers (e.g., Cosmides, 1989; Crawford & Anderson, 1989), Belsky et al. claim that we are a species that has evolved so as to be able to take our early experience into account in adopting strategies relevant to reproduction. These writers say that while such strategies are not consciously chosen, early experience may incline us toward one or the other of two major reproductive strategies: a "quantity" orientation, which involves investment in mating early and often and producing many offspring, and a "quality" orientation, which emphasizes investment in stable pair bonding and the rearing of a smaller number of offspring in a protected way that increases their chances of survival. They argue that it makes biological sense for people who have learned that they cannot rely for child rearing on an enduring pair bond with a mate to choose a strategy of having as many children as possible as early as possible. In other words, it is unwise for a person with a history of unstable interpersonal relationships to delay procreation in the interests of trying to find a single stable partner. They apply
677
this reasoning equally to persons of the two sexes. Adopting for the moment the evolutionary explanatory framework within which the paper by Belsky et al. is framed, I would like to argue that the hypothesis they propose needs to be examined separately for males and females. Considering the sex differences in reproductive strategies that characterize mammalian species, it would be reasonable to expect that if early experience influences such strategies, it may do so in quite different ways for the two sexes. Some general points need to be made before getting into the specifics of the issue. We should be av\'are of the distinction between evolutionary theory and behavior genetics. Behavior genetics deals with individual differences within a species and seeks to identify the genetic contributions thereto. Evolutionary theory is concerned with species-wide characteristics. Indeed, its proponents claim that the more within-species variation there is in any characteristic, the less important that characteristic must be for reproduction, since less adaptive variations would have been weeded out by natural selection if survival depended on it (Buss, 1990; Crawford & Anderson, 1989; Tooby & Cosmides, 1990). However, there is one individual-difference variable that evolutionary theorists bring into the forefront of their thinking, and that is sex. In dealing with sexual species, the two sexes are treated essentially as subspecies. No doubt humans, like other species, have evolved so that our individual behavior would have served three biological imperatives: to survive at least until reproductive maturity, to mate, and to raise our progeny to an age when they, too, can survive and reproduce. In humans, like other species with little or only moderate dimorphism, there is no evolutionary reason why the two sexes should differ in their striving for individual survival; and, indeed, they do not differ in the perceptual and cognitive structures that equip individuals to find food, detect danger, and solve problems that affect individual well-being. When it comes to procreation, males and females are both equipped with a strong sex drive and a readiness to respond to sexual signals from chosen partners. However, because of the different fitness consequences of certain reproductive behaviors for the two sexes, evolution has shaped them to take up different strategies for mate selection and parenting
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(Buss, 1989; Wittenberger & Tilson, 1980). As has been stated and overstated to the point of becoming a cliche, females in most sexual species make a far greater parental investment than do males. In mammals, the differentiation is especially great, since lactation involves transmitting even more calories to the young than does gestation (Jelleffe & Jelleffe, 1978) and may involve a considerably longer period of time. In species like humans in which young are usually produced one at a time, after long gestation, and in which the young require lengthy postnatal parental care, the number of offspring that a female can produce in her reproductive lifetime is strictly limited. For males in such species, the range in the number of possible offspring is much greater, and the limitations on a male's reproductive success are of a different kind. It is a major tenet of evolutionary theory that animals do not act so as to insure the survival of their species, but rather to promote the survival of their own individual genes in the next generation (Trivers, 1972). Parents treat their own young preferentially (see Daly & Wilson, 1987, for a review of evidence). Sex differences in reproductive strategies stem in part from the fact that the sexes do not have equal confidence in their biological tie with offspring. As Daly and Wilson (1988) have put it, "Maternity is a fact, paternity a conjecture" (p. 109). Females invest large resources of energy in whatever young they have produced, knowing they are their own. Males may or may not invest in the rearing of young once they are born. For nonhuman males in mammalian species, there is a close tie between paternal investment and exclusive mating (which usually means monogamy; see Dewsbury, 1988; Kurland, 1979). When males invest in the care of young, it is almost always only in those young they can be confident are their own. In species in which females mate with multiple males, males do not participate in care of the young. In other species, males have a variety of strategies for assuring paternity: preference for unmated females; driving other males away from their territory or their mate; killing offspring of other males. In the human case, Daly and Wilson contend, "men are universally concerned not merely to gain sexual access to attractive, fertile women but to guarantee the exclusivity of that access" (p. 111). One could easily argue, then, that the "quantity" strategy is not the only strategy men could adopt to promote their reproductive fitness.
Among men who have learned to have weak confidence in pair bonds, some—especially those who do not have easy access to multiple women and in situations of scarce resources—might well adopt a strategy of becoming intensely jealous, coercing a female into exclusive mating, and staying close to her and the children. Given reasonable assurance of paternity, males as well as females presumably need to behave so as to increase the chances that their progeny will survive to maturity. But parental investment by males depends in part on the availability of resources (Emlen & Oring, 1977; Lott, 1984). In species that can move into new territories with unexploited abundant resources, males usually display low paternal investment. Males are more likely to be monogamous and help to rear young if the ecological niche in which the animals live has limited resources or predators that are close and dangerous, so that the activities of both parents are needed to feed and protect the young. In cold climates when eggs or neonates need to be kept warm, males will either take their turn providing body heat to the nest while the female goes out to forage, or, as in the case of arctic wolves, will bring food to the den for both the female and the pups. But in species living where climates are benign and food plentiful, the care of young tends to be left to the females without male participation. Indeed, there are some birds (e.g., the white-fronted bee eater) that are monogamous only in years when food is scarce; under these conditions, the male stays near and brings food to the nestlings; in years of abundance, he disappears, and the female rears the young alone. However, most species do not show this kind of flexibility. They either have high sex dimorphism, high levels of male competition and fighting, corralling of a harem by a dominant male, and low paternal investment, or monogamy, low dimorphism, and high paternal investment (Dewsbury, 1988). Human males clearly do have considerable fiexibility, perhaps based on early experience, and can take up positions somewhere along the continuum defined by these two extremes. But judging by lower species, the availability of resources ought to be a potent factor. Contrary to the Belsky et al. position, one could argue that it should be among the poor, not the rich, that males would invest the most. Boys who have experienced the risks and disadvantages of not having a father's resources committed to the family
Eleanor E. Maccoby might want to invest in their own children to give them a better chance of survival. Perhaps personality factors (e.g., aggressiveness, risk-taking, impulsivity) that develop in individuals reared in high-stress families preclude high paternal investment even though such investment would be adaptive. If one accepts that choosing a "quantity" rather than a "quality" parental investment strategy is one possible outcome of an unstable childhood for males, it is not intuitively obvious that this is only strategy that such a childhood would promote. Most writings on evolved reproductive strategies have focused on males, since there is clearly great variability among species and subspecies in male parental investment. Female investment is less labile. Most evolutionary writers assume that mammalian females have no choice when it comes to making a heavy investment, at least not until the end of the lactation period, and that in the division of energy expenditure between the pursuit of mating opportunities and the rearing of young, females will always be biased toward channeling a higher proportion of energy into parenting than will males. There is no known Instance in which mammalian females abandon existing offspring and leave them to the care of males while they themselves pursue new mating and new procreation. When females abandon young who are not yet viable on their own, males do not take up the slack; rather, the young die. Furthermore, an existing offspring is always more valuable than a potential one. In times of scarcity and danger, a female may abandon a fiewbom in order to feed and protect an older toddler who has already survived the period of high infant mortality. This amounts to their rearing fewer children under stressful conditions, not more. And, indeed, under the arduous conditions of early human evolution, as Belsky et al. note, spacing between children was probably wider than it is in present-day preindustrial societies. When males make less investment, females must make more. If continued parental investment is needed after the end of lactation to enable the young to grow safely to maturity, the female often needs to recruit investment from others in her children (Irons, 1979). Her basic strategy, then, is to select a mate who has the resources and the motivation to make an effective paternal investment (Buss, 1989). This-becomes more important under conditions of resource scarcity, when it is especially difficult for the
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female to rear the young alone. Do females have any altemative strategies under conditions in which males cannot or will not invest? Irons (1979), drawing on anthropological evidence, shows that in preindustrial societies the altemative appears to be some form of matriliny, in which females rely on their mothers and sisters for support in child rearing. There are analogous developments in segments of industrial societies, where males become peripheralized and matrilines carry the responsibility for child rearing (Burton, 1990; Cray, 1990; Stack, 1974). We do not know what strategies would have been adaptive for females living in environments in which they could not rely on either a male mate or female relatives for support in child rearing. It does not seem plausible, however, that evolution would have shaped matemal psychological and physiological characteristics toward having more children in the absence of such support, when each new child increases the difficulty of raising those already bom. Cangestad and Simpson (1990) have recently suggested that while females do advance their fitness by adopting a strategy of "restricted sexuality"—in that this increases the probability of paternal investment by a male—there is an alternative strategy that they describe as unrestricted sexuality. This strategy, while it entails risks of losing paternal investment, has fitness advantages in that it is alleged to increase the chances that a female will mate with a "high-quality" male, whose genes are then passed on to higher-quality offspring. The argument is that high-status, exceptionally vigorous males will have access to many females and will not normally delay procreation for the sake of a monogamous relationship. Whatever the merits of this claim, it does not change the fact that the young, once bom, must be reared, and that in most situations females need social supports (either from a mate or from relatives in the female line) to carry out this part of the reproductive enterprise. I have pointed to reasons for being skeptical about the claim that females have evolved so as to take up a "quantity" reproductive strategy in response to instability in childhood relationships. Nevertheless, we must take into account the evidence that girls from divorced families or with weak attachments to parents may reach puberty sooner, and do tend to become sexually active sooner, than girls from stable and supportive two-parent homes. We may grant
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that whatever causes children to leave home early or psychologically distance themselves from their parents so that they spend less time in their presence facilitates the onset of puberty (or disinhibits the delaying factors). As Belsky et al. note, the fitness considerations here are obvious: such a mechanism minimizes inbreeding. And given earlier maturation, the probability of earlier sexual activity is increased. But such early activity need have nothing to do with girls' having adopted a "quantity" strategy based on the low probability of forming a stable pair bond with a male mate. On the contrary, we may be seeing here an instance in which one fitness consideration (to avoid inbreeding) runs counter to another (to have fewer children if paternal investment is unlikely). There is no difficulty in accounting for the sexual precocity of girls from disorganized and/or divided families. They are probably subject to looser surveillance (Dornbusch et al., 1985). They are usually aware of greater sexual activity on the part of their mothers—often with several partners—than is the case for children from stable, two-parent families, and this awareness may function to convey permission by example. Cirls with a history of family instability may want to get away from home, from mothers whom they do not trust, earlier than other girls, and liaisons with men may appear to be a viable avenue for escape. Or, it may be mainly fathers who "tame" and feminize girls, so that without their influence, girls become more sexually adventurous and uninhibited (Johnson, 1988). And, finally, there is the allegedly earlier pubertal maturation, which may in itself render girls more vulnerable to exploitation by older males. However these factors combine, they would appear to be adequate to account for the (rather weak) tendency for girls from divorced families to become sexually active early. No further assumption about an evolved adaptation for a "quantity" orientation is needed. Indeed, I have argued that such a strategy is not adaptive for females, in either the evolutionary or the more common social-psychological sense of the word. Recent sociobiological theorizing has opened the door wider to the effects of early experience. Does developmental psychology need to undergo a similar broadening to accommodate tbe constraints imposed by the species characteristics that have evolved in humans? Perhaps; but our species, more than any other, probably is prepared through evolution to adopt a large variety of alterna-
tive strategies under various conditions. The greater the variety, the less importance will such constraints have in terms of individual development. It is not an easy matter to identify whatever constraints there may be, and we are only at the beginning of our effort to understand their generality and/or specificity.
References Belsky, J., Steinberg, L., & Draper, P. (1991). Childhood experience, interpersonal development, and reproductive strategy: An evolutionary theory of socialization. Child Development, 62, 647-670. Burton, L. (1990). Teenage childbearing as an alternative life-course strategy in multigenerational black families. Human Nature, 1, 123-144. Buss, D. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12, 1-49. Buss, D. (1990). Toward a biologically informed theory of personality. Journal of Personality, 58, 1-16. Cosmides, L. (1989). The logic of social exchange: Has natural selection shaped how we reason? Cognition, 31, 187-276. Crawford, C. B., & Anderson, J. B. (1989). Sociobiology, an environmental discipline? American Psychologist, 44, 1449-1459. Daly, M., & Wilson, M. (1988). The Darwinian psychology of discriminative parental solicitude. In D. W. Leger (Ed.), Comparative perspectives in modern psychology. Nebraska symposium on motivation 1987 (Series 35, pp. 91-144). Lincoln: University of Nebraska Press. Dewsbury, D. A. (1988). The comparative psychology of monogamy. In D. W. Leger (Ed.), Comparative perspectives in modern psychology. Nebraska symposium on motivation 1987 (Series 35, pp. 1-49). Lincoln: University of Nebraska Press. Dornbusch, S., Carlsmith, J. M., Bushwall, S., Ritter, P., Leiderman, H., Hasdorf, A., & Cross, R. (1985). Single parents, extended households, and the control of adolescents. Child Development, 56, 326-341. Draper, P., & Harpending, H. (1982). Father absence and reproductive strategy: An evolutionary perspective. Journal of Anthropological Research, 38, 255-273. Emlen, S. T., & Oring, L. W. (1977). Ecology, sexual selection, and the evolution of mating systems. Science, 197, 215-223. Gangestad, S. W., & Simpson, J. A. (1990). Toward
Eleanor E. Maccoby an evolutionary history of female sociosexual variation. Journal of Personality, 58, 69—96. Cray, F. P. (1990). Soviet women: Walking the tightrope. New York: Doubleday. Irons, W. (1979). Investment and primary social dyads. In N. A. Chagnon & W. Irons (Eds.), Evolutionary biology and human social behavior (pp. 181-212). North Scituate, MA: Duxbury. Jelleffe, D. B., & Jelleffe, E. F. P. (1978). Human milk in the modern world: Psychosocial, nutritional and economic significance. Oxford: Oxford University Press. Johnson, M. M. (1988). Strong mothers, weak wives. Berkeley: University of California Press. Kurland, J. A. (1979). Paternity, mother's brother, and human sociality. In N. A. Chagnon & W. Irons (Eds.), Evolutionary biology and human social behavior (pp. 145-180). North Scituate, MA: Duxbury. Lott, D. F. (1984). Intraspecific variation in the social system of wild vertehrates. Behavior, 88, 266-325. Stack, C. B. (1974). All our kin: Strategies for sur-
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vival in a black community. New York: Harper & Row. Steinberg, L. (1988). Reciprocal relation between parent-child distance and pubertal maturation. Developmental Psychology, 24, 122128. Steinberg, L. (1989). Pubertal maturation and parent-adolescent distance: An evolutionary perspective. In. G. Adams, R. Montemayor, & T. Gullotta (Eds.), Advances in adolescent development (Vol. 1, pp. 82—114). Beverly Hills, CA: Sage. Tooby, J., & Cosmides, L. (1990). On the universality of human nature and the uniqueness of the individual: The role of genetics and adaptation, yournai of Personality, 58, 17-68. Trivers, R. (1972). Parental investment and sexual selection. In B. Camphell (Ed.), Sexual selection and the descent of man, 1871-1971 (pp. 136-179). Chicago: Aldine. Wittenberger, J. F., & Tilson, R. L. (1980). The evolution of monogamy: Hypotheses and evidence. Annual Review of Ecological Systems, 11, 197-232.
OPMENT, 1991, 62, 676-681. The claim for a connection between stressful, unstable childhood environments and early pubertal maturation has only modest empirical support. However, granting the claim for purposes of discussion, and taking an evolutionary perspective, it is argued here that early puberty need not imply a shift from a "quality" toward a "quantity" reproductive strategy. Indeed, for females to make such a shift when they cannot count on secure pair bonding or paternal investment from a male would not serve their inclusive fitness interests; indeed, probably the reverse. Delayed puberty among juveniles with secure, long-continued bonds with the parental generation may serve a different evolutionary function: to minimize inbreeding. Nonevolutionary factors are more than adequate to account for precocious sexuality in individuals with stressful childhood histories.
The Belsky, Steinberg, and Draper paper (1991, in this issue) has a very broad sweep. It points to several familiar links in a causal chain from early childhood to adulthood, adds one less familiar link, and then brings evolutionary theory to bear as an explanatory framework. The familiar links are these: (1) Stress factors impinging on parents tend to diminish the quality of their parenting; they become harsh, unresponsive, and/ or minimally involved (neglectful). (2) Such parenting increases the risks of insecure attachments in their children—an insecurity that may develop with increasing age into a generalized lack of trust in others. (3) Early insecure attachments are associated with a variety of problematic childhood outcomes, particularly in the interpersonal relationships of the children concerned. (4) Among children with elevated rates of "problem" behaviors, there is an increased risk of subsequent "deviance," including early and/or promiscuous sexual behavior.
the discussion below, I will accept them as established. The less familiar link is the alleged one between stressful rearing conditions and early puberty. This link has been pointed to in two earlier papers by Steinberg (1988, 1989), who has argued that we need to consider individual differences in pubertal timing not only as an antecedent factor affecting adolescent social behavior but also as a possible outcome of variations in individuals' social history. Specifically, he suggests that stressful rearing and insecure attachment lead to children's becoming detached, at an earlier than usual age, from the parental system of control and support, and that this psychological "distancing" allows puberty to occur earlier than it otherwise would.
Steinberg's hypothesis does seem counterintuitive on first hearing, but the evidence on subhuman primates is impressive. It appears that sexual maturation is indeed In the present article, these links are delayed if young monkeys and apes are artiquite tightly drawn. The authors give us a ficially kept closer to the parent generation careful review of much of the positive evi- than they would be in the wild, and that disdence for them, and touch only lightly on inhibition occurs quickly once the juveniles any contrary findings, thus painting a more are separated from dominant seniors. Some coherent picture than many knowledgeable human evidence is consistent with these readers will fully accept. But the authors are findings, in that "premature emancipation" careful to state that each link is only a proba- of children has been shown to be associated bilistic one, and that many other factors do with early sexual maturation, at least in girls. affect the developmental sequence. The au- The human findings are, of course, correlathors make a good case for these links, as tional, and so are less conclusive than the others have done, and for the purposes of experimental work with nonhuman pri-
[Child Development, 1991, 62, 676-681. ® 1991 by the Society for Research in Child Development, Inc. All rights reserved. {)009-3920/91/6204-0012$OLOO]
Eleanor E. Maccoby mates. And to date, there is not adequate replication. But the authors have laid out a good groundwork for the pubertal-timing hypothesis. Let us accept, for purposes of^ our discussion, that this link too has been established. We turn, then, to the main theme of the article—its effort to bring evolutionary principles to bear as an explanatory framework. We should note first of all that no such effort is needed to integrate the sequential picture outlined in the first four steps listed above. This sequence is perfectly understandable from the standpoint of social-psychological conceptualizations of socialization processes. (Altemative sequences might be equally defensible!) I will argue that the additional link to pubertal timing is flawed; a change in timing might promote the reproductive fitness of the individuals concerned but may serve a different survival function than the one Belsky et al. propose. Expanding on the earlier paper by Draper and Harpending (1982), the authors argue that sexual activity at an earlier than usual age—and with an increased number of partners—is adaptive for individuals who have grown up in a stressed early environment marked by untrustworthy human relationships. That is, it serves the inclusive fitness of such individuals by increasing the chances that they will produce viable progeny. Like other recent writers (e.g., Cosmides, 1989; Crawford & Anderson, 1989), Belsky et al. claim that we are a species that has evolved so as to be able to take our early experience into account in adopting strategies relevant to reproduction. These writers say that while such strategies are not consciously chosen, early experience may incline us toward one or the other of two major reproductive strategies: a "quantity" orientation, which involves investment in mating early and often and producing many offspring, and a "quality" orientation, which emphasizes investment in stable pair bonding and the rearing of a smaller number of offspring in a protected way that increases their chances of survival. They argue that it makes biological sense for people who have learned that they cannot rely for child rearing on an enduring pair bond with a mate to choose a strategy of having as many children as possible as early as possible. In other words, it is unwise for a person with a history of unstable interpersonal relationships to delay procreation in the interests of trying to find a single stable partner. They apply
677
this reasoning equally to persons of the two sexes. Adopting for the moment the evolutionary explanatory framework within which the paper by Belsky et al. is framed, I would like to argue that the hypothesis they propose needs to be examined separately for males and females. Considering the sex differences in reproductive strategies that characterize mammalian species, it would be reasonable to expect that if early experience influences such strategies, it may do so in quite different ways for the two sexes. Some general points need to be made before getting into the specifics of the issue. We should be av\'are of the distinction between evolutionary theory and behavior genetics. Behavior genetics deals with individual differences within a species and seeks to identify the genetic contributions thereto. Evolutionary theory is concerned with species-wide characteristics. Indeed, its proponents claim that the more within-species variation there is in any characteristic, the less important that characteristic must be for reproduction, since less adaptive variations would have been weeded out by natural selection if survival depended on it (Buss, 1990; Crawford & Anderson, 1989; Tooby & Cosmides, 1990). However, there is one individual-difference variable that evolutionary theorists bring into the forefront of their thinking, and that is sex. In dealing with sexual species, the two sexes are treated essentially as subspecies. No doubt humans, like other species, have evolved so that our individual behavior would have served three biological imperatives: to survive at least until reproductive maturity, to mate, and to raise our progeny to an age when they, too, can survive and reproduce. In humans, like other species with little or only moderate dimorphism, there is no evolutionary reason why the two sexes should differ in their striving for individual survival; and, indeed, they do not differ in the perceptual and cognitive structures that equip individuals to find food, detect danger, and solve problems that affect individual well-being. When it comes to procreation, males and females are both equipped with a strong sex drive and a readiness to respond to sexual signals from chosen partners. However, because of the different fitness consequences of certain reproductive behaviors for the two sexes, evolution has shaped them to take up different strategies for mate selection and parenting
678
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(Buss, 1989; Wittenberger & Tilson, 1980). As has been stated and overstated to the point of becoming a cliche, females in most sexual species make a far greater parental investment than do males. In mammals, the differentiation is especially great, since lactation involves transmitting even more calories to the young than does gestation (Jelleffe & Jelleffe, 1978) and may involve a considerably longer period of time. In species like humans in which young are usually produced one at a time, after long gestation, and in which the young require lengthy postnatal parental care, the number of offspring that a female can produce in her reproductive lifetime is strictly limited. For males in such species, the range in the number of possible offspring is much greater, and the limitations on a male's reproductive success are of a different kind. It is a major tenet of evolutionary theory that animals do not act so as to insure the survival of their species, but rather to promote the survival of their own individual genes in the next generation (Trivers, 1972). Parents treat their own young preferentially (see Daly & Wilson, 1987, for a review of evidence). Sex differences in reproductive strategies stem in part from the fact that the sexes do not have equal confidence in their biological tie with offspring. As Daly and Wilson (1988) have put it, "Maternity is a fact, paternity a conjecture" (p. 109). Females invest large resources of energy in whatever young they have produced, knowing they are their own. Males may or may not invest in the rearing of young once they are born. For nonhuman males in mammalian species, there is a close tie between paternal investment and exclusive mating (which usually means monogamy; see Dewsbury, 1988; Kurland, 1979). When males invest in the care of young, it is almost always only in those young they can be confident are their own. In species in which females mate with multiple males, males do not participate in care of the young. In other species, males have a variety of strategies for assuring paternity: preference for unmated females; driving other males away from their territory or their mate; killing offspring of other males. In the human case, Daly and Wilson contend, "men are universally concerned not merely to gain sexual access to attractive, fertile women but to guarantee the exclusivity of that access" (p. 111). One could easily argue, then, that the "quantity" strategy is not the only strategy men could adopt to promote their reproductive fitness.
Among men who have learned to have weak confidence in pair bonds, some—especially those who do not have easy access to multiple women and in situations of scarce resources—might well adopt a strategy of becoming intensely jealous, coercing a female into exclusive mating, and staying close to her and the children. Given reasonable assurance of paternity, males as well as females presumably need to behave so as to increase the chances that their progeny will survive to maturity. But parental investment by males depends in part on the availability of resources (Emlen & Oring, 1977; Lott, 1984). In species that can move into new territories with unexploited abundant resources, males usually display low paternal investment. Males are more likely to be monogamous and help to rear young if the ecological niche in which the animals live has limited resources or predators that are close and dangerous, so that the activities of both parents are needed to feed and protect the young. In cold climates when eggs or neonates need to be kept warm, males will either take their turn providing body heat to the nest while the female goes out to forage, or, as in the case of arctic wolves, will bring food to the den for both the female and the pups. But in species living where climates are benign and food plentiful, the care of young tends to be left to the females without male participation. Indeed, there are some birds (e.g., the white-fronted bee eater) that are monogamous only in years when food is scarce; under these conditions, the male stays near and brings food to the nestlings; in years of abundance, he disappears, and the female rears the young alone. However, most species do not show this kind of flexibility. They either have high sex dimorphism, high levels of male competition and fighting, corralling of a harem by a dominant male, and low paternal investment, or monogamy, low dimorphism, and high paternal investment (Dewsbury, 1988). Human males clearly do have considerable fiexibility, perhaps based on early experience, and can take up positions somewhere along the continuum defined by these two extremes. But judging by lower species, the availability of resources ought to be a potent factor. Contrary to the Belsky et al. position, one could argue that it should be among the poor, not the rich, that males would invest the most. Boys who have experienced the risks and disadvantages of not having a father's resources committed to the family
Eleanor E. Maccoby might want to invest in their own children to give them a better chance of survival. Perhaps personality factors (e.g., aggressiveness, risk-taking, impulsivity) that develop in individuals reared in high-stress families preclude high paternal investment even though such investment would be adaptive. If one accepts that choosing a "quantity" rather than a "quality" parental investment strategy is one possible outcome of an unstable childhood for males, it is not intuitively obvious that this is only strategy that such a childhood would promote. Most writings on evolved reproductive strategies have focused on males, since there is clearly great variability among species and subspecies in male parental investment. Female investment is less labile. Most evolutionary writers assume that mammalian females have no choice when it comes to making a heavy investment, at least not until the end of the lactation period, and that in the division of energy expenditure between the pursuit of mating opportunities and the rearing of young, females will always be biased toward channeling a higher proportion of energy into parenting than will males. There is no known Instance in which mammalian females abandon existing offspring and leave them to the care of males while they themselves pursue new mating and new procreation. When females abandon young who are not yet viable on their own, males do not take up the slack; rather, the young die. Furthermore, an existing offspring is always more valuable than a potential one. In times of scarcity and danger, a female may abandon a fiewbom in order to feed and protect an older toddler who has already survived the period of high infant mortality. This amounts to their rearing fewer children under stressful conditions, not more. And, indeed, under the arduous conditions of early human evolution, as Belsky et al. note, spacing between children was probably wider than it is in present-day preindustrial societies. When males make less investment, females must make more. If continued parental investment is needed after the end of lactation to enable the young to grow safely to maturity, the female often needs to recruit investment from others in her children (Irons, 1979). Her basic strategy, then, is to select a mate who has the resources and the motivation to make an effective paternal investment (Buss, 1989). This-becomes more important under conditions of resource scarcity, when it is especially difficult for the
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female to rear the young alone. Do females have any altemative strategies under conditions in which males cannot or will not invest? Irons (1979), drawing on anthropological evidence, shows that in preindustrial societies the altemative appears to be some form of matriliny, in which females rely on their mothers and sisters for support in child rearing. There are analogous developments in segments of industrial societies, where males become peripheralized and matrilines carry the responsibility for child rearing (Burton, 1990; Cray, 1990; Stack, 1974). We do not know what strategies would have been adaptive for females living in environments in which they could not rely on either a male mate or female relatives for support in child rearing. It does not seem plausible, however, that evolution would have shaped matemal psychological and physiological characteristics toward having more children in the absence of such support, when each new child increases the difficulty of raising those already bom. Cangestad and Simpson (1990) have recently suggested that while females do advance their fitness by adopting a strategy of "restricted sexuality"—in that this increases the probability of paternal investment by a male—there is an alternative strategy that they describe as unrestricted sexuality. This strategy, while it entails risks of losing paternal investment, has fitness advantages in that it is alleged to increase the chances that a female will mate with a "high-quality" male, whose genes are then passed on to higher-quality offspring. The argument is that high-status, exceptionally vigorous males will have access to many females and will not normally delay procreation for the sake of a monogamous relationship. Whatever the merits of this claim, it does not change the fact that the young, once bom, must be reared, and that in most situations females need social supports (either from a mate or from relatives in the female line) to carry out this part of the reproductive enterprise. I have pointed to reasons for being skeptical about the claim that females have evolved so as to take up a "quantity" reproductive strategy in response to instability in childhood relationships. Nevertheless, we must take into account the evidence that girls from divorced families or with weak attachments to parents may reach puberty sooner, and do tend to become sexually active sooner, than girls from stable and supportive two-parent homes. We may grant
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that whatever causes children to leave home early or psychologically distance themselves from their parents so that they spend less time in their presence facilitates the onset of puberty (or disinhibits the delaying factors). As Belsky et al. note, the fitness considerations here are obvious: such a mechanism minimizes inbreeding. And given earlier maturation, the probability of earlier sexual activity is increased. But such early activity need have nothing to do with girls' having adopted a "quantity" strategy based on the low probability of forming a stable pair bond with a male mate. On the contrary, we may be seeing here an instance in which one fitness consideration (to avoid inbreeding) runs counter to another (to have fewer children if paternal investment is unlikely). There is no difficulty in accounting for the sexual precocity of girls from disorganized and/or divided families. They are probably subject to looser surveillance (Dornbusch et al., 1985). They are usually aware of greater sexual activity on the part of their mothers—often with several partners—than is the case for children from stable, two-parent families, and this awareness may function to convey permission by example. Cirls with a history of family instability may want to get away from home, from mothers whom they do not trust, earlier than other girls, and liaisons with men may appear to be a viable avenue for escape. Or, it may be mainly fathers who "tame" and feminize girls, so that without their influence, girls become more sexually adventurous and uninhibited (Johnson, 1988). And, finally, there is the allegedly earlier pubertal maturation, which may in itself render girls more vulnerable to exploitation by older males. However these factors combine, they would appear to be adequate to account for the (rather weak) tendency for girls from divorced families to become sexually active early. No further assumption about an evolved adaptation for a "quantity" orientation is needed. Indeed, I have argued that such a strategy is not adaptive for females, in either the evolutionary or the more common social-psychological sense of the word. Recent sociobiological theorizing has opened the door wider to the effects of early experience. Does developmental psychology need to undergo a similar broadening to accommodate tbe constraints imposed by the species characteristics that have evolved in humans? Perhaps; but our species, more than any other, probably is prepared through evolution to adopt a large variety of alterna-
tive strategies under various conditions. The greater the variety, the less importance will such constraints have in terms of individual development. It is not an easy matter to identify whatever constraints there may be, and we are only at the beginning of our effort to understand their generality and/or specificity.
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