human adoption in evolutionary perspective

HUMAN ADOPTION IN EVOLUTIONARY PERSPECTIVE

Joan B. Silk University of California, Los Angeles

Exploitation is a fundamental element of the parental strategies of many species of birds. Cuckoos, for example, lay their eggs in the nest of other birds, who often unwittingly rear the alien nestlings as their own. Nest parasitism is an efficient reproductive strategy for cuckoos, who do not have to worry about building a nest, incubating their eggs, or feeding their nestlings. But not all hosts respond passively to such intrusions. In re- sponse to parasitic cowbirds, for example, robins have evolved the ability to detect and selectively eject alien young from their nests. Human parenting strategies differ sharply from the strategies of cuckoos and robins. Unlike cuckoos, we are reluctant to allow our children to be raised by others. Unlike robins, we knowingly rear strange young. What makes human behavior toward children so different from that of cuckoos and robins? Humans seem to share a number of predispositions that facilitate successful adoptive relationships, and the desire to raise children seems to be pervasive among modern humans. Despite these commonalities, patterns of adoption transactions vary greatly among contemporary human societies. This paper considers the origins and causes of cross-cultural variation in human adoptive behavior from an evolutionary perspective.

KEY WORDS: Adoption; Kin selection; Nepotism; Parental behavior

Received May 19, 1989; accepted June 5, 1989.

Address all correspondence to Joan B. Silk, Department of Anthropology, University of California, Los Angeles, CA 90024.

Copyright �9 1990 by Walter de Gruyter, Inc. New York Human Nature, Vol. 1, No. 1, pp. 25-52. 1045-6767/90/$1.00+.10

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26 Human Nature, Vol. 1, No. 1, 1990

ADOPTION Nurturing couple wants to shower newborn with love, laughter and security. Expenses paid. Call collect . . .

(The California Aggie, May 24, 1989)

This advertisement appeared in the classified section of the student newspaper of the University of California at Davis, and others like it are placed in newspapers throughout the country (Miller 1989). At one level it is easy to account for these appeals. The number of healthy children available for adoption in the United States has declined in recent years, partly because fewer mothers are giving their babies up for adoption (Bonham 1977; Silverman and Weitzman 1986). At the same time, infertility has increased (Cole 1984). Faced with long delays and high costs of adopting children through conventional channels, prospective adoptive parents have begun to solicit children directly from their natural parents.

At another level, it is more difficult to understand both why we are so reluctant to give up our own children for adoption and why we are willing to adopt and rear unfamiliar infants. Consider, for example, the cuckoo. Cuckoos do not build nests or rear their own nestlings. Instead, they lay their eggs in the nests of other birds. Cuckoo eggs hatch quickly, and the young cuckoo hatchling ejects other eggs and nestlings from the nest (Wickler 1968). Many host species unwitting rear the alien young as their own.

Biologists explain the evolution of nest parasitism in terms of the reproductive benefits achieved by the parasitic bird. Females who lay their eggs in the nests of others are able to devote virtually all of their energy to producing additional eggs, and none to building nests or raising young. What is beneficial for the parasite is clearly costly for the host. Birds whose nests are successfully parasitized may end up rearing none of the eggs from their own clutch. Not surprisingly, natural selection has favored both the evolution of an array of adaptations that en- able birds to resist parasitism and adaptations that make parasites more difficult to detect. American robins, for example, eject alien eggs depos- ited by parasitic cowbirds, leaving only their own eggs in the nest (Rothstein 1975).

The behavior of contemporary humans seems to flout the evolutionary logic that underlies the nest parasitism of cuckoos and the development of counterstrategies by their hosts. Unlike cuckoos, we are reluctant to allow our offspring to be raised by others. Unlike robins, we often knowingly rear strange young. Why should humans behave this way?

Current theories of the origins of human behavior suggest three possible answers to this question. One possibility is that our behavior is not directly influenced by evolutionary forces. Instead, contemporary adoption practices may reflect a particular constellation of cultural beliefs

Human Adoption in Evolutionary Perspective 27

about the value of children, parenthood, and the nature of kinship relationships. If this is the case, there is little reason to expect that parents will prefer to rear their own children, or that there will be any consis- tency in patterns of adoption among different human societies.

An alternative hypothesis is that both our desire to rear our own children and our willingness to rear strange young are the products of psychological and physiological mechanisms that were shaped by natural selection to meet conditions faced by Pleistocene food foragers but are no longer adaptive in the modern environment (Alcock 1979; Tooby and Cosmides 1989). If early hominids lived in small family groups, like many modern food foragers do, there may have been little opportunity for parents to take advantage of other group members and, conse- quently, little need to develop the ability to discriminate against strange young.

A third possibility is that there are important functional differences between nest parasitism and human adoption, and that contemporary adoption behavior may actually be adaptive. For reasons that will be discussed more fully, we might predict that adoption would be most common among genetic relatives or reciprocating partners (Alcock 1979; Silk 1980) and that adoptive parents would generally be those who are unable to produce children of their own.

These general predictions about the patterns of adoption practices within and between human societies can be partially evaluated with data on adoption in traditional societies and modern industrial nations.

EVOLUTION AND HUMAN ADOPTION

To develop predictions about the evolutionary origins of contemporary human adoption behavior, we must consider the kinds of forces that shape the evolution of behavior. All else being equal, traits that enhance foraging efficiency, improve the ability to detect predators, or directly increase fecundity will be favored by natural selection. Moreover, for animals for whom parental care enhances the probability that their offspring will survive and reproduce, competent and solicitous parental care is expected to be favored by natural selection.

Adoption interests evolutionary biologists because it appears to be a form of altruism. That is, it enhances the fitness of the recipient and lowers the fitness of the donor. Biologists have long recognized that many animals perform a wide variety of behaviors that are unlikely to enhance their fitness directly (Hamilton 1964). For example, vervet monkeys give alarm calls that warn their neighbors of the approach of predators (Cheney and Seyfarth 1981); chimpanzees (Hauser and Wrangham


1987; Wrangham 1975) and toque macaques (Dittus 1984) give loud food calls when they encounter rich food patches; and many primates form alliances during aggressive confrontations (e.g., macaques, Massey 1977; baboons, Cheney 1977; geladas, Dunbar 1984; vervets, Cheney 1983; howlers, Jones 1980). All of these behaviors seem likely to reduce the fitness of the actor and simultaneously increase the fitness of the recipient. Biologists define such acts as altruism.

Kin selection and reciprocal altruism are two processes that may account for the evolution of altruistic acts. The theory of kin selection rests on the insight that an individual's fitness is jointly based on (a) fitness gained through the direct replication of its own genetic material through reproduction, and (b) inclusive fitness gained as a result of its own actions from the replication of copies of its own genes carried by genetic relatives (Hamilton 1964). Thus, when a female flees from a predator, the individual component of her fitness is enhanced. When a female alerts her niece that a leopard is about to pounce, and her niece escapes, the female benefits by augmenting the inclusive component of her own fitness.

The theory of kin selection also specifies how much aid should be given to kin. Hamilton (1964) showed that altruistic acts increase the inclusive fitness of the actor only if the increment to the recipient's fitness (b) weighted by the coefficient of relatedness between them (r) is greater than the decrement to the actor's fitness (c), or b • r > c. The coefficient of relatedness is simply the probability that two individuals carry copies of the same gene through descent from a common ancestor. As kinship becomes more distant, the value of r declines.

Although ecological and demographic factors will alter the value of the parameters in the equation in specific cases, Hamilton's rule generally predicts that altruism will be directed selectively toward kin, and costly forms of altruism will be restricted to close kin. Thus, the vervet female who encounters a leopard should warn her sister (r -- 1/2) that a leopard is approaching, thereby calling the leopard's attention to her- self, only if the benefits of warning her sister are more than twice as great as the decrement to her own fitness, and she should warn her niece (r = 1/4) only if b > 4c.

Reciprocal altruism allows the evolution of altruism among unrelated individuals as long as three conditions are satisfied (Trivers 1971): pairs of individuals must have the opportunity to interact repeatedly, the benefits to the recipient must be much greater than the costs to the altruist, and individuals must be able to remember past interactions. This form of altruism is favored by selection because both of the participants in these reciprocal exchanges eventually obtain benefits that outweigh the costs of their altruism.


LEVELS OF ANALYSIS OF EVOLUTIONARY PROCESSES

Most biologists generally agree that evolution shapes the form of specific behavioral acts, as well as the cognitive and physiological mechanisms that produce a given behavior at a particular moment. Although some workers who are interested in predator avoidance focus on overt elements such as vigilance behavior, others study the physiological mechanisms that underlie arousal at the sight, smell, or sound of predators.

This distinction is paralleled among those who are interested in the evolution of human behavior (Richerson and Boyd 1989; Symons 1989). Some argue that evolution has shaped human behavior to maximize reproductive success, and they design empirical studies to identify differences in reproductive success among individuals practicing different forms of a given behavior. Others emphasize the importance of the psychological and physiological mechanisms that underlie behavior. They believe that natural selection has shaped these mechanisms in the "environment of evolutionary adaptedness" (Tooby and Cosmides 1989), the hundreds of thousands of years during which humans for- aged for food and lived in simple societies. Such adaptations may persist, even if they are not entirely appropriate for the environment in which humans now find themselves.

This issue is directly relevant to an understanding of the evolutionary basis of adoption in contemporary human societies. Adoption seems to be altruistic, an assumption that will be considered more critically be- low. If it is altruistic, then those who expect human behavior to be adaptive would expect adoption transactions to occur only among close kin or reciprocating families (Silk 1980). Evidence of widespread unreciprocated adoption among nonrelatives would provide a serious chal- lenge to their views.

Those who believe that contemporary behavior reflects adaptations that evolved in earlier times would focus on the psychological mechanisms that underlie adoption transactions. They would expect to docu- ment the existence of an array of psychological and physiological predispositions in contemporary humans that contribute to the decision to adopt and facilitate the establishment of successful adoptive relationships. Moreover, they would expect to find no clear patterning of adoption transactions, unless there are also psychological mechanisms that promote nepotism or reciprocity.

IS ADOPTION ALTRUISTIC?

Before we proceed further, it is important to question the implicit assumption that adoption is altruistic. In order to show that adoption is


altruistic, we need to be able to assess the impact of adoption on the genetic fitness of all of the participants in the adoptive transaction. Ge- netic fitness is the currency in which both costs and benefits must be measured. In practice, it is difficult and perhaps impossible to quantify the effects of a single behavioral act on the fitness of all of the individuals involved. Instead, we must rely on provisional qualitative estimates of the relative magnitude of the effects of particular interactions on the participants. Tidy conclusions rarely emerge from this process.

For adoption to qualify as a form of altruism, it must be costly for the adoptive parents. It seems likely that this is the case. Adoptive parents feed, house, and clothe their adoptive children; they nurture, comfort, protect, and educate them; and they often bequeath property to them. These activities may impose substantial economic burdens on adoptive parents. Since the economic resources of the parents are usually finite, adoption is likely to reduce the quantity or quality of resources that parents allocate to their natural offspring and their relatives. If access to resources influences the fitness of children, adoption may reduce the genetic fitness of adoptive parents.

It is important to assess the net effect of adoption on adoptive parents, however. If adoptive parents accrue benefits through adoption that outweigh their costs, care for adoptive children may not be altruistic. Adoption might enhance the fitness of adoptive parents directly. Many people believe that adoption is a reliable solution for infertility (Lamb and Luergans 1979). The probability of conceiving after adopting has been examined closely in the medical and public health literature. Although these studies are confounded by methodological problems with establishment of appropriate control populations, verification of clinical sources of infertility, and prevalence of small sample sizes, the general conclusion seems to be that adoption does not promote fertility (Aronet et al. 1974; Aronson and Glienke 1963; Banks et al. 1961; Hanson and Rock 1950; Humphrey 1969; Humphrey and McKenzie 1967; Lamb and Luer- gans 1979; Rock et al. 1965; Tartasky 1985; Tyler et al. 1960; Weinstein 1962; Weir and Weir 1966; but see Katz et al. 1985; Sandler 1965).

Adoptive parents might benefit economically by acquiring children who will eventually participate in their domestic enterprises. Because the value of children's labor varies greatly cross-culturally and histori- cally, it is not possible to draw any general conclusions about the economic benefits of raising adoptive children.

Natural parents may benefit by relinquishing their children to adoptive parents (Betzig 1988). Parents who arrange for one or more of their children to be raised by others can thereby increase the quantity or quality of resources allocated to each of their remaining children, which may in turn increase their other children's chances of surviving and


reproducing successfully. Although this may seem a radical means to manipulate investment in offspring, people sometimes resort to even more drastic measures. Evolutionary analyses of infanticide and fatal neglect of children indicate that parental solicitude is selective. For the most part, parents kill or abandon children that they cannot raise successfully. Infanticide is most common when twins are born or infants are congenitally deformed, born too soon after other siblings, or illegitimate (Daly and Wilson 1984; Dickemann 1975; Hrdy and Hausfater 1984). Historical accounts of childhood in medieval Europe suggest that ille- gitimacy and poverty motivated many parents to kill or abandon their children (Boswell 1988; Trexler 1973). Natural parents will generally benefit more by giving up their children for adoption than by killing or fatally neglecting them, particularly if their children receive good care in their adoptive homes.

The impact of adoption on adoptive children is difficult to assess in the abstract. Children might receive better nourishment, medical care, and education in their adoptive homes than in their natal households, and such benefits may ultimately be linked to fitness. In some human societies, economic status is positively associated with genetic fitness (Borgerhoff-Mulder 1987; Essock-Vitale 1984; Irons 1979), perhaps because wealthy people are better able to accumulate resources that enhance their own survivorship, their marriage prospects, or their ability to raise their children successfully.

It is important to question the assumption that adoptive children thrive in their adoptive homes. Several analyses of abusive and homi- cidal behavior toward children in North America suggest that the pres- ence of unrelated adults in the household significantly elevates the risk that children therein will be mistreated (Daly and Wilson, 1980, 1985, 1987, 1988; Lenington 1981; Lightcap et al. 1982; Wilson et al. 1980). If the degree of relatedness between children and their caretakers is the proximate factor underlying abuse, then children may also be at risk in adoptive households.

Barth and Berry (1988) review several studies that suggest that adoption does not elevate the risk of abuse toward children. Although adoptive children represent approximately 3% of the population, adoptive parents are suspected of being responsible for only 1% of reported cases of child abuse, and only a small proportion of placements is terminated because of suspected abuse to adopted children (Boneh 1979; Festinger 1986; Russell and Trainor 1984; Sack and Dale 1982; all cited in Barth and Berry 1988).

It is not clear that adoption is always altruistic. It seems reasonable to hypothesize, however, that adoption will be most costly to adoptive parents when the value of children's labor is low and children are rel-


atively expensive to rear. Adoption will be most beneficial to natural parents and their children when the natural parents have other progeny and their children are adopted by adults who are better off than their own parents and provide good care for them.

PREDISPOSITIONS THAT FACILITATE ADOPTION IN HUMANS

Humans seem to possess a constellation of psychological traits that enhance the probability of forming nurturant relationships with infants and children. Many of these traits are shared with monkeys and apes.

In some animals, the potential for establishing adoptive relationships is restricted by the existence of innate kin recognition mechanisms or stereotyped maternal bonding processes. Experimental studies indicate that some animals, such as ground squirrels (Holmes and Sherman 1982), tadpoles (Waldman 1981), and sweat bees (Greenberg 1979), can recognize close kin on the basis of physical cues alone. Some hoofed mammals learn to recognize their young during a critical period that follows parturition (Wood-Gush et al. 1986).

Current evidence suggests that neither innate kin recognition mechanisms nor very early discrimination of young have evolved among monkeys and apes (Fredrickson and Sackett 1984; Gouzoules and Gouzoules 1987). Macaque mothers do not learn to recognize their infants for several days after birth. In captive settings, female monkeys and apes often tolerate foster infants who are substituted for their own infants during the first few days of life (Thierry and Anderson 1986). Primate infants apparently learn to recognize their mothers and other maternal kin through their spatial associations and interactions with their mothers (Gouzoules and Gouzoules 1987). Although kin recognition mechanisms are more labile in primates than in some other animals, they do seem to be effective regulators of the distribution of altruistic behaviors. There is ample evidence that primates living in undis turbed social groups selectively groom, support, and associate with close kin (Gouzoules and Gouzoules 1987; Silk 1987a).

Like other primates, humans seem to lack innate kin recognition mechanisms and must learn who their relatives are. In medieval Europe, parents often left tokens with infants abandoned at the entrances of foundling homes, which were used for identification if the parents decided to retrieve their children in the future (Boswell 1988).

These data suggest that the attachment process in humans is relatively flexible. This view contrasts sharply with the belief that there is a sen- sitive period shortly after birth during which bonds between human


parents and their offspring are forged (Klaus and Kennell 1982). Accord- ing to this view, maternal contact during the first few hours after birth is a critical element of successful bonding between parents and offspring, and extended parental contact with newborn infants is beneficial to them. Most studies, however, have failed to demonstrate significant long-term effects of extended early contact with newborns (reviewed in Sluckin 1986; Sluckin et al. 1983). The available evidence suggests that adoptive parents can form close emotional bonds with children that they meet weeks, months, or even years after birth (Herbert 1984). Some workers argue that adoptive children are no more likely to suffer from attachment problems than children raised in natal households (Singer et al. 1985; Smith and Sherwen 1983).

Nonhuman primates display an intense interest in newborn young, and their interest may facilitate the development of adoptive relationships. In many species, group members cluster around mothers with newborn infants (Higley and Suomi 1986). Baboons direct elaborate greeting gestures toward newborn infants, and mothers of newborn infants are inundated by social overtures (Altmann 1980). In some monkey species (e.g., langurs, howlers, and vervets), mothers permit newborn infants to be handled extensively by others, whereas in other species, such as macaques and baboons, mothers resist efforts to touch and carry their newborn infants (McKenna 1979). There is also considerable variation within and between species in the quality of care that infants receive from nonmothers. Although allomaternal care is sometimes be- nign, infants are sometimes harmed by negligent or abusive allomothers (Hrdy 1976; Nicolson 1987; Thierry and Anderson 1986).

The ability to feed newborn infants may also facilitate the development of successful adoptive relationships in humans. Although nonlactating female monkeys sometimes begin to produce milk when they are presented with a young infant, infants adopted by nonlactating group members often die of dehydration or starvation (Thierry and Anderson 1986). In human societies, alternatives to breast milk are more readily obtained for adoptive infants. Where cows and goats are domesticated, infants can be fed their milk. Adopted Inuit infants were fed masticated meat, fat, and broth before canned milk and other trade goods became available (Guemple 1979). Like some other primate females, human women are sometimes able to lactate spontaneously, even if they have not been pregnant recently. Wieschhoff (1940) compiled a number of anecdotal reports of spontaneous lactation in traditional societies, often women suckling their grandchildren, although he doubted their credi- bility. In the United States, it has become relatively common for adoptive mothers to breast-feed their babies (Auerbach 1981; Auerbach and Avery 1981; Kleinman et al. 1980; Lancet 1985). For most lactating adop-


tive mothers, manual stimulation of the nipple is sufficient to stimulate milk production, and only modest maternal dietary supplements are required to maintain milk production.

NEPOTISM AND RECIPROCITY IN ADOPTION TRANSACTIONS

Those who expect the behavior of modern humans to be adaptive would expect the pattern of adoption transactions to fit predictions derived from the theories of kin selection or reciprocal altruism. If adoption is favored by kin selection, they would predict that children would be adopted by close kin. If adoption has evolved through reciprocal altruism, they would predict that families would adopt each other's children. In either case, adoption transactions would be expected to be uniformly patterned. On the other hand, those who believe that our behavior was shaped by social and ecological pressures acting upon Pleistocene food foragers living in small family groups might not expect contemporary adoption transactions to follow either of these patterns. A comparison of adoption practices in traditional societies and modern industrialized nations provides evidence in support of each of these views.

Adoption in Traditional Societies

There are two regions of the world in which a large fraction of children are raised in nonnatal households. In some of the societies of Oceania, 25% of all children are raised in nonnatal households (Brooks 1976; Kay 1963; Lieber 1970; Monberg 1970; Tonkinson 1976). In the North American Arctic, the frequency of adoption in traditional communities is roughly similar (Dunning 1962; Guemple 1979). In these societies, adoption seems to be an altruistic act. Adoptive parents bear most of the costs of housing, clothing, and feeding their adoptive children, whom they raise with considerable love and affection. Many elements of adoption transactions in these societies are consistent with predictions derived from kin selection theory (Silk 1980, 1987b). For example,

�9 Natural parents are apt to give up children when they cannot af- ford to raise them, and they rarely give up firstborn children for adoption. In the Arctic, where survival is tenuous, adoption is often an explicit alternative to infanticide.

�9 Adoptive parents are most often childless couples, but they may


also be older adults who have no young children in their home, couples who wish to balance the sex ratio among their progeny, or simply couples wealthy enough to raise another child.

�9 In Oceania and in the Arctic, natural parents who give up primary responsibility for raising their children typically delegate care of their offspring to close consanguineal kin.

�9 Natural parents are uniformly reluctant to give up their children to others permanently, and they often express regret at the necessity of doing so.

�9 Parental investment is not necessarily terminated when adoption and fosterage transactions are completed. The identities of the natural parents are usually known to the adoptive child. Natural parents may maintain contact with them, continue to contribute resources to their care, and retain their rights to retrieve their offspring if they are mistreated.

�9 The natural parents are often selective in their choice of adoptive and foster parents. They typically prefer adults who can enhance their children's economic prospects.

�9 There are sometimes asymmetries in the care of natural and adoptive children. Adopted children often inherit less property from their adoptive parents than do their siblings.

�9 Anthropologists have also noted that adoption sometimes rein- forces social relationships and establishes reciprocal alliances between families and kinship groups (Brady 1976a; Carroll 1970a; Guemple 1979; Morrow 1984).

It would be misleading to assume that all elements of adoption transactions fit the predictions derived from kin selection theory. In some societies in Oceania and the Arctic, adults seem to accumulate unrelated adoptive children to enhance their status and prestige. In many ethnographic accounts of adoption in Oceania, the desire to raise children and sentiment for particular children emerge as strong motives underlying decisions to adopt (Silk 1980).

It would also be misleading to think that the patterns of adoption in the societies of Oceania and the North American Arctic are representa- tive of all traditional societies. There are few other societies in the world in which adoption of young children from intact families is so prevalent, although foster care of older children is common in West Africa (Goody 1982). In other areas of the world, adoption functions primarily to meet the needs of orphans, legitimize children born out of wedlock, provide heirs for childless individuals, or incorporate the captives of war into the social community (Silk, Gayron, and Smucny in preparation).


Adoption in Modern Industrial Nations

Contemporary adoption transactions in the United States, Canada, and Western Europe seem to reflect a deep desire to raise children, with considerably less concern for nepotism or reciprocity.

The first adoption law was enacted in the United States in 1851, when Massachusetts passed "An Act to Provide for the Adoption of Chil- dren." Before that time, the custom of "putting out" children, particularly orphans, as domestic servants and apprentices predominated. Many of these transactions involved kin (Kawashima 1981-1982; Presser 1972). The Massachusetts law required that natural parents consent in writing to the adoption, that the court be satisfied that the adoptive parents were competent to raise the child, and that the court transfer all legal authority over the child to the adoptive parents. During the next 25 years, similar adoption laws were passed in 24 more states (Presser 1972). Modern adoption laws were subsequently adopted in Canada, Great Britain, and Europe (Andersson 1986; Baer 1986; Hall 1986; Hoks- bergen and Bunjes 1986; MacDonald 1984; Melchior 1986; Picton 1986).

The numbers of adoptions have risen steadily in the United States since World War II. In 1944, approximately 50,000 children were adopted in the United States (Schapiro 1956); 10 years later, the number rose to 90,000 (Bureau of the Census 1965); and in 1964, the number reached 135,000 (Bureau of the Census 1968). In 1957, there were 2.1 adoptions for every 100 children born in the United States. By 1974, this number had risen to 4.8 (Bonham 1977). The highest number of adoptions per annum was recorded in 1970, when 175,000 child adoption petitions were approved (Bureau of the Census 1972).

The number of adoptions in the United States has dropped since the early 1970s. This trend reflects a decline in the number of healthy infants available for adoption, as more mothers decided to keep their infants and as birth control and abortion became more accessible (Bonham 1977; Silverman and Weitzman 1986). There have been even more pronounced declines in the number of children relinquished for adoption in Sweden (Andersson 1986), Holland (Hoksbergen and Bunjes 1986), En- gland (Hall 1986), France (Pilotti 1986), Australia (Picton 1986), the Fed- eral Republic of Germany (Baer 1986), Denmark (Melchior 1986), and Canada (Bagley 1986).

At the same time the demand for adoptive infants continues to grow. The rising demand for adoptive infants is generally linked to a rise in infertility and changes in popular attitudes toward adoption. Substantial numbers of American couples experience problems with infertility, which are linked in turn to late age at marriage, postponement of at- tempts to start families, and an increase in the prevalence of sexually


transmitted diseases (Cole 1984). Many of those who cannot produce children decide to adopt.

All of these factors contribute to a substantial asymmetry in the availability and demand for adoptive infants. There may be as many as 100 prospective applicants for every healthy infant available (Cole 1987). This has produced lengthy delays in the adoption process. Faced with such delays, many couples turn to independent adoption agencies, at- tempt to adopt foreign-born children, consult attorneys who arrange private adoptions, or solicit adoptive infants from birth mothers through newspaper advertisements (Miller 1989; Silverman and Weitzman 1986).

The difficulty in obtaining healthy infants has also led some prospective adoptive parents to broaden their criteria for an acceptable child. Surveys indicate that many adoptive applicants are willing to adopt a child with one major handicap, an older child, or a child of another race or ethnic group (Chambers 1970). Although such children were once considered "unadoptable," a growing number of children with special needs are being placed successfully in adoptive homes (Barth and Berry 1988; Churchill 1984).

Adoption in modern industrial societies seems to be an altruistic en- deavor. Although adoptive parents seem to derive immense pleasure and personal satisfaction from their children, they also incur substantial material costs. In a survey of adoption fees charged by 120 private adoption agencies belonging to the National Committee for Adoption, the average fee was $7731 (Cole 1987). International adoptions generally cost slightly less (Cole 1987; Silverman and Weitzman 1986). These fees do not cover the costs of legal services to finalize adoption transactions. In addition, when adoptions are arranged through private parties, prospective adopters sometimes pay for the biological mothers' medical care during pregnancy (Brieland 1984).

It is also expensive to raise a child in this society. Parents will spend well more than $100,000 to raise a child born in the early 1980s (Es- penhade 1984). Although some states provide subsidies to adoptive parents, none reimburses parents for all of their expenses (Freeman 1984; Silverman and Weitzman 1986).

Adoptive parents incur more than economic burdens on behalf of their children. All children require substantial care and commitment, but those who adopt children with special needs face a remarkable chal- lenge. Consider the comments of this adoptive parent:

Within two weeks after we got him we realized he was deaf, although we had been told only about his hydrocele and hernia. When he started walking, he had turned feet and stiffness of the hips, then later dental problems and allergies. But he's a beautiful boy. I couldn't say I wouldn't want to do it again. (Franklin and Massirik 1975:51)


Adoption generally seems to be beneficial for adopted children, at least to the extent that it enhances their socioeconomic position and the sta- bility of their lives. In the United States and Western Europe, adoption is a unilateral transaction. Although the mother who gives her child up for adoption is likely to be young, unmarried, poor, and a member of a minority ethnic group, the adoptive parent is likely to be older, married, middle-class, and white (Mandell 1973; Pascall 1984). For children adopted from developing countries plagued by war or poverty, the dis- parity in economic and social status of children before and after adoption is likely to be even greater (Baker 1984; Silverman and Weitzman 1986).

Adoption generally provides a permanent home for children. In a survey of 15 adoption studies conducted over a 40-year period, Ka- dushin (1970) found that 74% of adoptions in 2236 families were "un- equivocally successful." Even for children with special needs, a great majority of placements are successful (Churchill 1984).

Nepotism and Adoption in the United States

From 1952 to 1971, nearly half of all adoptions in the United States were among nonrelatives. Approximately half of the remaining adoptions involved children who were legally adopted by their stepparents (Kirk 1981). Thus, perhaps as many as three of every four adoption transactions involve unrelated individuals.

Although many adoptions involve nonkin, it would be misleading to imply that nepotism is irrelevant in parenting decisions. In fact, there is little evidence that adoption is widely favored over biological parenthood. A relatively small fraction of adoptive parents have produced children of their own (Bonham 1977).

Moreover, even though adoption is widespread, it is not considered to be equivalent to biological relationships by the public at large. In a survey conducted by Kirk in the 1950s, citizens of a university commu- laity in New York were asked the following question:

A woman was once asked whether she preferred her son by birth to her adopted son. She said "If they were drowning and I could save only one of them, so help me, I don't know which I would choose." Do you think this mother's feeling is natural or not so natural? (Kirk 1964:25)

A substantial majority (76%) of the respondents thought that this mother's sentiments were natural. Only 19% thought her feelings were un- natural. The remainder did not respond to the question. Thus, people seem to feel that adopted children ought to be treated like natural chil-


dren. When the respondents were asked what a mother ought to do in such a situation, however, their answers were quite different:

Suppose a mother actually was faced with having to choose between two boys. If she could do nothing else, which of them would you say she should try to save first--the one by birth or the adopted son? (Kirk 1964:25)

Nearly half of the respondents to this question thought the mother should save the son by birth (46%), only 3% thought she should save the adopted son, and 17% thought she should save the nearest child. The remainder refused to answer the question.

Although attitudes may have changed to some extent in the interim (Kirk 1981), disparities in attitudes toward adoptive and biological children do not seem to have disappeared entirely. Moreover, the desire of many adopted children to learn the identity of their natural parents (Haimes and Timms 1985; Sachdev 1984; Triseliotis 1984), and the desire of natural parents to learn the fate of the children they have given up for adoption (Sachdev 1984), suggest that adoptive kinship relationships do not entirely supersede biological kinship relationships.

CONCLUSIONS

Although societies seem to vary widely in the frequency of adoption and fosterage, there seem to be no human societies in which biological parenthood is ignored or considered irrelevant. This fact alone suggests that cultural explanations are not sufficient to explain the origins of human adoption behavior.

For those who expect human behavior to be adaptive, adoption practices in Oceania and the North American Arctic fit a number of predictions derived from the theory of kin selection. In these societies, children are exchanged among kin, and surviving parents often maintain contact with their children after adoption. On the other hand, our own adoption practices, which often involve unreciprocated altruism toward strangers, seem to contradict predictions derived from the theory of kin selection and reciprocal altruism.

Those who believe that evolution has shaped the proximate mechanisms that underlie behavior would probably not be surprised to find high rates of adoption in any society in which rates of infertility are high. Humans seem to possess innate psychological predispositions that promote an intense desire for children and permit the formation of close relationships with infants and children of strangers. These underlying


behavioral mechanisms may jointly facilitate adoption among those who cannot produce children of their own.

It might be argued that patterns of adoption in modern industrial nations may reflect little concern for nepotism because opportunities to adopt nonkin were limited in Pleistocene environments, and mechanisms to discriminate between related and unrelated young would have had little adaptive value. In an insightful discussion of the origins of human adoption that has been widely overlooked, Alcock (1979:463) suggests:

In large industrial societies, children are made available to people who do not know the parents of the adoptee. Throughout the vast majority of human evolution, in which people lived in small bands or villages this event would be exceptionally rare. Adoption in this environment may have been practiced regularly to the genetic benefit of the adopter.

The problem with this hypothesis is that it does not explain the existence of nepotism in adoption transactions in some human societies. The Inuit typically live in small and isolated groups, but not all members of their communities are related to one another. Nonetheless, there is a pronounced tendency to adopt related children. The peoples of Oceania live on densely populated islands, in groups much larger than those of Pleistocene food foragers. Here too, adoption occurs mainly among close kin. In the highly stratified societies of West Africa, children are often reared by foster parents, who are often kin (Goody 1982). In the colonial period of the United States, orphans were often adopted by relatives (Presser 1972).

Variation in the extent of nepotism in adoption in traditional societies and modern industrial nations may partly reflect differences in cultural beliefs about the nature of rights and obligations among kin. In Oceania, natural parents are sometimes pressured to give up infants for adoption, and they do so with considerable reluctance (Carroll 1970b). It seems unlikely that members of our own society would make or grant such requests. When childless couples in industrialized nations want to adopt children, they turn to governmental agencies, independent institutions, or private individuals, not to their families.

Differences in the kinship ideology among societies are sometimes linked to differences in the level of sociopolitical organization. Kinship is a more prominent organizing feature in horticultural and agricultural societies than in industrialized societies (Johnson and Earle 1987; Kees- ing 1975). Some authors have argued that in Oceania, adoption is part of a larger system of sharing among kin (Brady 1976a; Carroll 1970a, 1970b). Carroll (1970a:13) writes, "all of the available evidence suggests


that specific obligations to give up children in adoption are merely part of more general obligations between certain categories of kin; this seems evident in all of the societies which practice adoption on a large scale."

It may be possible to test the hypothesis that differences in the degree of nepotism in adoption are linked to differences in kinship ideology and sociopolitical organization within Oceania. There is considerable variation in the level of political organization among the societies of Oceania (Brady 1976b). If the hypothesis is correct, then societies with greater degrees of stratification and complexity should exhibit less nepotism in adoption transactions than simpler societies.

There may be many other explanations for differences in the degree of nepotism in adoption transactions among societies. The extent of nepotism may be related to demographic, historical, or cultural factors that influence the availability of children to prospective adoptive parents and the demand for adoptive children.

We can derive some practical insights about adoption and parenting in our own society from the evolutionary analysis of adoption developed here. In particular, the analysis suggests that parenting is a fundamental element of human nature. Despite the major technological, demographic, and cultural changes that have transformed our enviroment from that of our hominid ancestors, we seem to have retained an intense desire to raise children.

Although nepotism influences many aspects of contemporary human social relationships (Essock-Vitale and McGuire 1985), genealogical relatedness does not seem to be a necessary condition for parental commitment or love for children. Moreover, the ability to develop strong bonds with children does not seem to depend very heavily on the details of early experiences of parents with their infants or on the children's physical characteristics. Adoptive parents seem to have little difficulty loving and caring for adopted children, even if they look very different from themselves, do not arrive in their homes as infants, or are seriously handicapped.

On the other hand, it may not be appropriate to apply all of the information derived from these analyses directly to our own lives. For example, in the adoption literature there is now considerable discussion of the merits of "open" adoptions (Barth and Berry 1988) and contro- versy about the rights of adoptees to learn the identity of their natural parents (Haimes and Timms 1985; Sachdev 1984; Triseliotis 1984). In Oceania and the Arctic, adoptive relationships are generally acknowl- edged openly and children often maintain contact with their biological parents throughout their lives. Acknowledgment of biological relationships and maintenance of ties with biological kin seem unproblematic in these societies, but that does not necessarily mean that it would be


unproblematic in our own. Open adoptions among relatives in a society characterized by expansive notions of the rights and obligations among kin may be perceived very differently than open adoptions among strangers in a society in which rights and obligations toward those out- side the immediate family are restricted.

Current evidence indicates that children are more vulnerable to abuse in households that contain stepparents than in adoptive homes. Since children are unrelated to both adoptive parents and to stepparents, the degree of relatedness among the members of the household does not seem to provide a direct explanation for variation in the rate of child abuse in these households. Daly and Wilson (1988) have suggested that ambivalent attitudes toward parenthood and investment in children may sometimes precipitate abuse toward stepchildren. Such conflicts may not arise among adoptive parents, who typically possess a strong desire to become parents. It is not clear whether such conflicts charac- terize households in which children are legally adopted by their stepparents. Comparisons of rates of abuse by adoptive parents, adoptive stepparents, and stepparents may help us to understand the proximate factors that contribute to child abuse more clearly.

The adoption literature also provides evidence of an intriguing contrast in attitudes toward male and female children. Although gifts are selectively killed or neglected in many societies, girls are prefer- entially adopted in our own. In many societies that traditionally practiced infanticide or fatal neglect of children, girls were killed considerably more often than boys (Johansson 1984; Scrimshaw 1984). The selective neglect and killing of females are generally thought to be a result of differences in the economic benefits associated with rearing sons and daughters and cultural differences in the value placed on males and females (Burgess and Draper 1989; Johansson 1984). In India, where preferential female infanticide was once common, female children are now given up for adoption more often than males (Jungalwalla 1986).

Kirk (1964) reviews a number of studies conducted over a 50-year period in the United States and Canada that demonstrate a bias in favor of adopting female children. This bias appears to persist. In 1980, 41% of all requests to adopt children received by the Adoption Desk of Health and Welfare Canada specified a preference to adopt a girl, 26% stated a preference for a boy, and 33% expressed no gender preference (Lipman 1984). At that time, only 39% of the children registered with the adoption desk were female. In a recent s tudy in Holland, one-third of all prospective adoptive parents expressed a preference about the gender of the child. Of those who expressed a preference, 69% wished to adopt a gift (Hoksbergen et al. 1987). Reasons for the preference to adopt gifts are not well-established, although some workers have spec-


ulated that the preference may arise because girls are more affectionate than boys, more helpful to their parents in old age, more readily accepted by adopt ive fathers, or m a y reflect the cultural r emnan t s of a patrilineal descent sys tem in which male descent lines mus t be kept genetically pu re (Kirk 1964).

We need considerably more in format ion about the pat tern , f requency , and context of adopt ion a round the wor ld to construct a sat isfying ex- planat ion of h u m a n adoption. A complete theory mus t describe bo th the proximate mechanisms that under l ie adopt ive decisions and the adaptive consequences of adopt ion behavior . Moreover , such a theory mus t be conce rned with variation in the pa t t e rn and f requency of adop t ion among con tempora ry h u m a n societies and historical changes in the pat- te rn and f requency of adopt ion wi th in societies. It seems part icularly impor tan t to unde r s t and the complex ideological, historical, cultural, and demograph ic factors that inf luence the degree of nepo t i sm in adop- t ion in the broad range of con t empora ry h u m a n societies.

Robert Boyd, Sarah Blaffer Hrdy, and Jane Lancaster provided helpful comments on earlier drafts of this paper. This paper was prepared while I was conducting research at the California Primate Research Center at the University of California, Davis; I thank the CPRC for their support.

Joan Silk is an assistant professor of biological anthropology at the University of California, Davis. She received her Ph.D. in Anthropology from the University of California at Davis in 1981. Dr. Silk's research has generally been concerned with the evolution of social behavior among nonhuman primates; she has conducted fieldwork on chimpanzees and savannah baboons and has been studying the social behavior of bonnet macaques at the California Primate Research Center since 1977.

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