Integrating Evolution, Culture and Developmental Psychology: Explaining Caregiver-Infant Proximity and Responsiveness in Central Africa and the United States of America


Barry S. Hewlett and Michael E. Lamb


For: Between Biology and Culture: Perspectives on Ontogenetic Development, Heidi Keller, Ype H. Poortinga, and Axel Schölmerich (Eds.). Cambridge UP

DRAFT: Please do not cite without authors' permission.


This paper describes three neo-evolutionary or neo-Darwinian approaches and their implications for developmental psychology. Many developmental psychologists view "evolutionary" perspectives as "hard-wired", "biological" or "genetic" explanations of human development largely because so much public and scholarly attention has been given to one of these neo-evolutionary approaches--evolutionary psychology. Cover stories about evolutionary psychology have appeared in Time and Newsweek, and evolutionary psychology courses now exist at several major universities. Although evolutionary psychology is described in this chapter, emphasis is given to two lesser known approaches to neo-evolutionary thought--evolutionary ecology and evolutionary cultural anthropology. Particular emphasis is placed upon the position and role of culture within neo-Darwinian thought. Few developmentalists are aware that the culture concept (i.e., culture as symbolic, historical, transmitted non-genetically generation to generation) exists within an evolutionary framework. Recent research on caregiver-infant proximity and responsiveness among Aka foragers, Ngandu farmers and urban industrialists from Washington DC are utilized to illustrate the neo-evolutionary approaches.

Evolutionary Approaches

There are many brands of evolutionary thought within psychology, ecology and anthropology but the neo-evolutionary or neo-Darwinian approaches described in this chapter emphasize relatively recent contributions to Darwin's theories of natural and sexual selection. Core neo-evolutionary theories includes kin selection theory (Hamilton 1964), parental investment theory (Trivers 1972), reciprocal altruism theory (Axelrod 1984) and life history theory (Charnov 1993) which built upon, clarified and expanded Darwin's ideas of natural and sexual selection. According to Darwin (1859), for instance, the measure of "fitness" in natural and sexual selection was the number of offspring an individual left behind. Hamilton's (1964) kin selection theory expanded this idea by pointing out that an individual's genes existed beyond self and offspring. As a result, an individual could enhance his or her reproductive fitness by helping any individual that shared genes with him or her (nieces, nephews, cousins). Several books describe these central neo-evolutionary theories (Daly and Wilson 1983, Trivers 1987). From this point on, we drop the "neo" prefix from "evolutionary" or "Darwinian" because our focus in on these recent conceptual frameworks.

Before describing the three evolutionary approaches it is important to mention briefly a few basic concepts that they have in common. First, the unit of natural selection and the focus of evolutionary studies are individuals rather than groups. Humans live in groups and have cultural practices and beliefs because those groups enhance the survival and reproductive fitness of individuals. Second, evolutionists are interested in ultimate rather than proximate explanations. Ultimate explanations focus on the ways in which particular behaviors enhance the reproductive fitness of individuals whereas proximate explanations focus on social, psychological, hormonal or cultural factors. They are different kinds of explanations and they are not necessarily contradictory or mutually exclusive. Developmental psychologists may attribute teenage males' risk-taking in sports or sexual relations by reference to identity formation processes or as a result of high levels of male testosterone. An evolutionist would be interested in explaining why particular males make these risky choices, why testosterone evolved to increase male risk-taking, or how the risk-taking relates to an individual's reproductive fitness? Ultimate and proximate explanations are different levels of explanation and are not necessarily contrary to one another. It is also important to remember that evolutionary theories were developed to explain cross-species behaviors and are therefore not anthropocentric.

Diversity within evolutionary thought

Table 1 summarizes some of the theoretical diversity within evolutionary thought (modified from Smith 2000 and Hewlett 2001). The core theories of evolutionary thought were developed in the 1960-70s and by the early 1980s clear differences emerged among evolutionary researchers working with humans. One group of scholars, including Nick Blurton Jones, Pat Draper, Melvin Konner and Magdi Hurtado, conducted quantitative behavioral field studies of infants and children among relatively remote small-scale cultures (foragers, pastoralists and horticulturalists) in Africa and South America. They hypothesized that many child care practices (e.g., frequent holding, birth spacing, timing of weaning, parent-child interactions) were "adaptive" in that they maximized the reproductive fitness of children or adults in particular natural and sociodemographic environments. By comparison, David Buss, Don Symons, Martin Daly and Margo Wilson, John Tooby and Leda Cosmides conducted research in contemporary complex cultures, often studying college undergraduates using pencil and paper questionnaires. They were critical of the first group's "fitness maximizing" research because it did not contribute to a better understanding of human nature and cognition, and they did not value efforts to demonstrate that specific parenting behaviors increased reproductive success. Instead, they wanted to identify specific universal modules, information-processing programs, or algorithms of the mind that helped Paleolithic hunter-gatherers solve recurring adaptive problems in the environment of evolutionary adaptation or EEA originally defined by John Bowlby (1969). These researchers eventually termed their approach evolutionary psychology (EP) while the first group called their approach evolutionary ecology (EE) or human behavioral ecology.

Table 1 identifies some of the distinctions between these two approaches. Most importantly, evolutionary psychologists identify several content-specific modules in the mind while evolutionary ecologists view the mind as a general-purpose fitness-maximizing mechanism. Evolutionary psychologists are interested in identifying biologically based (i.e., genetic) universals of the human mind that evolved during the EEA and continue to shape human behavior. Current research emphasizes identification of the universal behavioral modules while their biological bases (i.e., specific genes or location in brain) are often assumed. For instance, EPs describe a kin recognition module that contributes to the differential care of biological as opposed to step-children (i.e., step-children are more frequently neglected and abused [Daly and Wilson 1988]). The language acquisition device is another module of the mind that enables children to quickly acquire the structure and meaning of language (Chomsky 1965, Pinker 1994).

Evolutionary ecologists on the other hand, tend to view human behavior as more flexibly and adaptively responsive to diverse natural and sociodemographic environments and this focus on explaining behavioral diversity in varied environments. Individuals of any age have only so much time and energy and are believed to enhance their reproductive fitness by maximizing their time and energy in a given environment. For instance, Lamb et al. (1984), Belsky (1997) and Chisholm (1996) hypothesize that children's attachments styles (i.e., secure, avoidant, resistant) are adaptations to particular rearing environments (e.g., high mortality, family stress). Belsky and Chisholm hypothesize that attachment styles lead to fitness-maximizing reproductive strategies later in life. For EEs, most behavior (what individuals do) is culture; they make little effort to understand individuals' ideas about what they do. In their view, culture does not have particular or special properties and for some EEs the concept of culture is not necessary (Betzig 1998). EEs emphasize that there are no specific genes for the behaviors that they describe since the mind is a general-purpose fitness-maximizing mechanism that allows individuals to evaluate the fitness consequences of alternative behaviors in particular environments.

The final approach in Table 1 is referred to as evolutionary cultural anthropology (ECA)--an evolutionary approach that focuses on the evolutionary nature of culture and social reproduction. Others refer to this research tradition as the dual transmission (genes-culture) or coevolutionary approach (Smith 2000, Durham 1992). We prefer to call it evolutionary cultural anthropology because its proponents aim to understand specific properties of culture and cultural diversity (Cavalli Sforza and Feldman 1981, Boyd and Richarsen 1985)--the domains of cultural anthropology--and because all three approaches are "dual" to the extent that all consider biology-culture/behavior interactions. Although most ECA theorists are theoretical biologists or geneticists, they aim to understanding culture, often using analogies and models derived from population genetics and epidemiology. Culture is usually defined by ECAs as information or knowledge that is symbolic, socially transmitted and historic (i.e., modified from prior forms) (Durham 1992). The specific units of cultural communication are referred to as memes (Dawkins 1976, Blackmore 1999) or semes (Hewlett et al. in press). Cultural inheritance is, of course, quite distinct from genetic inheritance (e.g., culture is inherited from individuals other than parents, cultural inheritance takes place faster or more slowly than genetic inheritance) and ECAs aim to identify and describe the nature of cultural inheritance systems. The approach is evolutionary in that culture has all the elements under which natural selections takes place (e.g., production of variability, heritability, fitness effects) and cultural mechanisms contribute to more efficient learning than trial-and-error. Efficient mechanisms of transmission enhance reproductive fitness.

Many of these researchers have a background in population genetics and know very well that genetic mechanisms can produce genetic. For instance, if parents in West Africa are heterozygous for the sickle-cell trait, they have a 25% chance of producing an offspring who is homozygous for the sickle-cell trait which leads to death at an early age. If genetic mechanisms produce maladaptation, it seems reasonable to hypothesize that cultural mechanisms could produce maladaptive patterns as well. ECAs point out that the identification of mechanisms underlying genetic transmission revolutionized genetics and that a better understanding of cultural transmission mechanisms could do the same for cultural anthropology. Consequently, ECA research has focused on identifying, describing and modeling (often mathematically) mechanisms of cultural transmission. Table 2 briefly describes the basic characteristics of some of the key mechanisms identified by evolutionary cultural anthropologists. We have placed them in a developmental context.

The concept of "niche construction" (Lalande, Odling-Smee and Feldman 2000) is a recent conceptual contribution by ECA theorists. Some species, including humans, modify natural selection pressures within their environments by creating a niche (e.g., burrow, nest) that produces a separate constellation of selective pressures (the constructed niche creates its own set of problems), which may or not enhance reproductive fitness of the individuals. Over time the niches and the consequences of the selective pressures are inherited. This is a potentially important contribution to ECA because culture increases the abilities of humans to construct new niches. Current work focuses on how innovative artifacts or technologies can lead to dramatic changes in the social and natural environment, which in turn create other selective pressures.

The ECA approach has motiviated the smallest number of researchers and few empirical studies have been conducted in this area. Researchers have demonstrated that many aspects of kinship and family organization are a result of demic diffusion, vertical cultural transmission and group effect rather than adaptations to natural or social environments, or cultural diffusion (Burton et al. 1996, Gugliemino et al. 1995, Hewlett et al. in press,). Demic diffusion involves the movement of people with their culture; a new group replaces earlier populations often because of technological or other cultural innovations. The Bantu expansion in central Africa and European colonization of the Americas are classic examples of demic diffusion. The features of vertical transmission and group effect are described in Table 2. ECA theorists believe that the distribution and diversity in infant caregiving and caregiver-infant sensitivity are the result of demic diffusion and vertical transmission of these semes (i.e., culture history) rather than "adaptations" to particular natural or social environments.

In summary, proponents of all three of all the evolutionary approaches discussed here are interested in understanding interactions between culture and biology, but EPs emphasize universal biology, EEs emphasize the natural and social environment, and ECAs emphasize the nature and structure of culture. Each approach asks somewhat different questions and employs its own methodological toolkit to address those questions, so it is not surprising that at times there are conflicts between individuals affiliated with each of the approaches, but these three approaches provide an opportunity to examine developmental psychology from a holistic and integrative perspective.

Infant Caregiving in Three Cultures

In this section we examine caregiver-infant proximity and responsiveness in three cultures with three distinct modes of production. These aspects of infant care were selected because they are hypothetically linked to attachment theory and social-emotional development in infancy. In the next section of the chapter, we discuss the results of our study in the context of the three evolutionary approaches.

Our studies were initiated to describe the "ecology" of daily-life for infants in several cultures. Most studies of infancy are based upon remarkably few hours (usually two or less) of direct naturalistic observation. The brief observations lead to a characterization of infancy that may apply only to particular contexts--e.g., play, feeding, caregiving contexts. In addition, few researchers have studied infancy in small-scale "traditional" societies. Specific evolutionary hypotheses were not evaluated in the research, but an evolutionary view of the interactions between biology, ecology and culture guided the research design. Behavioral observations dominated the research (we wanted to observe each infant for at least 12 hours), but we also interviewed parents about their ideas about childhood and their explanations for the behaviors we observed. We also participated in the daily lives of families in two of the three cultures (Aka and Ngandu, but not Euro-Americans) so we were able to talk to parents about their children in a variety of informal contexts (e.g., washing clothes, hunting).

The study and the families

The study consisted of 20 Aka, 21 Ngandu and 21 Euro-American families with 3 to 4 month-old infants. Infants were observed for three hours on four different days for a total of 12 hours per infant. Aka and Ngandu were observed between 6 a.m. and 6 p.m. on all days of the week whereas Euro-Americans were observed between 8 a.m. and 8 p.m. on weekdays only. The naturalistic observational procedure and coding system were modified from the scheme originally developed by Belsky et al. (1984). The observer watched for 20 seconds and recorded for 10 seconds. After 45 minutes, the observer took a 15 minute break, and then resumed observation. Observers noted on a checklist the occurrence of 11 caregiver and 10 infant behaviors, 6 dyadic behaviors, as well as the location, position and identity of the caregiver and infant.

All of the Euro-American infants were first borns. Only about 15 percent of the Aka and Ngandu infants were first borns. About 15 percent of their fathers had more than one wife. None of the Aka had a formal education or were engaged in a cash economy. All Aka and Ngandu men and women engaged in subsistence activities during the observation period. Most of the Ngandu men and several of the Ngandu women had an elementary education. Men and women engaged in subsistence and market activities, but neither were employed outside of the household. All of the observations took place during the dry season.

All Euro-American parents were college educated and half of them had graduate degrees. Ninety-five percent of these Euro-American mothers were employed full-time before the birth; they took leave during the first few months after delivery, and all returned to work by the time the infants reached 12 months of age. Mothers were always in the house during observations and observations took place primarily during the summer months. All the fathers were employed full-time and all infants had their own crib in their own room. The mean family income in 1991 was about $80,000 per year.


The Aka foragers and Ngandu farmers in this study are neighbors in the rural southern region of the Central African Republic (population density less than one person per square km) when they make a living in the same dense humid tropical forest. The Aka and Ngandu have similar relatively high mortality (infant mortality is 10-20 percent) and fertility (about 4-6 live births per woman).

The Aka and Ngandu live in very different physical and social settings. The Aka live in camps of 25 to 35 related (by blood or marriage) individuals who live in 5 to 8 dome-shaped houses that occupy about a 400 sq. foot area. Ngandu villages consist of 50 to 400 related (including clan affiliation) individuals. Each house is at least 10 feet away from the next, but there are no walls or fences between houses. Polygyny is about 40% among the Ngandu and about 15% among the Aka. The EA households were all located in relatively wealthy suburban Washington DC. Each house had several bedrooms and a large backyard. Infant mortality is less than one percent and the total fertility rate is less than two.

Aka and Ngandu have frequent social, economic, and religious interactions and see each other's caring for infants on a regular basis, yet have distinct modes of production, male-female relations, and patterns of infant care. The Aka are net-hunting foragers (also known as hunter-gatherers), move their villages several times a year, have minimal political hierarchy (i.e., chiefs with little/no power over others) and relatively high gender and intergenerational egalitarianism, whereas the Ngandu are slash and burn farmers, relatively sedentary, and have stronger chiefs and marked gender and intergenerational inequality. Upper middle class Euro-Americans have the greatest level of political and socioeconomic hierarchy, relatively high gender equality and low intergenerational egalitarianism.

Aka and Ngandu cultures share more and are in many ways more egalitarian than Euro-American cultures, but Aka sharing and egalitarianism are also substantially greater than among the Ngandu. The Ngandu focus on maintaining egalitarianism and sharing between households; households that accumulate more than others and do not share with neighboring families are prime targets of sorcery, which is believed to cause illness or death. Sharing between households is not as frequent as it is among the Aka (i.e., not daily) and there is marked inequality within Ngandu households, with men and the elderly receiving more than others are. The Aka, on the other hand, share with many people in many households on a daily basis and there is greater gender and age egalitarianism. Upper middle class neolocal families rarely share with others outside of the household, but regularly share food and resources with household members (i.e., between husband and wife). Differences among individuals are evaluated on a near daily basis.

Aka infants are carried in slings on the left-hand side of the caregiversí body whereas Ngandu infants are tied rather snugly on the caregivers' backs. When the adults are sitting, both Aka and Ngandu careproviders place infants on their laps or between their legs facing outwards. When infants are laid down, they are always placed on their backs. Aka infants sleep with their parents and siblings whereas Ngandu infants often sleep with their mothers (or in separate cots, when husbands come to visit). EA infants sleep alone in their own room in a crib.

Upper middle class EA infants are placed in a variety of technological devices--infant seats, swings, etc.-- generally facing towards the parent. Caregivers are usually home alone with the infant and there are few adult or juvenile visitors. EA infants have the most caregiving devices (e.g., clothes, diapers, baths, and toys) whereas the Ngandu have more caregiving devices than Aka. Some Ngandu parents make small chairs, beds or mats for the infants to lie on. Ngandu infants also have more clothes than Aka, are often dressed more warmly than adults even in the middle of a hot day, and are washed once or twice a day. By comparison, Aka infants seldom have more than a protective forest cord around their waists and are infrequently given a complete bath. Both Aka and Ngandu caregivers carefully keep insects and debris off their infants.

Patterns of Caregiver-Infant Proximity and Responsiveness

Here we summarize some of our results on the holding, feeding, and fussing/crying experiences of Aka, Ngandu, and Euro-American three-to-four-month-olds. The data are described in greater detail elsewhere (Hewlett et al. 1998, 2000a, 2000b) and have omitted statistical tests of significance to make our discussion more readable. All "caregivers" in the EA sample were mothers or fathers, whereas the Aka and Ngandu had several other categories of caregivers. The majority of Aka and Ngandu caregivers were adults so the following descriptions of "caregiver" proximity and responsiveness refer primarily to adult proximity and responsiveness.


Figure 1 summarizes different proximal behaviors and demonstrates marked differences among groups in the frequencies with which infants were held, proximal (defined as within an armís reach of caregiver), or were alone (i.e., caregiver not in room or not in sight). Aka infants were almost always held and never alone, whereas Ngandu infants were held half as frequently as Aka but significantly more than EA infants. There were no significant differences between the Ngandu and EA samples with respect to how often the caregivers were within armsí reach of the infants.

The holding differences between Aka and the other two groups occurred, in part, because caregivers in the other two groups put their infants down when they fell asleep whereas the foragers continued to hold/touch their infants while they slept. Ngandu adults held their infants 44% of the time when they slept and 60% of the time while they were awake, whereas the Aka held their infants 94% of the time while they slept and 98% of the time while they were awake. The Euro-American infants were held 22% of the time while sleeping and 44% of the time while they were awake. The asleep-versus-awake holding differences are highly significant when the Ngandu and Euro-Americans were compared, but not when the Aka and Ngandu were compared.

If we estimate the time infants were held/touched over 24-hour periods, the differences between the groups become even greater. Aka infants sleep with parents, Ngandu infants sleep with their mother and EA infants sleep in their own beds in their own rooms. With this taken into consideration, Aka infants are held/touched 99% of the time, Ngandu infants are held/touched 79% of the time and EA infants are held/touched 18% of the time.


Table 3 summarizes the frequency and duration of infant feeding. There were no statistically significant differences among the three groups in the percentage of intervals in which infant feeding occurred, but there were significant differences between Aka and Ngandu, and between Aka and Euro-Americans in the frequency of feeding/nursing. Aka caregivers fed their infants about twice as frequently as did Ngandu or Euro-American caregivers. The Aka were also distinguished by the frequency with which women other than mothers breast-fed their infants--more than 50% of the Aka infants were breast-fed by someone other than their mothers sometime during the observational period. All EA infants received some bottle-feeding were sometimes fed by their father or other careproviders.

Caregiver-Infant Responsiveness

There were no differences among the three groups with respect to the frequencies with which caregivers showed affection (hugging, kissing) to their infants, but there were significant differences in the length of time infants fussed or cried and the frequency and nature of responsiveness to fuss/cry events. Table 4 summarizes the frequencies of fussing and crying in the three groups. The Aka infants fussed and cried the least, the EAs were intermediate and the Ngandu infants fussed and cried significantly longer than infants in both other ethnic groups.

Figure 2 indicates how caregivers responded to instances of fussing or crying. Fifty to sixty percent of the time caregivers in all groups tried to soothe fussing/crying infants by rocking, singing or verbalizing to the infants. Aka caregivers were more likely to respond by nursing the infants whereas EAs were more likely to respond by holding or stimulating the infant. Aka caregivers did not respond by holding because the infants were almost always already being held when the fussing/crying started. It was interesting to find that Ngandu caregivers were the least likely and the Aka the most likely to respond to fuss/cry events.

In summary, Aka caregivers were the most proximal and responsive to infant feeding and crying, Ngandu caregivers were more proximal to their infants than EA caregivers but less responsive than EAs to infant fussing and crying. Aka infants were the most likely to experience multiiple caregiving.

Applying Evolutionary Approaches

In this final section we examine the caregiver-infant proximity and responsiveness data in light of the three evolutionary approaches described earlier in this chapter. Proponents of each evolutionary approach ask somewhat different questions, but all help to provide an integrated understanding of infant caregving.

Evolutionary Psychology

Evolutionary psychologists have conducted few systematic studies of infant caregiving, so what we present here is based upon our understanding of this approach and how it might be applied to the analyses of infant caregiving.

Evolutionary psychologists would emphasize that, in order to explain caregiver-infant interactions, it is necessary to understand the adaptive problems (i.e., recurring conditions that affected reproductive fitness) of caregivers and infants in the EEA and the modules or design features of their minds that developed under EEA conditions. One way to identify modules is to look for cross-cultural commonalties, possibly paying more attention to hunter-gatherer groups like the Aka since their way of life is closest to the EEA. One adaptive problem faced by humans was the relative helplessness or altriciality of newborns. Due to the evolution of bipedalism and increased brain capacity, human infants are more immature at birth than newborn chimpanzees and other higher primates. In particular, they are unable to grasp and hold onto their caregivers at birth. In addition, several common patterns of caregiver-infant interactions are evident in this cross-cultural study: 1) infant fussing or crying gets the attention of caregivers and somebody responds most of the time; 2) mothers are most likely to be caregivers and most likely to respond, but several other people also respond, especially among the Aka; 3) mothers and several others know how to soothe crying infants; 4) most caregivers are genetically related to the infants; 5) infants are soothed, among the Aka in particular, by feeding, but they can be soothed by several other means; and, 6) infants respond to caregivers besides their mother.

Given the adaptive problem and the cross-cultural commonalties, EPs might suggest that human infants have a fuss/cry or infant vocalization module and an attachment or proximity module. The attachment or proximity module is the classic system described by Bowlby (1969). Infants cannot run, hide or protect themselves from predators or infanticidal males (Hrdy 1999) so they seek the proximity of particular others by crying for, reaching for, or smiling at these individuals. Infants who did this were more likely to survive in the EEA than infants who did not have these behaviors.

Indeed, fussing or crying might represent a separate module for infant communication, because although fussing and crying are linked to attachment, they also permit communication with others before infants can speak to or crawl to others. Crying can be helpful in communicating hunger, illness, temperature (e.g., too cold), discomfort (e.g., wet) or jealousy (e.g., caregivers spending too much time with other children).

Caregivers are believed to have their own suite of modules: a kin recognition module, an infant communication module, and a proximity module. First, caregivers interested in enhancing reproductive fitness must be able to identify their own infants. This module is common to many primates (Silk 2001) and most caregivers in all three cultures were genetically related to the infant. In the EEA, caregivers who responded to related infants who were fussing or crying would have experienced greater reproductive success than caregivers who responded to any crying infants.

Second, caregivers also need to know how to interpret and respond to crying and other aspects of infant communication. Experience and learning certainly enhance these abilities, but EPs argue that most individuals in these cultures learn rather quickly how to interpret, respond to, and communicate with infants. For instance, many of the mothers and fathers in the EA sample never took care of infants until they had their own, yet learned to respond to crying and other forms of infant communication. The use of "motherese" by caregivers is another example of the infant communication module in that it appears to be used "naturally" or with minimal training. Bornstein et al. (1992a and b) also describe several cross-cultural and developmental universals in maternal speech and responsiveness. Considerable intracultural and intercultural variability in sensitivity is expected, but most caregivers quickly learn how to communicate and respond to infants.

There is also cross-cultural evidence that kin, mothers in particular, try to maintain proximity to their young infants. Infants among most hunter-gatherer groups like the Aka are held in the caregivers' arms or laps all day and night (Hewlett et al. 2000). Although proximity-seeking behaviors encourage caregiver proximity, hours of behavioral observations indicate that hunter-gatherers maintain proximity with no or very few signals from their infants (e.g., they continue holding infants when they fall asleep). This makes evolutionary sense as mothers and other kin would be better able to protect, feed and transmit culture when their infants were nearby.

Recent research indicates that 3-4 month-old infants are able to recognize faces, but unlike older infants they seldom fuss/cry when held by a stranger and can be soothed by several other individuals besides mother (Lamb, Bornstein and Teti, in press). This suggests either that the brain is not developed enough to establish stranger anxiety or that it was important in the EEA for young infants to compensate for their helplessness by soliciting care from many caregivers.

The infant and caregiver modules described above are speculative, but an EP approach emphasizes the importance of considering evolved design features of the mind that were selected for in the EEA. Humans have a long history and it is likely that the human mind has adapted to deal with recurring problems. Our interpretation and understanding of caregiver-infant interactions is likely to be constrained without some consideration and understanding of evolved templates of the mind.

Evolutionary Ecology

The EE perspective emphasizes they ways in which rearing conditions influence infant caregiving. What are the reproductive costs and benefits of different forms of parental investment in this environment? What are the fertility and mortality parameters? How many other caregivers are available? What reproductive tradeoffs are associated with investing now as opposed to later in children? EEs assume that individuals (caregivers as well as infants) try to maximize their reproductive fitness in specific environments.

For instance, kin selection theory states one is more inclined to help out those who are biologically related. EEs also use parent-offspring conflict theory to help explain parent-child interactions (Trivers 1972). Parent-offspring conflict theory emphasizes that parents and their offspring do not have identical reproductive interests--they share half of each others' genes and they are most concerned about their own interests. Given this, the care infants receive may not be optimal from the infants' perspective because parents are also concerned about their own health as well as the well-being of other offspring. Infants thus try to extract more time and energy from caregivers than caregivers are willing to provide. Caregivers and infants are interested in their own fitness and in particular social contexts these interests may conflict.

By comparison to EPs, EEs have developed several hypotheses to explain caregiving in the three modes of production represented in the three cultures in this study (i.e., foraging, farming and urban industrialism). This is not unexpected as EEs are interested in explaining behavioral diversity rather than human universals.

Table 5 summarizes three models used by EEs to explain caregiving in the three modes of production. Draper and Harpending (1982) were among the first to use evolutionary theory to explain differences in parental sensitivity to their children, suggesting that individuals raised in an "intimate" husband-wife context, with low marital stress and contributions to subsistence by both parents, developed a reproductive strategy that emphasized parental effort (proximal caregiving that was very responsive to infant needs). Individuals raised in "aloof" father-absent households developed reproductive strategies that emphasized mating effort (i.e., keeping and finding mates and less proximal and sensitive caregiving).

This work provided the basis for later work by Belsky (1997) and Chisholm (1996) on the relationships between family rearing environments, attachment, and parental reproductive strategy. They hypothesized that families with stable ("intimate") husband-wife relations and minimal socioeconomic stresses provide sensitive caregiving which in turn provided "secure" attachment patterns in infancy as well as parenting effort (i.e., greater investment and responsiveness with fewer children) reproductive strategies in adulthood. By comparison, families with father absence and greater social and economic stress are less proximal and sensitive to their infants. This promotes "insecure" attachments in infancy and a mating effort reproductive strategy in adulthood. It is important to emphasize that both parents and infants try to maximize their reproductive fitness in the different environments in which they live.

LeVine et al. (1994) also advanced an approach consistent with evolutionary ecology. He indicated that parental goals were adaptations to different levels of infant mortality. Agrarian (e.g., the Ngandu in this study) parental goals focused on the survival, health, and physical development of infants because infant mortality levels were high--often only half of the children survived to reproductive age. In order to monitor and respond to health and survival indicators, agrarian parents were expected to keep their infants close (holding or keeping them in proximity), respond quickly to fusses or cries, and feed infants on demand. This contrasted with urban industrial parental goals that emphasized active engagement, social exchange, stimulation, and proto-conversations with infants. LeVine reasoned that urban parents were concerned with the acquisition of cognitive skills essential for survival when infant mortality is low, children cost more and contribute less, and there is a competitive labor market operating through an academically graded occupation hierarchy. He predicted that urban industrial parents would be less proximal and responsive to fussing and crying since infant health was not a priority.

The third model in Table 5 identifies three basic parental investment strategies characterized, respectively, by "survivorship", "production", and "offspring reproductive success" (ORS) enhancers (Blurton Jones 1993). The survivorship-enhancing and production-enhancing patterns are similar to the parental and mating effort patterns described by Draper and Harpending in the first model. Blurton Jones suggested that forager-farmer differences in caregiving were due to differential environmental hazards (e.g., predators); he proposed that forager parents invest more time and energy in each child to ensure "survivorship" in an especially hazardous environment whereas farmers emphasize "production". Parents who adopted the ORS enhancing pattern, like LeVine's prototypical urban industrial parents, were expected to have very few offspring, invest heavily in their children's cognitive development and minimize the importance of caregiver proximity and responsiveness. Blurton Jones associated the survivorship-enhancing strategy with low fertility hunting and gathering groups, the production-enhancing strategy with high fertility agricultural societies, and the ORS-enhancing strategy with high socioeconomic status parents in industrial societies (1993:311).

There are several similarities and clear differences among the modelsí assumptions and predictions. Most important, from an evolutionary ecology approach is that local ecologies--rearing environment and health risks--are central to each of the models. LeVine used differences in infant mortality to explain differences between the agrarian and industrial parental infant caregiving, whereas Blurton Jones emphasized threats to infant and child survival (e.g., predators) to explain the proximal and sensitive caregiving practices of survivorship-enhancing hunter gatherers.

Important differences exist between the three models in their characterization of group differences in caregiving, however. Most importantly, LeVine described farmers, such as the Ngandu, as proximal (e.g., holding or staying close to their infants), responsive, and sensitive to their infants whereas the other two models suggested that farmers should provide relatively harsh, rejecting and insensitive parenting. Draper and Harpending proposed similarities between foragers and EAs whereas LeVine and Blurton Jones suggested that these groups should differ.

The data presented in the previous section raise serious questions about LeVineís depiction of "indulgent" (i.e., frequent holding and feeding, rapid response to fuss/cry) agrarian parents. Infants among the agrarian Ngandu fussed and cried substantially more than the EA infants, they were fed almost as frequently (bouts per hour) as EA infants, and there were no differences between EA and Ngandu infants with respect to the amounts of time that infants were near caregivers during the day. EA infants were not held as often and they were more likely than Ngandu infants to be alone. Such findings support Draper and Harpending's and Blurton Jones' characterizations of agrarian caregivers, with parents emphasizing "production" and minimal responses to infant demands. Analyses of the distal-verbal measures in previous papers (Hewlett et al. 1998) tended to support LeVine's and Blurton Jones' predictions regarding urban industrial caregivers emphasis on cognitive development, however.

In another paper (Hewlett 2000) we identified several problems with EE explanations of forager infant proximity and responsiveness. Some of them are listed below.

1. Most studies reveal few differences in fertility and mortality between foragers and simple farmers (Bentley et al. 1993, Hewlett 1991,). Specifically, Aka women have more live births than do Ngandu or EA women (6.2, 4.3 and 1.8 respectively), but hold their infants more often and respond to fussing and crying more quickly and consistently.

2. A study of the causes of death among Aka infants indicated that infants and young children never died from snakebites or predator attacks (Hewlett et al. 1986).

3. Aka and Ngandu both have high (by Western standards) infant mortality rates (both above 10%). If one caregiving pattern did lead to greater survival and better child health parents should adopt the behaviors that increased survivorship (i.e., towards Aka frequent holding and sensitivity). Aka and Ngandu see each other's caregiving on a regular basis and are therefore clearly aware of alternative patterns.

4. Aka allow their infants to play with "dangerous" sharp instrument such as digging sticks, spears, axes, knives and machetes while Ngandu and EA parents do not.

5. Aka caregivers are not as vigilant as Ngandu and EA caregivers in trying to keep older infants and young children away from hot pots and pans. Young Aka are allowed to crawl around hot pots and fires.

6. Interviews with 20 Aka and 20 Ngandu parents revealed that: a) parents felt there were just as many poisonous snakes in the village as there were in the forest; b) snakes, insects and predators were not identified by Aka or Ngandu parents as important health risks for infants; c) both Aka and Ngandu parents felt that Ngandu infants were more fragile but this did not lead Ngandu parents to hold their infants or respond to them more frequently; and, d) both groups of parents felt that crying could lead to illness, but this did not lead the Ngandu to respond as quickly as Aka parents to crying infants.

We thus suggest a few alternative hypotheses. The first is consistent with EE and will be discussed here while a second hypothesis is consistent with ECA and will be mentioned in the following section. Our EE hypothesis suggests that holding and sensitive infant caregiving among foraging groups like the Aka contribute to the development of individuals who are more trusting, autonomous and generous (Hewlett et al. 2000). Hunter-gatherer life is characterized by its depth and breadth of sharing and egalitarianism. Studies among the Aka and other hunter-gatherers indicate that it is not unusual for individuals to share 80 percent of the food they obtain by hunting or collecting with most of the families in a camp (Gurven et al. in press, Kitanishi 1996). If children learn to trust themselves and others, they are more likely to give and share extensively with others. Individuals trust that others will be generous with food, caregiving and other resources. A secure sense of self also contributes to autonomy, another common feature of forager life (Gardner 1991), and egalitarianism. A secure sense of self and trust in others decreases the potential for ranking and evaluation of others by gender, age or features. This hypothesis helps to explain why EA and Ngandu caregivers may be less responsive to infant needs--it is less important to promote trust and sharing in these cultures--but does not help to explain why the Ngandu are least responsive to crying.

In summary, the EE perspective emphasized the impact of sociodemographic factors (e.g., availability of fathers, predators, and socioeconomic conditions) and the importance of cooperation and sharing between individuals in explaining the three patterns of caregiver proximity and responsiveness in the three cultures.

Evolutionary Cultural Anthropology

The final evolutionary approach examines the ways in which specific features of culture and social reproduction shape caregiver-infant proximity and responsiveness. The ECA approach suggests that culture exerts it own set of unique forces on infant care. By comparison to juveniles in higher primate species, human children have a remarkable ability to imitate and acquire knowledge, information and behaviors from others--i.e., to acquire culture. ECAs are particularly interested in the processes of cultural transmission and acquisition. They pay close attention to the history and transmission of caregiving ideologies, artifacts of caregiving, and cultural practices or "habits" associated with caregiving. Like the EP approach, specific ECA studies of infant care do not exist so this description should be considered exploratory.

The ECA approach is similar to recent analyses of the relationships between attachment theory and culture (Rothbaum et al. 2000). Their analysis suggests that maternal sensitivity and infant attachment patterns are substantially different in and the US because of dramatic differences in what cultural anthropologists call "culture cores"--general schema that pervade and shape developmental trajectories throughout the lifespan. The Japanese schema places priority on collectivism, interdependent conceptions of self, empathy, compliance and propriety towards others (called symbolic harmony). By comparison, middle-class US parents emphasize individualism (independent conceptions of self) and exploration (called generative tension). The preferred patterns of social relations affect parent-infant proximity and responsiveness. According to Rothbaum et al., Japanese parents are more proximal and responsive in order to promote infant dependency and sense of interrelatedness. "Japanese mothers meet their infants' needs even before they are expressed, thereby blurring the self-other distinction" (ibid.: 1126). Selfless devotion and indulgence (amae) is expected of Japanese mothers. US parents, on the other hand, prefer to wait for the infant's cues before responding. Rothbaum et al. cite a number of studies showing that Japanese parents are more proximal than US parents: Japanese mothers hold and carry their infants more than US parents, practice cosleeping more frequently, and are more likely to use snugglies to carry infants rather than strollers or walkers. Rothbaum et al. suggest that Japanese parents are more responsive to infant needs such as hunger and fussing and crying, but unfortunately, no data were presented to support the statement. This is problematic because both Aka and Ngandu caregivers in this study were more proximal to their infants than the US parents were, while US parents were more responsive to their infants' fussing and crying than Ngandu parents were.

Proponents of ECA argue that culture and cultural history have substantial effects on caregiver-infant proximity and responsiveness, but the ECA approach is distinct from Rothbaum et al. and other "cultural" approaches to infant care (e.g., Super and Harkness 1986; Harwood, Miller and Irizarry 1995) in that it tries to identify specific evolutionary- based processes and mechanisms that lead to the prominence of culture in explaining human behavior (see Table 2). The work by Freedman and Gorman (1993) on attachment is probably one of the best examples of an ECA approach to infant care. Freedman and Gorman describe how the attachment process and the associated development of internal working models (IWMs) help to explain the emotional basis of conservative vertical transmission--i.e., IWMs shape how we feel about relations with others and this in turn shapes our responsiveness to infants needs. This early patterning of social relations (a type of schema) becomes what ECAs call "marker traits" (Boyd and Richerson 1984)--patterns of language, dress, and social relations that are acquired in infancy and early childhood. ECAs point out that individuals are more likely to acquire culture (or semes) later in childhood and adolescence from people with marker traits similar to their own.

ECAs hypothesize that peoples with a common history are likely to have common patterns of infant care due to particular patterns of transmission--vertical transmission and group effect. Bantu peoples in Africa, Polynesians in Oceania, and Indo-Europeans in Europe and the Americas share a common history, in that all expanded their ranges and displaced previous inhabitants. The culture cores of these peoples are conserved due to vertical transmission and group effect mechanisms. Individuals in each of these expansionist groups are expected to have similar patterns of infant proximity and responsiveness, and this implies that infant care may or may not be reproductively adaptive to particular natural or social environments. To better understand the phylogenetic history of caregiver proximity and responsiveness, we need comparable systematic data from many cultures around the world, and unfortunately these data do not exist.

Systematic interviews with Aka and Ngandu children and adults indicated that infant caregiving practices (how to soothe a fussy infant, how to hold and carry an infant) were learned from their parents (Hewlett and Cavalli Sforza 1988; Hewlett, unpublished data). Most individuals, especially among the Aka, reported that they learned these skills from both their mothers and fathers, although some mentioned only their mothers or fathers. Aka and Ngandu respondents usually knew these skills by age ten. Behavioral observations were consistent with interview data; when young children cared for infants their parents usually supervised them. Vertical transmission and group effect are thus the principle mechanisms by which infant care is transmitted in these two groups. Although comparable systematic data was not obtained from EA parents, many EA parents today rely upon the advice of pediatricians or family doctors, "how to" books, and close friends who have children. Parents (mothers, in particular) may be consulted, but they seldom live nearly (especially in upper middle-class families). Infant care among EAs appears to be transmitted primarily by way of horizontal and one-to-many mechanisms. These differences in primary transmission mechanisms imply that Aka and Ngandu caregiving is more conservative and is likely to have a deeper phylogenetic history, whereas the transmission of EA caregiving is consistent with rapid change and short histories in a highly variable environment. ECAs point out that horizontal and one-to-many types of transmission are more likely to lead to maladaptation (i.e., to reduce reproductive fitness of individuals) because there may be less time to respond to feedback from the environment.

The mechanisms of transmission for EAs help to explain some of the relatively rapid changes in EA infant care practices. For example, US parents today are encouraged to hold infants more often, feed more on demand rather than on a schedule, and to respond quickly to fussing, crying or other infant needs. This contrasts with recommendations offered by experts in infant care manuals in the 1950s (e.g., Spock 1946). The changes in recommended practices are integrated and take meaning within the culture core, which is transmitted more vertically and through group effect (also called frequency dependent bias). This is one limitation of the analysis conducted by Rothbaum et al. (2000)--they emphasize that differences between Japanese and US cultural cores, but give the impression infant care practices in Japan and the US have not changed much since the 1950s.

In the previous section, we questioned EE explanations for the high levels of infant proximity and responsiveness among the Aka. The frequent holding and feeding as well as the sensitive responsiveness to fussing and crying cannot be explained entirely by high infant mortality and dangers in the environment. We hypothesized that extensive sharing is another important "ecological" factor to consider, but it is also important to consider the role of culture. Although our analysis of parental ideologies is not complete, Aka parents clearly do not view their environment as particularly dangerous for their infants. Parents are preoccupied with infant health and survival--indeed they allow infants and young children to play with all kinds of sharp objects and to wander around and touch hot pots and pans. Aka parents talk about keeping their children physically and emotionally close to them throughout the lifespan. Aka respondents say that Ngandu parents may love and keep their infants and young children close to them, but the Aka are different in that they love and keep their children close at all ages. The Aka and many other foragers have vertically transmitted internal working models that value remarkable responsiveness and sensitivity to infants' needs. Even the most engaged EA parents are unlikely to hold their infants all day and to nurse their infant four times an hour. Ngandu parents, on the other, hand value respect and deference towards others. One may hypothesize that Ngandu caregivers let infants cry relatively long to help develop deference to the needs of older individuals. But deference and respect to the group are the reasons given for the reverse--immediate response to infant's needs--among the Japanese.

Finally, the recent work on niche construction within the ECA approach also suggests that we should pay particular attention to the material culture of caregiving. The Aka have few material artifacts associated with infant care while EAs have several. Our observational data indicate that 3-4 month-old EA infants spend about 55 percent of their daylight hours in some sort of device (infant seat, car seat, crib, etc.). Niche construction theorists indicate that such artifacts or particular technologies can lead to separate selective forces. For instance, infant seats are constructed niches and which affect the infant's temperature, movement, eye contact, etc. Over time, they could lead to changes in either biological or cultural evolution.

In summary, the ECA approach indicates that the caregiver-infant proximity and responsiveness are influenced by a variety of cultural transmission mechanisms and that both the phylogenetic history of a culture and ontogenetic enculturation play a role in explaining these behaviors.



The evolutionary perspective is undergoing an evolution of its own! At first it was thought to be a unified perspective, often called "sociobiobiology". As more researchers from a variety of disciplines and backgrounds applied the evolutionary perspective, they began to distinguish among the three approaches described in this chapter. Although considerable work is still needed within each of these approaches, there is increasing interest in trying to integrate the three approaches.

Figure 3 is a heuristic and integrative model for the study of human development. The model has several distinguishing features. First, biology, environment and culture are defined and conceptually separated from the behavior that one wants to understand (e.g., caregiver proximity and sensitivity in this study). If one wants to consider alternative hypotheses about the impacts of biology, environment and culture on particular behaviors, it is essential to clearly define and distinguish these factors. This is particularly important for "cultural" approaches as most socio-cultural researchers do not distinguish behavior and culture. ECAs define culture and describe its unique features. Second, the model and all the approaches are based on a unified body of theory--all are "evolutionary". The underlying unity enables those that focus on biology or culture to communicate with each other. Researchers using different evolutionary approaches often disagree, but an underlying commonality exists which enables communication and the potential for synthesis and theoretical development. This does not mean that all other theoretical approaches should be incorporated into an evolutionary approach. Evolutionary approaches are limited in that they focus on individuals and ultimate kinds of explanations. Finally, the model is integrative in that it assumes that it is unlikely that any behavior is entirely biological, cultural or environmental. Biology, culture or environment may explain a certain percentage of the variability, but it is necessary to integrate the three approaches. For instance, evolved modules associated with the attachment process (e.g., proximity) contribute to the emotional basis for conservative vertical cultural transmission. It is also likely that evolved modules exist for cultural transmission mechanisms, such as, indirect bias where individuals adopted semes from people of status (see Table 2). Individuals in the EEA that adopted semes from reproductively successful collectors, archers, or caregivers probably experienced greater reproductive success than individuals who learned only through trial-and-error. Likewise, it is also true that aspects of culture--artifacts and vertical transmission mechanisms--can lead to changes in genes frequencies (e.g., the adoption of yam cultivation in West Africa leading to changes in gene frequencies of the sickle cell trait) and the evolution of modules. Also, the modules that the EP researchers describe are "evoked" under specific conditions, which means it is essential to understand the social (EE) and cultural (ECA) contexts of these behaviors.

The natural and social "environment" in EE is a large and complex set of factors, but it is likely that culture (semes) and evolved psychological modules shape and influence decisions and behavior in that environment. For instance, EEs might argue that the nature and frequency of caregiver-infant responsiveness is linked to the number and type of alternative caregivers available. Aka fathers are often around their infants and are highly involved in caregiving, but Ngandu fathers are also often around their infants but seldom participate in infant care, in part, due to vertically transmitted gender roles.

Summary and Conclusions

In this chapter, we have described three emergent approaches within evolutionary theory and have illustrated how they might be applied and integrated to understand issues and problems in developmental psychology. The chapter focused on caregiver-infant proximity and responsiveness because these behaviors have implications for understanding social and emotional development. The EP approach indicated it was essential to understand underlying biologically-based modules of the mind that evolved in the EEA due to recurring adaptive problems. Infant communication and proximity modules in infants, and kin recognition, proximity and infant communication modules in adults were identified as potentially important modules contributing to caregiver-infant proximity and sensitivity. The EE approach emphasized the importance of infants' rearing environments (e.g., availability of alternative caregivers, social stress, environmental hazards, child and adult mortality rates, frequency of sharing). The EE approach is similar to ecological and systems theories in developmental psychology, but distinct in that reproductive fitness and strategies of both caregivers and infants are the assumed bases for explaining caregiver-infant behaviors in a particular ecology. Finally, the ECA approach indicated that caregiver proximity and sensitivity are influenced by caregivers' feeling that specific styles of caregiving (e.g., immediate response to a fuss/cry) are natural are appropriate. Caregivers feel they are natural because they have been culturally transmitted and acquired through relatively conservative mechanisms of transmission--vertical and group effect transmission of internal working models and culture cores. This approach is also similar to existing "socio-cultural" theories in developmental psychology, but it is distinct in that it focuses on specific evolutionary-based mechanisms of culture.

As mentioned several times, the approaches are relatively recent developments so few systematic studies within developmental psychology have been conducted. Developmental psychologists are in an excellent position to make substantial contributions to each approach, however, as all of these approaches seldom take ontogenetic development into consideration. Current evolutionary theory focuses on the minds, environments, and cultures of adults; discussion and consideration of children and their development are few and far between. When, how and why do evolved modules emerge? Table 2 is the first attempt to try and place the ECA mechanisms of transmission in a developmental context. Developmental psychologists have the knowledge and methodological tools to make important contributions.

In conclusion, evolutionary theory provides one way in which several approaches can be unified. The integrative approach illustrated in Figure 3 also makes it clear that biology, environment and culture are mutually constituted. In particular, we have tried to emphasize that the culture concept is alive and well within an evolutionary framework and that many who adopt an evolutionary perspective (EEs and ECAs in particular) do not feel that human behavior is necessarily hard-wired or biologically-based (genetic).


We are grateful to the Aka, Ngandu and Euro-American families for so graciously allowing impersonal behavioral observations by strange anthropologists and psychologists. We want to thank Donald Shannon, Patricia Evans, Hope Hallock, Nan Hannon, Nancy Kimmerly, Christina Larson, Laura Scaramella for their assistance in data collection and analysis. We acknowledge and thank the government of the Central African Republic for authorizing the research. The National Institute of Child Health and Human Development supported the research.


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Table 1. Three evolutionary approaches.



Evolutionary Psychology



Evolutionary Cultural Anthropology





Tries to explain

Human universals, human nature

Human behavioral diversity, reproductive tradeoffs in different environments

Cultural diversity, culture change, gene-culture interactions





Key constraints

Genetically-based cognitive modules

Natural and social/demographic environments

Cultural mechanisms





Time for adaptive change to take place

Long-term (genetic)

Short-term (phenotypic)

Varies by cultural mechanism





View of culture

Culture is the manufactured product of evolved psychological mechanisms

Culture is the product of individuals trying to enhance reproductive fitness, culture as epiphenomena

Culture has its own properties, can take its own course and can drive or co-direct genetic evolution





View of child


Development influenced by specific genetically-based modules of the mind that emerge during development

Children of all ages are trying to maximize their reproductive fitness in their various "niches"

Cultural mechanisms and histories influence parental ideologies, caregiving practices

Topics of study in child development

Language acquisition, attachment, cooperation, aggression, maternal sensitivity

Parenting strategies, child abuse and neglect, attachment, father
















Note: Modified version of Smith (2000) and Hewlett (2001).


Table 2. Mechanisms of cultural transmission identified by evolutionary cultural anthropology






Age Most Frequent





Vertical Transmission

Child adopts semes of parents

Genetic transmission analogy, contributes to conservation of culture, semes slow to change

Infancy and early childhood







Child adopts semes of unrelated individuals

Epidemiological transmission analogy, frequency of interaction with individuals with the semes determines likelihood of transmission (e.g., high frequency of interaction leads to greater likelihood of transmision and rapid change)

Early childhood (btw generations); Late childhood and adolescence (within generations)





Group Effect

Child adopts commonly observed semes

Also called frequency dependent bias, contributes to conservation of culture; semes slow to change

Late childhood and adolescence





One-to-Many Transmission

Child adopts semes of teacher, leader, mass media

Contributes to rapid culture change, common today but rare in most of human history

Late childhood and adolescence





Indirect Bias

Child adopts semes of individuals with status

Children adopt semes of individuals with status (TV and sports stars,etc.) because the individuals believe the seme may lead to reproductive success







Child adopts semes imposed upon him/her

Leader imposes laws regulating aspects of culture/semes (e.g. marriage); assumes high stratification; emphasis on lack of free choice






Many-to-One Transmission

Child adopts semes from group organized to transmit these semes

Initiation ceremonies, most conservative form of transmission, semes slow to change

Late childhood and adolescence



Table 3. Infant feeding among three month-old Aka foragers, Ngandu farmers and Washington DC urban industrialists.






Number of Infants








Percentage of daylight hours feeding








Mean number of feeding bouts per hour








Mean number of minutes per feeding bout








Percentage of infants that received non-maternal breastfeeding










Mean percentage of time infants received non-maternal breastfeeding








Table 4. Mean percentage of time and frequency of fussing or crying among Aka foragers, Ngandu farmers, and Euro-American urban industrialists.





Mean percentage of time fussing








Mean frequency of fussing bouts per hour








Mean percentage of time crying








Mean frequency of crying bouts per hour






Table 5. Evolutionary Ecology Characterizations of Parent-Child Sensitivity in Three Modes of Production.









Urban-Industrialists (middle or high SES)





Draper and Harpending 1982, 1987; Belsky et al. 1992

Parents have fewer children due to demands of mobile life so parents are sensitive, supportive, positively affectionate and responsive to infant demands.

Parents want many children and are less responsive, relatively harsh, rejecting, insensitive or inconsistent in their care.

Parents want fewer "quality" children so parents are sensitive, supportive, positively affectionate and responsive to infant demands.





LeVine et al. 1994

does not discuss

Parents are concerned about survival so parents maintain physical proximity (hold), are quick to respond to fuss/cry and breastfeed frequently.

Parents are concerned about cognitive development because mortality is low so they hold infants less, and are not as responsive. Frequent face-to-face interaction and stimulating verbal interaction are emphasized.





Blurton Jones 1993

Parents are concerned about survival so are nurturant and warm, quick to respond to infant requests and hold them frequently to protect infants from danger and exposure.

Parents are "production enhancers" and consequently unresponsive to infants' demands. Sibling care is utilized to decrease demands on mother.

Parents want to increase the reproductive success of offspring so begin to teach children cognitive skills--frequent face-to-face interaction, stimulation. Responsiveness not important.




Legends to Figures

Figure 1. Percentage of time Aka, Ngandu and US infants were held, within proximity (i.e., arm's reach) and left alone.

Figure 2. Types of caregiver responses to Aka, Ngandu and US infant fussing or crying.

Figure 3. Heuristic model for interpreting and analyzing behavior from an evolutionary perspective.