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Parental impacts on development: How proximate factors mediate adaptive plans

Abstract and Keywords

In the light of the theory of evolution, parental impacts, like other environmental stimuli, are hypothesized to be processed by children's evolved psychological mechanisms. Various behavioural algorithms, decision rules, learning programs, and epigenetic rules transform input from the family context into behavioural output that presumably solved adaptive problems in evolutionarily relevant environments. This is why evolved physiological and psychological processes, considered as proximate mechanisms, are so crucial in the evolutionary explanations of human behaviour. This article clarifies the specific ways in which these mechanisms channel behavioural acts into adaptive decisions. A central task is to gain insights into the details of socialisation: how children process the information that comes from their parents and how this shapes adaptive patterns and states during development.

Keywords: evolution, parental impacts, children, psychological mechanisms, proximate mechanisms, human, behaviour, adaptive decisions, socialisation

19.1. Introduction

Individuals' reproductive interests are most directly served by having children who grow up and reproduce. From a Darwinian perspective, evolution has operated to select certain forms of parental care in order to ensure that offspring reach sexual maturity and have the potential to carry on the parents' reproductive lineage. This includes not only the physical means necessary for survival, but also the means by which a child develops competencies in his or her social group (Bjorklund and Yunger, 2000).

The impact of parental care on development is especially large and extended in the case of human beings. One of the most striking differences between humans and the rest of the primates is the extent of delayed maturation. Human development is not only prolonged relative to other primates, but also possesses a stage of the life history, namely adolescence, that is unique to Homo sapiens. In contrast to other animals, human brain growth continues until adulthood and neural connections continue to be created under the influence of experience (Bogin, 1999).

An extended period of youth and an enlarged brain are needed to master the increasing complexity of the social environment. Human children are physically dependent on their parents for a long period of time during which they learn vital social skills and practice adult roles. Parents not only provide offspring with resources, but also have a major impact on their social and cognitive development, which deeply influence offspring attitudes and behavioural styles in adulthood (Bowlby, 1965; Bjorklund and Pellegrini, 2002).

In the light of evolutionary theory, parental impacts, like any other environmental stimuli, are hypothesized to be processed by children's evolved psychological mechanisms. Various behavioural algorithms, decision rules, learning programs and epigenetic rules transform input from the family context into behavioural output that presumably solved adaptive problems in evolutionarily relevant environments (Buss, 1995). This is why evolved physiological and (p. 256) psychological processes, considered as proximate mechanisms, are so crucial in the evolutionary explanations of human behaviour. In the present chapter, an attempt is made to clarify the specific ways in which these mechanisms channel behavioural acts into adaptive decisions. A central task is to gain insights into the details of socialization: how children process the information that comes from their parents and how this shapes adaptive patterns and states during development.

19.2. Early mother–child relationships and long-term developmental effects

Well-known (even notorious) experiments with animals have revealed that an infant's need for contact with a familiar figure, especially the mother, is crucial for normal growth and development (Harlow and Mears, 1979). Studies in the 1970s and later suggested that early physical contact facilitates a powerful emotional attraction of a human mother to her child, and influences the child's later development. Mothers who experienced extensive contact with their children immediately after birth displayed more nursing behaviour several months later (Klaus and Kennel, 1976) specifically, they made visual contact with their 3-month-old children more often, as well as kissing them and suckling them more frequently than those who—according to ‘traditional’ hospital practices—were allowed to see their babies only 12–18 h after delivery. Even 1 year later, these ‘early contact’ mothers spoke with greater inflection to their children, using more questions and adjectives than commands. The researchers' interpretation of these patterns was that early contact brings about a harmonious relationship and stimulates growth, and perhaps even mental development. In the light of a classical ethological framework, we would say that infants imprinted on the specific cues associated with their mothers—and mothers also imprinted on their infants—during this early, sensitive period, which then ensured the development of long-lasting bonds between them.

Several authors have concluded from these observations that very early contact has a crucial and irreversible effect on a child's later development. However, it has been found that early contact in the first hours after birth has no measurable effects on the mothers' emotional bonds with their infants a year later. In fact, only one group of mothers benefited from early contact; those who were already at risk of abandoning their babies (Hrdy, 1999). Among these women, readiness to accept their babies increased, with the subsequent advantages of later contact, compared to those who did not develop an early bond. In the case of mothers generally, however, a lack of physical contact with their infants immediately after birth does not influence the subsequent security of mother—infant attachment. This suggests that preprogrammed imprinting mechanisms occurring during short sensitive periods, of the kind peculiar to many species of birds and mammals, cannot be fully extended to humans.

Although efforts to relate directly aspects of very early parenting to developmental outcomes have generally met with limited success, several forms of early attachment have been found to predict children's later development. For instance, specialized vocal stimulation from the parent (so-called infant-directed speech) was found to be correlated with 3–4-month-old infants' growth rate, which is, in turn, likely to be associated with health (Monnot, 1999). Young infants who received insensitive maternal care experienced difficulty in recalling representations of attachment as young adults (Beckwith et al., 1999). Prolonged and total emotional deprivation in the first year may lead to further behavioural difficulties in relationships with adults and peers outside the family; deprived children, aged between 7 and 14 years, show over-friendliness toward strangers and more aggression towards class-mates (Hodges and Tizard, 1989). Furthermore, the effects of early adversity do not manifest themselves uniformly across all domains of behaviour. In a study of Romanian orphans, perceptuo-motor skills were found to be less vulnerable to the effects of early deprivation than cognitive and language development (Rutter et al., 1998).

Other studies have indicated that the consequences of parental practices can only be understood by taking into account children's individuality: both their physical condition and various temperamental traits. There is agreement (p. 257) that the interaction of parental characteristics and child characteristics is likely to provide greater explanatory power than either of these two factors alone (Schaffer, 2000). Therefore, just as the mother's condition and psychological preparedness for childbirth influences her relationship with the newborn and its subsequent development, so the child's health and physical condition, as well as their temperament and responsiveness, have a great impact on early attachment and subsequent developmental patterns. For instance, under the influence of risk factors (poverty, parent's mental illness, divorce), children with an ‘easy’ temperament (high activity and sociability, low anxiousness) develop greater stress resistance than ‘difficult’ children, probably because they are able to establish supportive relationships with adults more easily, which is beneficial in an adverse environment (Masten, 1994).

Low birth weight and prematurity may be another factor of the infant's initial character that frequently leads to discriminative parental care, which in turn influences development. Mothers tend to smile at and touch such infants less than healthy ones (Goldberg et al., 1986). This may be a response to the infants' inability to participate in some of the mother—child interactions that normally elicit and facilitate maternal attachment (Daly and Wilson, 1987). Premature infants have been found to contribute to difficulties in arousal regulation by providing less clear and easily interpretable behavioural cues to their parents (Bigsby et al., 1996). They tend to have an aversive cry, show poorer motor coordination, smile less, and exhibit more gaze aversion (Furlow, 1997). The physical condition and health status of the newborn appear to strongly influence the parents' perception of the infant and their decision regarding levels of investment. In a study of seven pairs of twins, the healthy twin was found to receive more positive parental behaviour (holding, soothing, gazing) than the poorly twin (Mann, 1992).

This parental discrimination exacerbates the effects of genetic and physiological impairments, such as neurological abnormalities, on the handicapped infant's physical and cognitive development. Low birth weight and premature infants older than 1 year of age show less exploration, play, and verbal interaction, have more frequent distress behaviour, and engage in more anxious and resistant attachment patterns compared to normal birth weight children, especially if prematurity is associated with another risk factor, such as illness or economic stress (Wille, 1991). Developmental delays and cognitive disabilities, measured at the age of 6 years, have been related to both biomedical disadvantages, such as intrauterine growth retardation, and a disruption of bonding processes (Korner et al., 1993; Shenkin et al., 2004).

The evolutionary strategy underlying this pattern of parental care and the development of handicapped children is considered as a tradeoff between current and future reproductive interests. Parental psychology is shaped by selection to make adaptive decisions about the timing and amount of investment in offspring (Clutton-Brock, 1991; Surbey, 1998; Bjorklund and Pellegrini, 2002). Mothers can optimize their reproductive output with regard to maternal and child health, as well as to their access to resources for child rearing. The reproductive value of a child, i.e. its own expected fitness, is considered to be the primary factor influencing parental investment. Given that an offspring's poor genotypic and phenotypic quality is unlikely to enhance the parents' reproductive prospects, the rejection or neglect of an unhealthy newborn could be a fitness-enhancing strategy. Indeed, there is a considerable amount of evidence that these children are likely to become victims of abuse and infanticide (Daly and Wilson, 1987, 1988). The optimal strategy for parents that have a high-risk child is to reduce investment in that offspring, while increasing long-term fertility by saving resources for a subsequent child (Burgess and Drais, 1999; Chisholm, 1999). As a result, the retention of resources for future reproduction lowers the reproductive value of the current offspring but increases the parents' subsequent reproductive success.

Low birth weight delivery and prematurity have been found to influence strongly a child's reproductive value (Peacock, 1991; Mann, 1992). In the USA, 73.7% of neonatal deaths among white infants and 83.4% among black infants occurred in babies who either weighed less than 2500 g or were born prior to 37 weeks (p. 258) of gestation (Paneth, 1995). Furthermore, low birth weight infants have three times the risk of developmental handicaps and twice the risk of serious congenital anomalies than normal birth weight infants (Abell, 1992). As low birth delivery and prematurity imposes severe costs on mothers, they are expected to reduce, or even terminate, their investment, and minimize their costs in rearing offspring with relatively low prospect of survival and reproduction (Daly and Wilson, 1998).

One longitudinal study has revealed that mothers of high-risk (premature and low birth weight) infants shortened the duration of breastfeeding and interbirth intervals, compared to those with a healthy infant (Bereczkei et al., 2000; Bereczkei, 2001) (Figure 19.1). As a response to the children's lower survival prospect, they diminished birth spacing, gaining 2–4 years across their reproductive lifespan for having additional children. This study also gives some insight into the adaptive significance of this parental bias, especially in the group of subjects living in the most risky environment (in terms of low resource levels, low income and high social uncertainty). Due to their relatively short interbirth intervals, mothers with one or two low birth weight infants were found to have significantly more children, compared to other women with healthy infants. Thus, the mothers appear to compensate for handicaps associated with low birth weight and prematurity by having a larger number of closely spaced children following the birth of one or more infants with reduced probability of survival. The shift to giving birth to another child in the future might be a beneficial strategy in an environment where morphological and behavioural cues associated with low birth weight (and the possible related prematurity and retardation) signal a high mortality risk.

 Parental impacts on development: How proximate factors mediate adaptive plansClick to view larger

Fig. 19.1 Duration of breastfeeding for high-risk infants in terms of birth weight and health status.

However, even if mothers are ready to ‘replace’ high-risk infants, they do not necessarily take less care of them. On the contrary, they can even intensify parental care towards handicapped children. A number of studies have found that mothers of infants with moderately low birth weight make heightened efforts in their interaction with infants. They paid more attention to them, and showed enhanced maternal warmth, as a compensatory strategy against the infant's passivity (Barratt et al., 1996; Tessier et al., 2003; Tully et al., 2004). In general, severe and easily detectable diseases are likely to shift the mother towards a low-investment pattern, while less obvious and more correctable disabilities may encourage them to continue caring behaviour (Mann, 1992). Intensive care for handicapped children is obviously very costly to the parent, but the returns on their investment may be (p. 259) reproductively rewarding in certain cases. Therefore, mothers closely monitor their infant's health status during the early stages of infancy. Low birth weight and premature infants with no signs of severe, discernible deformities may appear ‘normal’ and healthy, and mothers may not be aware of their handicap. At the same time, because of the infant's unusual responses and unreliable behavioural cues, mothers can feel that something is wrong with the baby. Under such uncertain circumstances, mothers may steadily test the infant's development and continue to provide maternal care, including breast-feeding, up to a point when she can assess accurately the health status of her infant. The point is that mothers monitor infants and make decisions to reduce investment only if these initiatives fail and normal bonding processes cannot be established or restored.

19.3. Evolutionary and ontogenetic pathways of child social development

Experiences in the family during childhood have been shown to have an impact on later behavioural styles and attitudes. Children are sensitive to the cues coming from family arrangements and parental behaviour (MacDonald, 1997). Natural selection has favoured learning biases in children for the acquisition of certain forms of social behaviour (Draper and Harpending, 1988). As a result, they have the capacity to adjust life histories in response to environmental conditions, and variations in the life cycle are considered as adaptive answers to different circumstances.

Belsky et al. (1991; see also Belsky, this volume, Chapter 18) argued that the availability of resources, the stability of pair bonds, and the trustworthiness of others during early childhood would affect later mating and parenting effort. Individuals who develop insecure attachment to their parents, experience opportunistic relationships between family members, grow up in a father-absent family, and who face scarce resources and family stress, will shorten the age of biological maturation, accelerate sexual maturation, and tend to engage in short-term pair bonds. Those, on the other hand, who have more reliable relationships with others and less conflicts in the family will show more stable pair bonds and invest more in child rearing. Indeed, studies have revealed that young children exposed to emotional and financial deprivation and to negative parenting practices show earlier pubertal maturation (e.g. earlier menarche), more non-compliant and ‘problem’ behaviour, earlier onset of sexual activity, and more frequent marital dissolution than children coming from secure, harmonious, and stable family households (Graber et al., 1995; Bereczkei and Csanaky, 1996, 2001; Belsky, 2004; Ellis, 2004; Bogaert, 2005).

At the beginning of the chapter, I emphasized that an analysis of the proximate mechanisms that mediate evolutionary strategies at the level of manifest behaviour should play a crucial role in evolutionary psychological studies. Several factors have been described that are likely to link family relationships to pubertal development. Cortisol is a key hormone produced in response to physical and psychosocial stressors, and poorer family relations may produce physiological stress responses (Haggerty et al., 1994). A study in a rural village of Dominica indicated complex, sex-specific effects of the family environment on endocrine function. Children living in households with unstable caretaking were more likely to have abnormal cortisol levels. Adult males whose fathers were reportedly absent during their early childhood have higher cortisol levels and lower testosterone levels than father-present males. High levels of family conflicts during childhood were also associated with elevated cortisol levels in adults (Flinn et al., 1996, see also Flinn, Chapter 20, this volume).

The hypothalamic—pituitary—gonadal (HPG) axis is considered as the path linking family stress and pubertal maturation. Persistent activation of the stress response system is associated with immune deficiency, cognitive impairment, inhibited growth, and delayed sexual maturity. Psychosocial stress suppresses activity of HPG, and thereby impairs ovarian functioning in adult women (Marcus et al., 2001). At the same time, stress can stimulate maturation of the HPG axis in prepubertal females. It is possible that there is a curvilinear, U-shaped relation between early exposure to adverse conditions and the development of the stress-reacting system, with higher stress reactivity to both highly (p. 260) stressful and highly protected early social environments (Ellis, 2004).

Several studies have found that single-parent status, especially father absence, is a stronger predictor of daughters' pubertal timing and sexual maturation than conflicts and stress within two-parent families (Ellis et al., 1999). This is because girls may have evolved to show high sensitivity towards the father's role in the family. Over the course of human evolution, the high variability in male reproductive strategies may have afforded important cues to the reproductive opportunities and constraints that young girls were likely to encounter later in life (Ellis et al., 1999). It is unlikely, then, that the same evolved psychological mechanisms have been selected as a response to a father-absent environment and to the physical and psychosocial stressors in a two-parent family. Separate and largely independent paths of the timing of sexual development are involved in these different cases.

One of the causal mechanisms related to the effects of father absence is the daughter's consequent exposure to unrelated adult males. Consistent with this hypothesis, one study found that the length of exposure to stepfathers and mother's boyfriends, rather than years of biological father absence, best accounted for the onset of menarche and other signs of pubertal maturation in girls (Ellis and Graber, 2000). It may be that girls from paternally deprived homes are more likely to become exposed to pheromones of unrelated adult males, which in turn accelerates pubertal development. Research on a variety of mammalian species indicates that exposure to pheromones produced by unfamiliar male conspecifics accelerates female development. Experimental studies have shown that exposure to pheromones produced by men's axillary sweat glands reduced variability in women's ovarian cycles (Cutler et al., 1986). An alternative mechanism may be that increased exposure to biological fathers inhibits sexual maturation. Accordingly, it has been found that higher father—daughter interaction is associated with later pubertal maturation (Ellis et al., 1999). It is highly probable that both mechanisms—inhibition induced by biological father and stimulation caused by stepfathers—play a role in daughters' sexual development. Similarly, the effect of stress and father absence on pubertal timing—as separate mechanisms—frequently reinforce or counteract each other. A study revealed that girls in families with stepfathers tended to experience early pubertal development only when the relationship between the mother and the stepfather was stressful (Ellis and Graber, 2000).

The above-mentioned hormonal processes (HPG, cortisol, pheromones) are considered as proximate mechanisms by which parental impacts on child development manifest in family environments characterized by stress, insecure attachment, father absence, etc. However, these components of early family environment are also regarded as proximate factors under a slightly different evolutionary explanation, namely that they mediate and express the adaptive problem associated with local mortality rates. In their life-history model, Promislow and Harvey (1990) stated that life-history traits and developmental trajectories vary in accordance with local death rates. If the chances of survival are good, a mother can afford to invest heavily in a limited number of offspring with high competitive ability. If survival is unpredictable or unlikely, a relatively large number of offspring with low levels of parental investment promote the possibility of high fitness in good years but minimize maternal losses in bad years.

Chisholm (1993) argued that children have been selected to be sensitive to environmental cues associated with a high probability of juvenile and adult death. High stress in the family, insensitive and rejecting childrearing practices, insecure attachment and parental feelings of anger, fear, and despair were likely to have been associated with high mortality rates during a major part of human evolution. Facing these cues (or indices) of local mortality, humans have been selected to adopt high-mating-effort reproductive strategies as a form of compensation for increased mortality. Although the relatively low probability of juvenile or adult death is no longer expected to influence the offspring's reproductive value, differential mortality seems to have an impact on developmental trajectories, even in a society following the demographic transition. Humans may be sensitive to the distribution of deaths among relatives and acquaintances, calculate survival probabilities, and make decisions, although not necessarily (p. 261) consciously, on pair-bond and reproductive strategies. On the one hand, sadness and grief represent a kind of psychosocial stress that may lead to the subjective feelings of insecure attachment in adulthood. On the other hand, perceptions of mortality risk, that may signal a shorter expected lifespan, may result in an earlier onset of sexual maturity. Negative correlations were found between early stress and expected lifespan and positive correlations between expected lifespan and age at menarche and first birth (Chisholm et al., 2005)

Among studies that have measured fertility and mortality rates, one found that age of first reproduction, number of children born per woman, mortality risks, and local resource availability were all interrelated in modern-day Chicago (Wilson and Daly, 1997). In neighbourhoods with low resource availability, shorter lifespans were associated with both an earlier age of first reproduction for both men and women and nearly twice as many children born to women, compared with a more favourable neighbourhood. In a more recent Hungarian study, an attempt was made to investigate the possible association between childrearing practices and mortality rates (Bereczkei and Csanaky, 2001). We predicted that differences in family environment would be associated with differential mortality even in an industrialized culture. (Mortality rates were measured as the number of deaths among the subjects' sisters and brothers.)

In accordance with theoretical considerations, higher mortality rates were found for the siblings of 732 Hungarian subjects, close to completed fertility, coming from families with high stress and rejecting, cold parental attitudes when compared with subjects from more favourable family conditions. Parental affection and emotional atmosphere were found to be strongly correlated with adult mortality rates (Figure 19.2). The less parental love the children perceived during childhood, the higher life-long mortality rates they experienced until the age of 45 years. Similarly, high levels of stress during childhood proved to be a strong determinant of the children's low life expectancy. Furthermore, we have found a strong relationship between the subjects' family conditions during childhood and their reproductive output. Mothers from a family environment with a low level of parental affection and a high level of self—parent conflicts started reproduction earlier and gave birth to more live-born infants than those growing up in more favourable households.

 Parental impacts on development: How proximate factors mediate adaptive plansClick to view larger

Fig. 19.2 Mortality rates for siblings of subjects aged more than 45 years with a particular birth order.

These results suggest that children are sensitive to cues in their family environment that are (p. 262) associated with a high probability of juvenile and adult death. Under the influence of adverse childhood experiences, they are inclined to adopt an opportunistic mating pattern with early maturation and high fertility as a form of compensation for increased mortality. It is remarkable that humans in contemporary societies still seem to follow their reproductive interests, with evolved psychological mechanisms transformed into reproductive output. A possible explanation of the strong correlations between childhood experiences, mortality and fertility patterns is that conditional strategies are involved that enable individuals to follow environmental changes and make reproductively optimal decisions in various environments (Barrett et al., 2002). Both American and Hungarian studies suggest that early and frequent reproduction in certain social contexts might be, at least in part, a facultative response to high mortality rates (Wilson and Daly, 1997; Bereczkei and Csanaky, 2001).

19.4. Sexual imprinting: parental influence on mate choice

Freud (1905) suggested that during the so-called Oedipal phase of development, boys take their mother as their first and primary sexual object, and speculated that, as adults, they look for someone who can represent their mother. Whereas the assumptions on incestuous drives and sexual desires between mothers and infant sons have been falsified by recent theoretical and empirical findings (Daly and Wilson, 1990), the hypothesis that their intimate relationship has a large impact on the child's later sexual development remains plausible. It appears especially true for shaping mate preferences and attitudes.

Imprinting-like mechanisms have been suggested as an explanation for the impact of early experiences on mate criteria in both humans and other animals. Ethologists stated several decades ago that, in addition to fixation on the parents, imprinting also plays a crucial role in shaping sexual behaviour during adulthood (Bateson, 1964; Bolhuis and Horn, 1992; Lorenz, 1965). In the case of sexual imprinting, early exposure to a set of species-specific characteristics is thought to shape mate preferences that persist until late adulthood. Cross-fostering experiments with various species have revealed that during pair formation adult males tend to prefer sexual partners that are similar to the female that reared them (Immelmann et al., 1991; Oetting et al., 1995; Vos, 1995).

Bateson (1983) argued that sexual imprinting enables individuals to learn the characteristics of their close kin and subsequently to choose mates that appear slightly different, but not too different, from their parents and siblings. This is because both inbreeding and outbreeding have obvious reproductive costs and benefits (Read and Harvey, 1988; Ridley, 1994). On the one hand, inbreeding increases the degree to which parents share genes with offspring, thereby enhancing genetic representation in future generations. It might also prevent genetic complexes co-adapted to the local environment from being disrupted, thereby enhancing reproductive success. On the other hand, an extreme degree of inbreeding can impose serious inbreeding depression effects, and various kinds of outbreeding strategies help to prevent reproductive costs associated with incest. It has been argued that an adaptive compromise (optimal outbreeding) has evolved between the opposing selection pressures of inbreeding and outbreeding, with individuals choosing a mate with a moderate degree of similarity (Bateson, 1983). Indeed, experiments have revealed mating preferences for slightly familiar conspecifics in various species (Bateson, 1988; Alcock, 1998).

Several studies have also revealed imprinting-like effects of human parents on children in their later mate choice. People born of mixed ethnicity marriages show a tendency to marry into the ethnic group of the opposite-sex parent more often than into that of the same-sex parent (Jedlicka, 1980). Two studies have shown greater importance of the opposite-sex parent over the same-sex parent in predicting the hair and eye colour of actual partners (Zei et al., 1981; Little et al., 2002). Similarly, women were found to prefer the odours of men with HLA alleles that resembled her father's HLA alleles but not her mother's (Jacob et al., 2002).

Although these studies suggest that imprinting-like mechanisms occur in mate choice, they do not provide evidence of parental impact and (p. 263) developmental processes leading to mating preferences for individuals with similar appearance. A hypothesis was put forward that sexual imprinting during a sensitive period in early childhood is responsible for shaping one's later mate choice criteria through the observed features of the opposite-sex parent (Daly, 1989; Bereczkei et al., 2002). Children are expected to prefer potential mates who are sufficiently similar to the representation of their opposite-sex parent's appearance. During their first 6–8 years of life, children internalize this parent's phenotype as a template for acquiring mates with shared genes. In the process of attachment, young men and women may shape a mental model of their opposite-sex parent's physical appearance and use it in mate choice after reaching adolescence. In the light of this hypothesis, a genetically canalized learning process, rather than direct genetic similarity detection, is responsible for the perceived similarity between spouses.

In a study in which subjects compared more than 300 facial photographs of family members and controls, the subjects correctly matched wives to their mothers-in-law at a significantly higher rate than expected by chance (Bereczkei et al., 2002). Furthermore, using a retrospective attachment test (EMBU), it was found that the more physical and emotional distance from their mothers that sons had experienced during childhood, the less similarity was perceived between their wives and mothers. A regression analysis revealed that men who had been more frequently rejected by their mother during childhood were less likely to choose mates who resembled their mothers in physical appearance. These results suggest that males build up an image of their mother's appearance and search for a partner who fits that perceptual schema.

However, a crucial limitation of these investigations is the difficulty of separating the effects of sexual imprinting from a more direct detection of similarity between mates themselves. It has been suggested that phenotype matching could be responsible for controlling mate choice without the help of learning from familiarity or proximity. That is, individuals may be guided to respond to specific phenotypic cues in others and direct altruism selectively towards individuals with shared genes (Dawkins, 1982; Hepper, 1991). Obviously, this can only occur if there is a high correlation between genetic similarity and phenotypic similarity on traits that individuals use to distinguish potential mates. Individuals, equipped with some specific innate algorithm, could then detect some aspect of their own phenotype, match it to new, unfamiliar individuals, and prefer those who possess the same or similar phenotype. Much experimental evidence shows that both lower and higher animals are able to recognize genetic similarity on the basis of shared olfactory and visual cues (Blaustein et al., 1991; Holmes, 1995; Pfennig and Sherman, 1995).

Several authors argue that this mechanism is responsible for homogamy or assortative mating in humans (Rushton, 1989). A number of studies have shown that the majority of mates resemble each other in a high number of traits, such as socioeconomic status, age, intellectual ability, education, personality variables, physical attractiveness, vocational interest and anthropometric measures (Mascie-Taylor, 1988, 1995; Jaffe and Chacon-Puignau, 1995; Bereczkei and Csanaky, 1996; Keller et al., 1996; Bereczkei et al., 1997). However, it is not known what perceptual mechanisms are involved in feature detection associated with phenotype matching, and what part of the similarity detection system is based on innate mechanisms versus associative learning. Several recent studies on the relationship between genetic similarity and mate preferences have yielded controversial results. One has shown that women prefer the odour of men who have significantly more HLA allele matches to her own alleles than to men with the least preferred odour (Jacob et al., 2002). Another study revealed that female students tended to prefer the scent of men who possessed dissimilar HLA genotypes (Wedekind and Furi, 1997; see also Wedekind, this volume, Chapter 22). A very recent study suggested that females find the faces of HLA-heterozygous men more attractive than faces of homozygotes (Roberts et al., 2005), whereas another study detected no such effect (Thornhill et al., 2003).

Despite these controversies, phenotype matching remains plausible as an account of the resemblance between mating partners, and can therefore be considered as a rival explanation of sexual imprinting. This is because similarity between one's spouse and his/her opposite-sex (p. 264) parent may be an artefact, given the 50% overlap between the parents' and offspring's genetic material. Therefore, if homogamy works via phenotype matching, it would be responsible for the similarity between spouse and opposite-sex parent. In this case, our results about resemblances between family members may be due to innate similarity detection between spouses, not sexual imprinting on the mother. In order to disentangle these effects, we conducted another study investigating the mate choices of women from adoptive families (Bereczkei et al., 2004). In light of the sexual imprinting hypothesis, early experiences with the opposite-sex parent will have a long-term effect on one's mate choice, whether the child and the caring adult were relatives or not. Women are expected to choose a mate who resembles their father even though he is not a biological relative. Alternatively, if phenotypic matching theory is correct, then women would prefer mates similar to themselves but not to their adoptive father (Bereczkei et al., 2004).

In accordance with the imprinting hypothesis, subjects found a significant resemblance in facial traits between a daughter's husband and her adoptive father, but no resemblance between a daughter's husband and her adoptive mother. This effect is likely to be modified by the quality of the father—daughter relationship during childhood. Daughters who received more emotional support from their adoptive father were more likely to choose mates similar to their father than those whose father provided a less positive emotional atmosphere (Figure 19.3). These results suggest that individuals acquire mate choice criteria at least partly from the quality of social contact with their opposite-sex parents during childhood, rather than using genetically prescribed mechanisms to detect particular phenotypic cues in unrelated individuals.

 Parental impacts on development: How proximate factors mediate adaptive plansClick to view larger

Fig. 19.3 Percentage of husband—wife's adoptive father matching as a function of the father's emotional warmth.

However, the operation of phenotype matching in various interpersonal relationships cannot be ruled out. It is highly probable that phenotype matching plays an important role in kin recognition, but that it has less influence on interpersonal relationships beyond the circle of relatives. It may mediate positive affiliation between relatives who share similar detectable facial features or who smell similar. A study found that mothers who had limited contact with their newborns immediately after birth could recognize them by olfactory cues alone (Porter, 1987). (It is worth noting, however, that mothers can also learn the smell of their child during pregnancy, which represents associative (p. 265) learning, rather than a purely genetic phenotype-matching mechanism.) Adult subjects could also match the odours of mothers with those of their infants, but were unable to match husbands with wives (Porter et al., 1985). Using visual cues, adult subjects could match mothers', fathers', and their newborn infants' facial photographs (Christenfeld and Hill, 1995). In another study, participants were able to identify the odour of most of their first-degree relatives, but mothers could not recognize their stepchildren, nor could children recognize their stepsiblings (Weisfeld et al., 2003).

Recently, DeBruine (2004) and DeBruine et al. (2004) argued that humans use facial resemblance as a cue of kinship. She found women's preference for self-resemblance increased during the luteal phases of their cycle, when they were less fertile, and hypothesized that this was a byproduct of a hormonal mechanism for increasing affiliative behaviour towards kin during pregnancy. Furthermore, she revealed that facial self-resemblance increased attractiveness judgments of same-sex faces more than other-sex faces, which may indicate a non-sexual response to family-, kinship- and in-group-related cues. The preference for facial resemblance also appears to influence decisions about social contracts: similarity to the subject's own face raised the incidence of trusting a partner in a two-person sequential trust game (DeBruine, 2002).

In sum, humans may have separate domain-specific mechanisms to deal with social and mating contexts. A preference for self-resemblance seems prevalent in social contexts, and may result from phenotype matching (recognizing certain traits of own phenotype in others). Mate preferences, on the other hand, may lie in mechanisms different from phenotype matching, that is sexual imprinting (exposure to the opposite-sex parent early in life). Alternatively, humans might have been selected for a multiple mechanism that enables individuals to detect similarity in others, and this complex psychological mechanism—that contains both phenotype matching and sexual imprinting—can be used in various contexts of kin recognition and mate choice. Specific conditions, like family circumstances, life-history factors and personality traits can influence the extent to which one matches one's own phenotype to others or uses a template based on experiences with family members.

19.5. Cultural and individual differences of socialization

During socialization parents often teach their children in a way that promotes children's adjustment to the cultural environment. Parents may have evolved psychological mechanisms, expressed as teaching biases, which convey adaptive information to offspring (Barrett et al., 2002). They use socialization techniques and educational rules that increase their children's access to social resources and status. This parental effect, rather than hard-wired, genetically encoded information, would allow the production of diverse cultural variants. A cross-cultural analysis has revealed that boys are taught to be more aggressive, self-reliant and competitive, whereas girls are taught to be more sexually restrained and obedient in cultures where polygamy prevails (Low, 1989). Especially large differences have been found in unstratified highly polygynous societies, where men are expected to compete for resources necessary to have wives, and women are expected to show chastity and obedience as desirable traits for wives who are encouraged to marry up the social scale (hypergyny).

 Parental impacts on development: How proximate factors mediate adaptive plansClick to view larger

Fig. 19.4 Mean lifetime fertility of Gipsy and non-Gipsy mothers as a function of the sex of their first-born children.

In Hungarian Gypsy villages, daughters are traditionally socialized to help with raising children, and their parents normally expect them to engage in various kinds of duties such as looking after babies, playing with children, cooking for them, etc. (Bereczkei and Dunbar, 2002). In accordance, girls and boys are treated differently; prior to marriage, girls are prepared for maternal roles and provide a valuable service as ‘helpers-at-the-nest’. Indeed, studies in rural Gypsy communities have found that (i) firstborn girls engage in substantial help in housework related to childcare of their younger siblings; (ii) this assistance both increases the length of the mother's reproductive career and reduces inter-birth intervals for subsequent children; and (iii) Gypsy women with first-born daughters have significantly more children than those who gave birth to sons first (Figure 19.4). These results suggest that a parental bias towards daughters is adaptive in these rural populations (p. 266) because a first-born daughter reduces the mother's costs of rearing her current infant, thus allowing her to transfer the resulting spare time and energy to producing and raising additional children.

Teaching offspring to behave differently according to their sex is only one kind of differential parental investment. In general, parents apply specific techniques and socialization practices in order to enable their children to adopt the adult roles, values and norms prevalent in a given culture. Ireneus Eibl-Eibesfeldt (1989) found that, in warfaring cultures, such as Eipo, Himba, Yanomami, children are educated early to toughen themselves up by learning to bear physical pain and to meet aggression with aggression. Children also develop fighting skills in their games in which they frequently imitate the behavioural models displayed by the adults. Adults reinforce certain behaviour through praise and scolding. By contrast, in the more peaceful cultures, as among Bushmen of the Kalahari, parents promote other models for their children. They do not encourage vengeful behaviour, often reprimand the attacker, and comfort the one who was attacked. They rely on appeasement, and rarely use physical force in disciplining their children.

These investigations reveal that in every culture, during socialization, parents provide their (p. 267) children with all the necessary information for them to be able to adapt to the specific social environment. Cross-cultural differences in behaviour patterns are certainly not due to genetic differences, they are rather induced by the diversity of parental rearing strategies. However, within a culture, the diversity of behavioural styles results from socialization processes in which the children's own needs and innate capacities play a more important role. This is important to emphasize, as according to the traditional concept of socialization, developing children are flexible, entirely malleable beings, who—through the impact of their parents—gradually acquire behavioural norms and roles prevailing in the given adult society. While this chain of causation has an undeniable role in children's development, most recent research—evolutionary studies among them—suggest a more complex, multi-causal model. They argue that parents not only shape their offspring's personality, but they also respond to the offsprings' various needs and propensities, and base their decisions concerning their rearing strategies on these endowments (Rowe, 1994; MacDonald, 2005). During this process, individual differences increase in the family.

Lalumière et al. (1996) argue that, in accordance with a Darwinian view, within multi-child families, siblings compete for the same resource: parental investment. They have initial differences due to genetic differences, age, order of birth and their sex. Initial differences on a given characteristic, such as verbal or athletic ability, may give one child an advantage over the other, and the less skilled child may have to display a different or additional type of ability or propensity to receive parental attention and care. Specific sibling characteristics may elicit specific parental reaction that reinforce and extend the original differences. This process places siblings on different developmental trajectories. Socialization processes producing developmental specialization may have been favoured by selection, for two reasons. First, specialization would reduce competition for extrafamilial resources among siblings. Second, specific individual capacities and achievements may make competition with others more efficient in a society that contains a number of niches.

In accordance with this theoretical model, it has been found that siblings grow more dissimilar the more time they spend together, which contradicts most contemporary socialization theories (Bouchard and Loehlin, 2001). Several twin studies have revealed that the length of time twins spend reared in the same family is negatively related to similarity in cognitive abilities and some personality traits, such as extraversion, aggreableness, and openness to experience (Pedersen et al., 1992). Unexpectedly, those twins who had been separated later were more dissimilar than those pairs separated earlier.

This evolutionary explanation closely fits to the recent results of human behavioural genetic studies (Rowe, 1994; Plomin and McClearn, 1997). Theoretically, socialization is not a one-way process in which parents shape children's temperament and personality. Rather, each socialization process involves an interaction between genetically canalized abilities and propensities and environmental effects, such as parental influence, cultural norms, etc. During this process, children play a decisive role in their relationship with their parents and in shaping their own personality characteristics later. One area of the child's activity is the so-called ‘evocative’ gene-environment correlation that involves an interaction, whereby the innate propensities and temperaments of children influence parental behaviour, which, in turn, affect various aspects of their development. In other words, siblings are treated differently because their genetically influenced patterns elicit or evoke different responses from their parents (Plomin, 1994, see also Sulloway, Chapter 21, this volume). For instance, a highly active child triggers more parental surveillance than an inactive one.

In one study, parent—child dyadic mutuality (cooperation, emotional reciprocity, maternal responsiveness), as an important component of family socialization processes, was measured by both parents and independent observers (Deater-Deckard and OʼConnor, 2000). Identical twins were found to be more similar in their relationship with mothers than were fraternal twins, suggesting that child genetic variance was present. Heritability and non-shared environment each accounted for about half of the variance in mother—child dyadic mutuality, whereas the effect of shared environment was negligible. (p. 268) Furthermore, full siblings (who share 50% of their genes, on average, as do fraternal twins) correlated at the same level as the fraternal twins, whereas adoptive siblings (who are genetically unrelated) were uncorrelated for parent—child mutuality. Since cooperation and emotional reciprocity were rated by parents and observers (not the siblings), the measured similarities and differences between siblings emerge only if parent's ratings are responsive to genetically influenced characteristics of children. This result can be interpreted in the evocative gene—environment correlation by which parents' actual behaviour is affected by genetically influenced characteristics of the children. As a result of this interaction, siblings' developmental paths would diverge and their cognitive and personality traits would be increasingly different.

In general, human behavioural genetics presents a picture of development, learning, and education that is different from the way these processes have been assumed to occur. Most theories of socialization assume that parental effects—and family environment, in general—will mean that children growing up in the same family will be similar to one another. On the contrary, genetic research has shown that environmental influences that affect behavioural development operate to make children in the same family different. There is little evidence that environmental influences shared by siblings, such as global parental style, have a crucial impact on later development. We know this, for example, because genetically unrelated children growing up in the same adoptive family hardly resemble each other for personality, cognitive abilities, and psychopathology in early adulthood (Plomin, 2000). However, this finding does not mean that family environment is unimportant. Rather, it means that environmental influences on development have their effect on an individual level rather than at the family level, and that every moment of human development is the result of an interaction between genetic propensities and environment.

19.6. Conclusions

Parental impacts have a great effect on the child's physical, social, and cognitive development. The main goal of this chapter was to analyse the proximate mechanisms that translate the underlying evolutionary logic into manifest behaviour. An explanation of human behaviour that focuses only on outcome and ultimate gain, and that does not take into account the environmental conditions and the participants' abilities, cannot explain how parents influence their children's development. Thus, the search for psychological mechanisms and their interaction with environmental factors is indispensable to the study of socialization.

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