David C. Geary
Reproductive effort involves trade-offs between mating and parenting (Trivers, 1972; Williams, 1966), and the attendant conflicts between the best interests of males and females and between parents and offspring (Krebs & Davies, 1993; Trivers, 1974). Conflicts emerge as each sex and each parent invests limited resources in self-interested ways that are not always in the best interest of the other sex or of offspring. Males and females and parents and offspring also have overlapping interests, and thus the evolution and proximate expression of reproductive effort reflects a coevolving compromise between the best interest of the two sexes and of parents and offspring. For the majority of species, the evolutionary result is that males invest more in mating (typically competition with other males for access to reproductive females) than in parenting, and females invest more in parenting than in mating (Andersson, 1994; Darwin, 1871), although there are readily understandable exceptions (Reynolds & Székely, 1997). Females benefit from male-male competition and the male focus on mating effort, because their offspring are sired by the most-fit males, and successful males benefit because they produce more offspring by competing for access to multiple mates than by investing in parenting.
The basic pattern is especially pronounced in mammals, where male parenting is found in less than 5% of species and where females invest heavily in offspring (Clutton-Brock, 1989). The reasons for these differences are found in the biology of internal gestation and obligatory postpartum suckling, and the associated sex differences in the opportunity and potential benefits of seeking multiple mating partners (Clutton-Brock & Vincent, 1991; Trivers, 1972). Given this pattern, the phenomenon of human paternal investment is extraordinary and the focus of this chapter; extended discussions can be found elsewhere (Draper & Harpending, 1988; Flinn & Low, 1986; Geary, 2000; Geary, Bailey, & Oxford, 2011; Geary & Flinn, 2001).
To understand the evolution and expression of men's parental investment, we must consider both the benefits to children and the costs to men, as well as the cost-benefit trade-offs to women. I begin with an overview of the cost–benefit trade-offs of paternal investment in nonhuman species, and discuss these as they relate to the evolution and proximate expression of men's parenting in the second section. I then turn to the relation between men's investment and women's reproductive strategies, and close with a review of the proximate correlates of men's parenting.
Although uncommon in mammals, paternal investment is found in many species of bird, fish, and in some insects (Perrone & Zaret, 1979; Thornhill, 1976; Wolf, Ketterson, & Nolan, 1988). For some species, this investment is obligate, meaning that male care is necessary for the survival of his offspring and will thus favor paternal males. For other species, this investment is facultative, meaning that it benefits offspring but it is not always necessary for their survival (Westneat & Sherman, 1993). In these species, the degree to which males invest in parenting reflects trade-offs between the costs and benefits of this investment in the contexts in which the male is situated.
Our focus here is specifically on male facultative investment, because this is the pattern found in humans (Geary, 2010). The trade-offs found with this type of investment are summarized in Table 20.1, and involve balancing the benefits to offspring against the cost of lost mating opportunities and the risk of cuckoldry. Benefits to offspring are lower mortality due to provisioning and protection from predators and may be uncommon in mammals because suckling provides most or all of the offspring's early nutrition (Clutton-Brock, 1991).
Table 20.1 Factors Associated with the Evolution and Facultative Expression of Male Parenting
Offspring Survival
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Mating Opportunities
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Paternity Certainty
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| Adapted from “Evolution and Proximate Expression of Human Paternal Investment,” by D. C. Geary, 2000, Psychological Bulletin, 126, p. 60. Copyright 2000 by the American Psychological Association. Reprinted with permission. |
As an example of trade-offs, consider that male parenting in fish is most common when males externally fertilize eggs and defend nesting sites from predators (Perrone & Zaret, 1979). Under these conditions, paternity certainty is high and males are able to fertilize the eggs of several females and thus investment does not reduce mating opportunities. Paternal investment is uncommon in fish with internal fertilization, because paternity is not certain and because males can abandon females after fertilization and avoid the cost of investment. Paternal investment does occur in some species with internal fertilization, including most species of bird and a few mammals. Again, the degree of paternal investment varies with potential benefits to offspring, availability of other mates, and paternity certainty.
The trade-offs are illustrated by the relation between level of males' parental investment and the likelihood of paternity or conversely the risks of cuckoldry (Birkhead & Møller, 1996). With facultative male parenting, cuckoldry rates often vary with male quality (e.g., as indicated by plumage color); an example is provided by the barn swallow (Hirundo rustica) whereby females often risk loss of male investment and copulate with healthier and more attractive males, if they are paired with a low-quality mate (Møller & Tegelström, 1997). Males counter cuckoldry risk by monitoring their mates' activity and adjusting investment accordingly, as illustrated by Ewen and Armstrong's (2000) study of the socially monogamous stitchbird (Notiomystis cincta). In this species, males provide between 16% and 32% of the food to the nestlings. Extra-pair copulations occur in the pair's territory and are easily monitored. Males counter this paternity threat by chasing off intruding males, but extra-pair copulations still occur. In this study, male provisioning of the brood decreased as the frequency of female extra-pair copulations increased (r = −0.72).
Paternity certainty and an improvement in the survival rate of his offspring are not sufficient for the evolution or facultative expression of paternal investment. The benefits of investment must also be greater than the benefits of siring offspring with multiple females (Dunbar, 1995). For instance, social monogamy and high levels of paternal investment are common in canids (e.g., coyotes, Canis latrens), which tend to have large litters (Asa & Valdespino, 1998). Large litter sizes, prolonged offspring dependency, and the ability of the male to provide food during this dependency (through regurgitation of meat) result in canid males being able to sire more offspring with a monogamous, high parental investment strategy than with a polygynous strategy. Paternal investment might also evolve if females are ecologically dispersed and consequently males do not have the opportunity to pursue multiple mating partners, as with callitrichid monkeys, such as marmosets (Callithrix; see Dunbar, 1995).
As noted, men's investment in children is facultatively expressed and thus subject to the same trade-offs found in other species. These are the physical and social benefits to their children balanced against the costs of lost mating opportunities and the risk of cuckoldry.
Mothers invest more in children than do anyone else. Across cultures, maternal investment in children is supplemented by that of other kin (Sear & Mace, 2008). Whether this investment comes from maternal or paternal grandparents or the children's father varies across cultures and contexts (Hrdy, 2009; Sear & Mace, 2008). The focus here is on when and to what extent fathers' investment improves the physical well-being of their children, but unfortunately, information on whether fathers reduce mortality rates in infancy and childhood is scant, in comparison to the literature on the psychological correlates of paternal investment. Fortunately, there is some information on the relation between paternal factors (e.g., occupation) and childhood mortality rates in preindustrial Europe and the United States, and a few studies of this relation in extant developing and traditional societies. The gist is that paternal investment can lower infant and child mortality risks in some human groups, but the magnitude of this effect cannot be determined.
Hill and Hurtado's (1996) extensive ethnography of the Ache (Paraguay) provides one of the most extensive assessments of the relation between paternal investment and child mortality in a traditional society. For forest-dwelling Ache, 1 out of 3 children died before reaching adolescence, with significant differences for father-present and father-absent children. Father absence due to death or divorce tripled the risk of child death due to illness, and doubled the risk of being killed by other Ache men or being kidnapped—and presumably killed or sold into slavery—by other groups. Overall, father absence at any point prior to the child's 15th birthday was associated with a mortality rate of more than 45%, as compared to 20% for father-present children.
Death due to sickness is related, in part, to the adequacy of the child's diet and in many traditional societies paternal provisioning provides an important component of this diet. The Ache share hunting proceeds among all members of the group and thus fathers do not directly provision their children with meat. Nevertheless, the children of skilled hunters have lower mortality rates than children of less-skilled hunters (Hill & Kaplan, 1988); this is also true in other hunter-gatherer societies (Smith, 2004; Wiessner, 2002). It appears that these children are better treated than the children of less-skilled hunters, including greater tolerance “of food begging by the children of good hunters” (Hill & Kaplan, 1988, p. 283), a greater willingness of band members to stay in one location to nurse the ill child of a good hunter, and greater alloparenting of these children.
Across a variety of other cultures, Sear and Mace (2008) found no consistent relation between father's investment and mortality risks for infants and young children. Sometimes fathers mattered, and sometimes they did not. With the death of the father or following a divorce, other kin—typically maternal grandmothers—are often able to compensate for the lost paternal investment (see also Hrdy, 2009; O'Connell, Hawkes, & Blurton Jones, 1999). Moreover, even when a father's skill at provisioning his family is related to child mortality risks, a causal link cannot be made. This is because culturally successful men tend to marry competent women who will improve the well-being of their children and it may be the mother's contributions that have the strongest effects on child mortality (e.g., Blurton Jones, Hawkes, & O'Connell 1997).
Protection from other men may be the one area in which other kin may not be able to compensate for loss of a father. As with the Ache, the presence of a stepfather is associated with increased mortality of young children in some human groups (Sear, Steele, McGregor, & Mace, 2002), and is associated with ongoing low levels of conflict and poor health in many other contexts (Flinn, 1992).
In developing countries in South America, Africa, and Asia there is a consistent relation between marital status and infant and child mortality rates; “mortality of children is raised if the woman is not currently married, if she has married more than once or if she is in a polygamous union.…Overall, it appears that there is a strong, direct association between stable family relationships and low levels of child mortality” (United Nations, 1985, p. 227). Indonesian children of divorced parents, for instance, have a 12% higher mortality rate than children of monogamously married couples. The same relation was found in 11 of the 14 developing nations surveyed.
The same pattern was evident in preindustrial Europe. During the 19th and early 20th centuries in Sweden, infant mortality rates were 1.5 to 3 times higher for children born to unmarried mothers than children born to married couples (Brändström, 1997). The same was true of the Netherlands from 1885 to 1940 (Kok, van Poppel, & Kruse, 1997). The direct importance of fathers is confirmed by the finding that the mortality of “illegitimate” children was lower if the father provided economic support to the child and mother and by the finding of higher mortality of “legitimate” children if the father died. A relation between paternal provisioning and infant and child mortality risks has in fact been reported throughout preindustrial and industrializing Europe and the United States (e.g., Klindworth & Voland, 1995; Morrison, Kirshner, & Molho, 1977; Schultz, 1991).
Even with vastly lower child mortality, the relation between socioeconomic status (SES) and the physical well-being of children remains true in modern societies (Reid, 1998). Adler and her colleagues concluded, “individuals in lower social status groups have the highest rates of morbidity and mortality within most human populations. Moreover, studies of the entire SES hierarchy show that differences in social position relate to morbidity and mortality even at the upper levels of the hierarchy” (Adler et al., 1994, p. 22). The relation between SES and health holds for all members of the family, not just the primary wage earner, and is not simply related to healthcare access. Members of high-status families are treated better than those of lower-status families and they have more control over the activities of everyday life, both of which influence physical health. In modern societies, paternal income and occupational status are an important, and sometimes the sole, determinant of the family's SES. As a result, paternal investment is correlated with the physical well-being of his children, even in contexts with low infant and child mortality rates.
Flinn and his colleagues provide clues about the potential relation between paternal investment and children's physical health (Flinn & England, 1997; Flinn, Quinlan, Decker, Turner, & England, 1996). In one assessment, family environment and cortisol (a stress hormone) and testosterone were assessed for children and adults in a rural village in the West Indies. Fathers' presence or absence was related to the cortisol and testosterone levels of boys, but not girls. In comparison to boys residing with their biological father, father-absent boys and boys living with a stepfather had either unusually low or highly variable cortisol levels and weighed less. Men who grew up in father-absent homes had higher cortisol levels and lower testosterone levels than did their father-present peers. The endocrine profile of father-absent men suggests chronically high stress levels, which can increase health risks (e.g., Sapolsky, 2005). Related studies suggest that prolonged parental conflict increases girls' and boys' risk for a variety of health problems in childhood and when they become adults (Troxel & Matthews, 2004).
Human paternal investment is puzzling when it occurs in contexts with low infant and child mortality. Under these conditions, selection would favor men who reduced or eliminated parental effort in favor of mating effort, but many men still invest in their children. The question is why? Geary and Flinn (2001; also Geary, 2010; Geary et al., 2011) proposed our australopithecine ancestors evolved in a gorilla-like family structure, whereby males maintained long-term polygynous relationships with several females and protected and behaviorally engaged with their offspring. If so, human paternal investment has a very long evolutionary history. Still, even with an evolved bias to provide paternal investment, high-investing men may no longer benefit from this investment, particularly in contexts in which most women only have a few children. All other things being equal, culturally successful high-investment men may be disadvantaged in terms of lost mating opportunities. Another possibility is that paternal investment in low-risk environments provides social-competitive advantages to children–investment designed to improve the “quality” of offspring (Davis & Daly, 1997)—and long-term reproductive benefits (e.g., number of grandchildren) to men.
In modern societies, men's investment—including income, play time, and support of mother—is correlated with better social and academic functioning of their children and higher SES when these children reach adulthood (Kaplan, Lancaster, & Anderson, 1998; Pleck, 1997).
There are also unique relations between men's investment and some child outcomes. Kaplan and his colleagues found that fathers' investment of time (e.g., helping with homework) and income (e.g., paying for college) was associated with the upward social mobility of children, when maternal characteristics (e.g., years of education) were controlled (Kaplan, Lancaster, Bock, & Johnson, 1995; Kaplan et al., 1998). Amato (1998) found a similar pattern. The common finding that withdrawal of paternal investment is associated with decrements in children's later social and cultural success is consistent with these results.
Divorce is the most common reason for reduction or withdrawal of paternal investment. Children from intact families are consistently found to have social and educational advantages over children from divorced families. Many of the differences between children from divorced and intact families are, however, found before the divorce (Cherlin et al., 1991). Still, some differences emerge, after controlling for predivorce levels of family functioning. It appears that divorce results in small to moderate increases in aggressive and noncompliant behaviors in boys, and an early onset of sexual activity and lower educational achievement for both sexes (Amato & Keith, 1991; Belsky, Steinberg, & Draper, 1991). Genetically informed studies confirmed a likely causal effect of divorce on these outcomes, although some of the effects were smaller with control of genetic factors (see D'Onofrio et al., 2006). These findings are consistent with the view that paternal investment can improve children's later social competitiveness, given the strong relation between delayed sexual activity, educational outcomes, and later SES in these societies (Belsky et al., 1991).
Fathers can also directly influence the social and psychological well-being of children. Children with fathers who regularly engage them in physical play are more likely to be socially popular—chosen as preferred playmates by their peers—than are children who do not regularly engage in this type of play (Carson, Burks, & Parke, 1993; Parke, 1995). In a longitudinal study, Lindsay, Colwell, Frabutt, and MacKinnon-Lewis (2006) found that boys with a high-quality father-son relationship reported more friends and better relationships with these peers than did other boys. Other qualitative features of fathers' relationships with their children, such as affection, are also associated with greater social and academic competencies in children (Parke & Buriel, 1998), and with fewer behavioral (e.g., aggression) and psychological (e.g., depression) difficulties (e.g., Sheeber, Davis, Leve, Hops, & Tildesley, 2007).
Many of the relations are, however, confounded by genetic and child evocative effects, in addition to maternal effects (Park & Buriel, 1998; Scarr & McCartney, 1983). Motivated and intelligent children are more likely to receive education-related paternal investment than are other children (Kaplan et al., 1998), and even these effects might simply be related to shared genes (e.g., for intelligence). Studies that incorporate genetic influences and simultaneously assess maternal and paternal effects are needed to more firmly establish a causal relation between paternal investment and child outcomes (Parke & Buriel, 1998; Rowe, 1994), as D'Onofrio et al. (2006) did for the relation between divorce and child and adolescent outcomes.
In all, it is likely that paternal investment improves children's social competencies and their later cultural success in contexts with low infant and child mortality rates (Nettle, 2008).
In fact, paternal investment may improve the cultural success of children in many contexts, not just modern ones, through payment of dowry and bride price and inheritance of wealth and social title (Hartung, 1982; Irons, 1979; Morrison et al., 1977). For Martu Aborigines living in Australia, fathers contribute substantially to the initiation of their sons into the social structure of adult men, and this is associated with sons' earlier age of marriage and higher reproductive success (Scelza, 2010). Winking, Gurven, and Kaplan (2011), however, found no such effect for the Tsimane (Bolivia). Clearly, much remains to be learned about when, where, and how men can effect the social development and competitiveness of their children.
Reduction of child mortality and improvement of social competitiveness are not sufficient to explain the evolution and maintenance of men's parental investment. As we know from Table 20.1, these benefits have to be balanced against the costs to fathers; specifically, the loss of potential mating opportunities and the risk of cuckoldry.
There are several aspects of women's sexual and social behaviors that are potential adaptations that reduce the men's mating opportunities, and thereby decrease the opportunity cost of paternal investment. The first is women's aversion to casual sex (Symons, 1979). Men prefer, on average, more sexual partners than do women (Buss & Schmitt, 1993; Clark & Hatfield, 1989; Ellis & Symons, 1990), meaning that most men have fewer partners than they would prefer. The critical result here is that most men's mating opportunities—and the attendant opportunity cost of parental investment—are substantially reduced by women's aversion to casual sex. A second factor is relational aggression, that is, female-female competition that involves social and psychological harassment of potential competitors (Campbell, 1995, Geary, Winegard, & Winegard, 2014). One function appears to be to exclude potential competitors from the social group, thereby reducing the mating opportunities of men and lowering the opportunity cost of paternal investment.
Relatively concealed ovulation and sexual receptivity throughout the ovulation cycle are other features of women's sexuality that may promote paternal investment. To ensure conception, concealed ovulation requires a longer relationship than is necessary for most other primate males (Dunbar, 1995), but this is not sufficient to ensure paternal investment. Once physical signs of pregnancy are evident, men could easily abandon women and avoid the cost of parenting. The combination of relatively concealed ovulation and continuous sexual receptivity (i.e., frequent intercourse) may foster another proximate mechanism, namely pair bonding (MacDonald, 1992), which maintains men's investment in their partner and children. Pair bonding and women's satisfaction with the relationship are also likely to reduce the risk of cuckoldry; that is, increase paternity certainty.
It cannot be stated with certainty, but it appears that men may be cuckolded 3% of the time, perhaps slightly less so in some contexts, on average, and more so in others (Anderson, 2006; Bellis & Baker, 1990; Bellis, Hughes, Hughes, & Ashton, 2005). As with other species, cuckoldry rates can vary substantially across contexts and social status. Sasse, Muller, Chakraborty, and Ott (1994) reported nonpaternity rates of 1% in Switzerland, but others have reported rates greater than 20% in low SES settings (Cerda-Flores, Barton, Marty-Gonzalez, Rivas, & Chakraborty, 1999; Potthoff & Whittinghill, 1965). It is, of course, possible that some of these men are aware of the nonpaternity of the children they are raising, and thus have not been technically cuckolded.
Moreover, it remains to be resolved whether women's extrapair relationships are explicitly to cuckold their partners or if cuckoldry results from failed attempts to switch mates. Support for the latter mate comes from Banfield and McCabe's (2001) survey of 112 women, 44 of whom were followed longitudinally. Less than 2% of these women had ever engaged in a purely sexual affair, but 12% reported a sexual affair when romantically attached to the extrapair man; romantic attachment suggests the pair-bonding mechanisms that support long-term relationships and biparental care are operating in these women. The issue is further complicated in contexts with high male mortality. Women in these cultures often maintain sexual relationships with several men, one of whom is considered to be the primary father and the others secondary fathers (Beckerman et al., 1998; Hill & Hurtado, 1996).
Men differ considerably in how much they invest in their children, but this follows naturally from an evolved facultatively expressed bias. The interesting question is why some men invest substantially in their children, whereas others invest little to nothing.
Most of the research on the biological factors associated with men's parenting is focused on the hormonal profiles that are associated with interest in or reactivity to infants. There is also research on the heritability of men's engagement with children, but this has not yet been well linked with the hormone studies.
In species in which male parenting is facultative, higher levels of testosterone are associated with a focus on mating effort, and lower levels of testosterone and higher levels of prolactin with a focus on parenting (e.g., Reed et al., 2006). Trade-offs in men's focus on mating or parenting also appear to be reciprocally related to testosterone and prolactin (Delahunty, McKay, Noseworthy, & Storey, 2007; Fleming, Corter, Stallings, & Steiner, 2002; Gray, Parkin, & Samms-Vaughan, 2007). In North American samples, men in long-term committed relationships have lower testosterone levels than other men (Mazur & Michalek, 1998), consistent with the prediction that these men are allocating less effort to competing for mates (Gray, Kahlenberg, Barrett, Lipson, & Ellison, 2002). Important tweaks to this relation provide further support: Men in committed relationships but who are open to an extra-pair affair have higher testosterone levels than their monogamous peers (McIntyre et al., 2006).
Men in a committed and monogamous relationship who wish to become fathers may have a different hormone profile than other men, including married men who do not wish to become fathers (Berg & Wynne-Edwards, 2001; Hirschenhauser, Frigerio, Grammer, & Magnusson, 2002). Expectant fathers who respond to infant distress cues (e.g., crying) with concern and a desire to comfort the infant have higher prolactin levels and lower testosterone levels than other men (Storey, Walsh, Quinton, & Wynne-Edwards, 2000). “Men with more pregnancy symptoms (couvade) and men who were most affected by the infant reactivity test had higher prolactin levels and greater post-test reduction in testosterone” (Storey et al., 2000, p. 79). Higher paternal (and maternal) cortisol levels are also correlated with more attentive and sensitive parenting of newborns (Corter & Fleming, 1995).
Cause and effect are not certain, however. Lower testosterone levels are associated with greater sensitivity to infant cries among men who are not fathers, suggesting that hormone levels may influence the tendency toward paternal investment (Fleming et al., 2002). It is also possible that the lower testosterone associated with a committed relationship bias men toward parenting, that men prone to parental investment are preferred as long-term partners, or some combination. The relation between men's parenting and prolactin is also complex. Close contact with infants appears to result in decreased prolactin levels when fathers hold their first-born but not later-born children (Delahunty et al., 2007). A combination of little contact with their infant during the past several hours and stated concern for the infant is related to increasing prolactin levels in men and heightened reactivity to infant crying. On average, men's prolactin levels appear to be more sensitive to development experiences (e.g., having younger siblings) and social context than women's levels (Delahunty et al., 2007), perhaps reflecting the facultative nature of men's parenting.
Kendler (1996) estimated that about half of the variation among fathers in warmth toward their children was due to genetic influences (see also Pérusse, Neale, Heath, & Eaves, 1994). More modest genetic influences were found for parental protection/control and authoritarian parenting (e.g., the parent making decisions for child). There is also evidence that men's (and women's) parenting is influenced by heritable traits of their children and adolescents (Feinberg, Neiderhiser, Howe, & Hetherington, 2001; Neiderhiser, Reiss, Lichtenstein, Spotts, & Ganiban, 2007), as well as by the unique experiences of mothers and fathers, including in their family of origin.
Firm conclusions cannot yet be drawn, but it may be that paternal behavior is more strongly influenced by context, prior experience, and child-evocative effects than maternal behavior, consistent with a facultatively expressed suite of behaviors. Research on heritable and environmental influences on men's basal and reactive (e.g., in presence of children) testosterone and prolactin levels are needed to more fully understand the proximate biological mechanisms potentially linking genetic and environmental influences to the expression of paternal behavior. Studies of the relation between personality and parenting are also needed. This is because the results reported here could reflect genetic influences on personality that are not directly related to the evolution of paternal care but still influence parenting behavior. For instance, individual differences in personality traits, such as conscientiousness are moderately heritable and associated with the stability of long-term spousal relationships and marital quality and could affect responsiveness to children (Graziano & Eisenberg, 1997).
For men in monogamous relationships in modern societies, the quality of the marital relationship is a critical influence on their engagement with children. Men in these contexts also face a trade-off between focus on cultural success and time with children.
The quality of the spousal relationship is a key influence on men's parenting (Feldman, Nash, & Aschenbrenner, 1983; Lamb, Pleck, & Levine, 1986). Although the quality of this relationship can influence how both parents interact with their children (Amato & Keith, 1991), “paternal parenting is more dependent on a supportive marital relationship than maternal parenting” (Parke, 1995, p. 37). In a set of observational studies, for instance, Belsky, Gilstrap, and Rovine (1984) and Lamb and Elster (1985) found that fathers' engagement with children was related to the quality of the marital relationship but found little relation between the level of marital interaction (e.g., degree of communication) and mothers' involvement with their children.
The finding that men with satisfying spousal relationships invest more in parenting suggests women's efforts to maintain an intimate relationship with these men is, in part, a strategy to induce more paternal investment. It is also possible that men biased toward paternal investment are more cooperative and prone to monogamy—and thus less likely to incite conflict with their wives—than are other men, and that the relation between martial satisfaction and paternal investment reflects genetic and not social effects. It is most likely that a combination of heritable biases in both parents (e.g., personality; Spotts et al., 2005), reactivity to martial dynamics, and hormonal mechanisms (earlier) influence paternal investment.
The extent to which fathers' are directly engaged in parenting is also related to the nature of his work and his personal ambition. Fathers in demanding and stressful jobs are less involved in infant caretaking, less playful with their infants, and less engaged with their toddlers than are fathers in less salient occupations (Feldman et al., 1983). Lamb et al. (1986) suggested there is a trade-off between family involvement and commitment to work. In comparison to men who were more focused on work than on family, “family-oriented accommodators…[were] more professionally passive and less successful professionally. They also tended to be in less prestigious jobs…, although it is not clear whether this was a cause or an effect of the family-oriented accommodative strategies” (Lamb et al., 1986, p. 79).
A similar relation between a man's success in culturally important endeavors and caretaking of children has been found in some traditional cultures (Hewlett, 1988; Hill & Hurtado, 1996). For instance, high-status Aka (west central Africa) men—those with large kin networks and, therefore, high hunting success—hold their infants less than half as frequently as men with few kin: Men without male kin hunt either alone or with their wives and are generally less successful hunters. The less direct care provided by these high-status men appears to be balanced by the provisioning of their families with diets that consist of a high proportion of fat and protein (Hewlett, 1988). It is not clear, however, whether efforts to obtain more income or other indicators of cultural success are components of paternal investment or if they are components of mating effort; successful men have more mating opportunities.
Draper and Harpending (1982), Belsky and his colleagues (Belsky et al., 1991), and Chisholm (1993) proposed that parents' social experiences (e.g., degree of conflict with other adults) and the availability of resources and risks in the wider community influence the attachment between parents and their children. In risky, low-resource environments, the stressors on parents are high and result in less attentive and more conflicted marital and parent-child relationships. These relationships are predicted to increase the risk of insecure parent-child attachments and through this bias the later relationship dynamics of these children. In less risky, high-resource environments, parent-child relationships are warmer and secure parent-child attachments are more common. Secure attachments are predicted to result in a tendency to form trusting and stable relationships later in life, including the spousal relationship. A history of a warm parent-child relationship during development and a stable spousal relationship is predicted to bias men toward higher levels of parental investment (MacDonald, 1992).
Del Giudice and Belsky argued the mechanisms related to later reproductive strategy include the type of parent-child attachment and the age of onset of adrenarche (prepubertal hormonal secretions from the adrenal gland) (Del Giudice, 2009; Del Giudice & Belsky, 2010; also Ellis & Essex, 2007). The combination of a conflicted family life and early adrenarche are associated with heightened risk of ambivalent (e.g., emotionally insecure, dependent) attachment styles in girls and women and avoidant (e.g., emotionally distant) styles in boys and men (Del Giudice, 2008). In adolescence and adulthood, an avoidant attachment style is associated aggression and dominance striving in boys and men, emotionally distant and frequent short-term sexual relationships, as well as little investment in parenting in early adulthood (Del Giudice, 2009).
Conditions in the wider community may also bias reproductive strategy. Wilson and Daly (1997) found age of first reproduction, number of children borne per woman, mortality risks, and local resource availability were all interrelated in Chicago. In neighborhoods with low resource availability, men compete intensely for these resources. The corresponding increase in mortality rates translated into an average lifespan difference of 23 years comparing the least and most affluent neighborhoods. A shorter lifespan, in turn, was associated with earlier age of first reproduction for both sexes and nearly twice as many children borne per woman comparing the least and most affluent neighborhoods. In other words, the early and frequent reproduction of women and men in these contexts might be, at least in part, a facultative response to high mortality rates, or at least a response to the perception that the future is uncertain and not likely to bring a better life (Davis & Werre, 2008).
However, for Ache and Mayan (Central America) men, Waynforth and colleagues found that “measures of family stress and violence were unsuccessful in predicting age at first reproduction, and none of the psychosocial stress indicators predicted lifetime number of partners” (Waynforth, Hurtado, & Hill, 1998, p. 383). Father absence was, however, related to less “willingness to pay time and opportunity costs to maintain a sexual relationship” (Waynforth et al., 1998, p. 383), although this could easily reflect genetic and not psychosocial effects. The final word on these issues will require genetically informed studies conducted in contexts that vary in risk factors (e.g., premature mortality).
The key cultural and ecological influences on men's bias toward parenting or mating are the social constraints on polygyny and the operational sex ratio (OSR), that is, the ratio of women to men seeking romantic relationships.
Father-absent societies are characterized by aloof husband-wife relationships, a polygynous marriage system, local raiding and warfare, male social displays, and little or inconsistent direct paternal investment in children (Draper & Harpending, 1988). These conditions “are particularly prevalent in so-called middle-range societies, that is, those in which agriculture is practiced at a very low level” (Draper & Harpending, 1988, p. 349) and in resource rich ecologies. In the latter, women can often provide adequate care to their children—for example, through small-scale agriculture—without substantial direct contributions from the father (Draper, 1989). If they are able to accumulate resources beyond what is needed to attract a single wife, these are conditions and social mores that allow polygyny and provide men with the opportunity to invest time and wealth into either parental investment or mating effort. Most men opt for the latter.
Father-present societies, in contrast, tend to be found in harsh ecologies and in modern or other relatively large, stratified societies (Draper & Harpending, 1988). These are societies that are sometimes characterized by ecologically—or socially—imposed monogamy (Flinn & Low, 1986). Under harsh conditions, few men are able to acquire the resources needed to support more than one wife and family. The reproductive aspirations of most men are thus ecologically restricted to monogamy. In most modern societies, monogamy is socially imposed; there are formal laws that prohibit polygynous marriages. The result is at least a partial suppression of men's mating efforts, and, through this, reduced opportunity cost to paternal investment. In these cultures, investing “excess wealth” in the well-being of children is a viable reproductive strategy for men, especially when child mortality risks fluctuate greatly and vary inversely with level of paternal investment (e.g., during epidemics).
The OSR has a strong influence on men's relative focus on mating or parenting (Guttentag & Secord, 1983; Pollet & Nettle, 2008). In modern societies, expanding populations, for instance, result in an “oversupply” of women, because women prefer slightly older marriage partners and men slightly younger ones (Kenrick & Keefe, 1992). With an expanding population, the younger generation of women will be competing for marriage partners from a smaller cohort of older men. The resulting imbalance in the numbers of marriage-age men and women is correlated with changes in divorce rates, sexual mores, and levels of paternal investment (Guttentag & Secord, 1983).
When there is an oversupply of women, men's mating opportunities increase, and these historical periods are generally characterized by liberal sexual mores; high divorce rates; an increase in the number of out-of-wedlock births and the number of families headed by single women; an increase in female participation in the workforce; and, generally lower levels of paternal investment (Guttentag & Secord, 1983). The bottom line is that men are better able to express their preference for a variety of sexual partners and relatively low levels of paternal investment. A very different pattern emerges when there is an oversupply of men (Guttentag & Secord, 1983). Here, women are better able to enforce their preferences for a monogamous, high-investment spouse. As a result, these periods are generally characterized by an increase in the level of commitment of men to marriage, as indexed by declining divorce rates and greater levels of paternal investment.
Given the biology of mammalian reproduction, it is unremarkable that mothers throughout the world show a much greater availability for and engagement with their children than fathers. The most remarkable feature of human reproduction is that many fathers show some degree of direct and indirect investment in their children. Although the level of paternal care may not always be satisfactory from the perspective of the wives of these men, it is nonetheless remarkable in comparison to the little paternal care found in most mammals (Clutton-Brock, 1989).
Men's investment or lack of investment in their children reflects the same cost–benefit trade-offs found with facultative paternal investment in other species (Table 20.1). The benefits of paternal investment include reductions in infant and child mortality rates in high-risk environments and improvements in children's later ability to compete for essential social and material resources (Kaplan et al., 1998). As found with other species with high levels of paternal investment, men's parenting is associated with relatively high—roughly 95%—levels of paternity certainty and with restricted mating opportunities. The combination reduces the costs of paternal investment.
The facultative expression of men's parenting is correlated with many factors, including heritable individual differences, hormonal profile, the quality of the spousal relationship, and child characteristics (Neiderhiser et al., 2007; Storey et al., 2000). Childhood experiences may also bias men toward mating effort or parental effort through the nature of the parent-child attachment, the level of parent-parent conflict, and the age of onset of adrenarche (Belsky et al., 1991; Del Guidice, 2009). An insecure, avoidant parent-child attachment and early adrenarche may bias boys and later men to exploit social relationships (Del Giudice, 2009). Among the consequences are an increased frequency of short-term sexual relationships and little parental investment. Wider social and ecological factors, especially laws against polygynous marriages and the OSR, also influence the degree to which men invest in the well-being of their children (Draper & Harpending, 1988; Flinn & Low, 1986; Guttentag & Secord, 1983). The goal for future studies is to uncover the relative contribution of each of these factors in general, and to better understand individual differences in men's responsiveness to factors such as marital quality and the OSR as they influence paternal investment.