Chapter 21
Parental Investment and Parent-Offspring Conflict

Catherine Salmon

Many species do not engage in parental care (Alcock, 2001). Part of the reason is that parental care is costly. By investing in offspring, parents lose out on resources that could be devoted to themselves, channeled toward securing a larger territory, finding additional mates, or future offspring. Some parents even risk their own lives in an effort to improve the survival of their offspring. So when we do see parental care, the reproductive benefits must have been great enough to outweigh the costs of providing not only the physical means for survival but also fostering the development of the skills required for success across the lifespan.

From the parental perspective, each individual's overall reproductive effort is a combination of mating effort (courtship, etc.) and parental effort or investment. Trivers (1972) defined parental investment as any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive potential) at the cost of the parent's ability to invest in other offspring (either current or future). In many species, it involves such things as food provisioning, and protection from predators. In humans, it involves a great deal more, ranging from providing food and shelter to an education, music lessons, taking the kids to hockey or soccer practice, or providing them with braces. In general, an offspring's fitness increases with the amount of parental investment it receives. We can assume that, in species that have parental care, extremely low levels of parental investment may result in the loss of offspring because a certain amount of investment is required for survival, but a point of diminishing returns is also eventually met at very high levels of parental investment because the offspring are unable to capitalize on investment over and above a certain amount.

Hamilton's rule (1964) can shed light on how parents and offspring behave with regard to parental investment. Hamilton developed the concept of inclusive fitness, noting that when we assess the fitness of a trait or behavior, we need to consider its contribution to the reproduction of that individual and to whether it influences the reproductive prospects of its kin. The inequality that sums up the conditions under which a particular behavior would be expected to spread is c < rb, where c equals the fitness cost of the action (such as providing food) to the actor, b is the fitness benefit (getting to eat) to the recipient, and r is the degree of relatedness between the actor and recipient (0.5 for parent-offspring, 0.5 for full siblings, 0.25 for half-siblings, etc.). Obviously, a parent's investment in its offspring provides a benefit to the offspring, which increases the parent's inclusive fitness. As long as the cost of parental investment doesn't begin to outweigh the benefit to the offspring times the degree of relatedness, it should continue.

Similarly, in a brood of two equal siblings, A and B, from Hamilton's rule (1964), A should continue to take resources until its marginal gains drop to one-half those of B, who gets the remainder (Parker, Mock, & Lamey, 1989). For half-siblings, marginal gains drop to one-quarter.

The key to this is the degree of relatedness. A child shares a given gene with itself with a probability of 1.0, but it shares the same gene with its sibling with a probability of only 0.5. For this reason, a child is expected to try to obtain resources (or continue to monopolize them in the case of nursing, for example) unless the value of that resource to that child drops below the value, multiplied by the degree of relatedness, of giving that resource to its sibling. Parents, in contrast, are equally related (0.5) to each of their offspring. As a result, they are motivated to distribute resources equally unless one child is better able to benefit from the resources than others. Our offspring are the way our genes get into the next generation but not all offspring are equally good fitness vehicles. Some offspring will be better able to survive or be more likely to mate. Certain offspring may be more likely to benefit from some forms of parental care than others (an infant compared to a teenager, perhaps). As a result, selection has favored mechanisms of parental care that have the effect of increasing the fitness of the parent by favoring offspring who are likely to provide a higher reproductive return on their parents' investment (Daly & Wilson, 1995). But the costs, degree of relatedness, and benefits to parents can be influenced by a variety of factors that, in turn, influence the amount of parental investment given. The conflict this can cause between parent and offspring will be discussed later.

Factors Affecting the Amount of Parental Investment

Factors influencing the amount of parental investment include the costs to parents, the benefits to parents, the circumstances influencing costs and benefits, and the degree and probability of relatedness to offspring.

Factors Influencing Costs to Parents

Parental age is one factor influencing maternal investment. In species in which the probability of death increases systematically with age, a parent is selected to give an increasing proportion of parental investment to older offspring. And some data shows that older parents do invest more than younger parents (Salmon & Daly, 1998; Voland & Gabler, 1994). This is often particularly true of older mothers who face more of a reproductive constraint due to menopause. In humans, the age of the mother is also a significant factor in the likelihood of perpetrating maternal infanticide (Daly & Wilson, 1988). Young women, those likely to have many future opportunities to reproduce, might be expected to be more willing to sacrifice a current child when conditions for successfully raising the child are poor. Older women, close to the end of their reproductive years, who pass up the opportunity to invest may never have that opportunity again. As the likelihood of future reproduction decreases, delaying childbirth becomes more costly. Selection should favor substantial immediate investment in children by older more than younger women, rather than delaying investment. The dramatic cross-culturally observed decrease in the rate of maternally perpetrated infanticide with increasing maternal age reflects the change over time of the weight the maternal psyche places on a current offspring versus possible future offspring (Daly & Wilson, 1995; Lee & George, 1999; Overpeck, Brenner, Trumble, Trifiletti, & Berendes, 1998).

The number of offspring at any given time is also expected to have an impact on parental investment. As parental investment is a limited resource (food, time, money) that must be allocated among offspring, it seems clear that with the possible exception of protection from predators, most parental resources will be in shorter supply when there are multiple young (not necessarily all the same age) present at the same time (Daly & Wilson, 1995). More children means fewer resources for each one.

Parental resource circumstances are also predicted to have an impact on the amount of parental investment. Clearly, when resources are in short supply or difficult to obtain, any particular investment is more costly from the parent's perspective than when resources are abundant. Davis and colleagues (Davis & Todd, 1999; Davis, Todd, & Bullock, 1999) modeled the success of a variety of parental investment decision rules in the Western bluebird and found that the success of different rules is highly dependent on the amount of resources available to parents. The less parents have, the more biased they ought to be in their allocation of investments. Parents faced with extremely poor resources ought to invest heavily in a single offspring, ignoring the others. As resources become more abundant, parents will do best by becoming more egalitarian. At a very general level, one could argue that the degree to which parents divide current investment unequally among offspring is a function of the amount of resources available to them.

Mating opportunities would also be expected to influence the cost of investing in offspring. Mating opportunity costs are missed mating opportunities resulting from effort devoted to parental care. Females and males experience missed mating costs but they are higher for males due to the fact that male reproductive success is largely limited by access to females, whereas for females reproductive success is not increased by sexual access to a variety of males. As a result, we expect males to provide less parental care, and studies suggest that when mating opportunities are frequent, men invest less (Magrath & Komdeur, 2003).

Factors Influencing Benefits to Parents

The age of the child can have a significant impact on the benefit of investing to parents. In many ways, one would predict a greater payoff from investing in older children. One's expected contribution to parental fitness resides mainly in one's reproductive value (expected future reproduction), and this quantity increases with age until at least puberty, making an older immature offspring more valuable from the parental perspective than a younger one. This increase occurs primarily because in nontechnological societies some percentage of children die (Volk & Atkinson, 2013). As a result, the average 14-year-old, for example, has a higher reproductive value than the average infant, because some infants don't survive to their teenage years. However, the older an individual offspring gets, the less valuable parental investment (especially certain kinds of investment) may be in terms of the offspring's ability to utilize it when compared to its utility to other offspring. In particular, a great deal of parental investment is often critical to the survival and future of young offspring. For them, significant parental investment can make a huge difference.

Parents clearly respond to the changing needs and abilities of their children. But when one child must be sacrificed so others can be saved, it is apparently a cross-cultural universal that the youngest is the likeliest victim (Daly & Wilson, 1984). Data on Canadian homicides also suggest that older children are more highly valued. When Daly and Wilson (1988) looked at the risk of the homicide of a child by a biological parent in relation to the child's age, infants were at a much higher risk of being killed than any other group of children. After 1 year, the rates drop off dramatically until they reach zero at age 17. And it is not only that infants are easier to kill, as the risk of a child being killed by a nonrelative shows a different pattern, with 1-year-olds more likely to be killed than infants, and teenagers being the most likely to be killed.

A child's expected future prospects will also be expected to have an impact on the benefits of parental investment. In other words, future survival and reproductive success influence the benefit to parents. If there is unlikely to be a fitness return on their investment, natural selection would be unlikely to favor mechanisms to invest in such offspring. Like offspring age, offspring expected future prospects are related to an offspring's ability convert parental investment into fitness. Thus, one would expect evolved psychological mechanisms of parental care to be sensitive to cues of offspring “quality” or ability to convert parental care into future reproductive success. For example, children who are disabled in some way, all else being equal, are less likely to have future reproductive success than children who are healthy. In humans, poor infant quality clearly has an impact on parental investment. Offspring born with a severe physical deformity are likely to be the victims of infanticide, especially in traditional societies where institutional care of the handicapped is not available (Daly & Wilson, 1984, 1988). Selection favors adaptations for investing where the return on investment will be highest relative to alternative forms of investment. This can shape differential investment between siblings, or it can direct investment toward other kin, or toward mating effort (as seen in some divorced men).

The effect of maternal condition/resources on the sex ratio at birth has received some attention in demographic studies of modern societies (Almond & Edlund, 2007; Gibson & Mace, 2003). Trivers and Willard (1973) argued that when one sex has a greater variance in lifetime reproductive success than the other and parents (specifically mothers) vary in their physical condition or access to resources, differences in preferences for offspring of the two sexes are likely to evolve. And some studies have demonstrated maternal condition predicting sex biases in infant mortality (Almond & Edlund, 2007; Voland, Dunbar, Engel, & Stephan, 1997); however, effect sizes tend to be small and a number of studies have failed to find Trivers-Willard effects (e.g., Beaulieu & Bugental, 2008; Guggenheim, Davis, & Figueredo, 2007). Cronk (2007) has highlighted the difficulties, both theoretical and methodological, of testing the Trivers-Willard hypothesis in large industrialized societies. The strongest evidence for Trivers-Willard effects comes from studies of small-scale societies.

Dickemann's (1979) review of historical data on infanticide and the Indian caste system reveals that infanticide was extremely common among the highest castes prior to the 20th century, with female infants the victims. These daughters had very few marriage options (had to marry within own subcaste). Among high-caste Indian families, investment in males (who could marry females from lower subcastes) paid larger dividends in terms of grandchildren, and parents heavily biased their investment toward males (Gupta, 1987). For lower castes, the tendency toward males marrying down meant daughters out-reproduced sons and parents biased their investment toward daughters (lower rate of female infanticide). Studies in the United States (Gaulin & Robbins, 1991) and Kenya (Cronk, 1989) have suggested that female infants from low-income families are nursed more than infant boys. Hungarian Gypsy populations also show a female-biased sex ratio (Bereczkei & Dunbar, 1997, 2002). Like lower-caste Indians, Gypsies are at the bottom of the social scale in Hungary, and Gypsy women are more likely to marry up the social scale than men and, in doing so, provide their parents with more surviving grandchildren. Gypsy women who marry up have babies with higher birth weights, lower mortality rates, and lower rates of birth defects than Gypsy women who marry within their social group. Bereczkei and Dunbar (1997) found that compared to native Hungarians, Gypsy women were more likely to suckle their firstborn daughters for longer than sons, abort a subsequent pregnancy after a daughter than after a son, and allow their daughters to stay in school longer.

There are also examples in which investment favors sons. In societies where the possession of resources has a significant impact on male reproductive success, a preference for sons will be seen among the affluent. This has been the case in 18th-century northern German villages (Voland, 1998) and has been noted in the records of probated wills among Canadians living in British Columbia (Smith, Kish, & Crawford, 1987). In terms of the parental pay-off, Cameron and Dalerum's (2009) study of Trivers-Willard effects in Forbes' list of billionaires indicated that people in the top economic bracket have more grandchildren via their sons than daughters and that mothers at this highest socioeconomic status have more sons.

Offspring need is also a relevant factor. Although offspring prospects, or the ability to turn maternal investment into future reproductive success, has always been assumed to be a strong predictor of maternal care, it has also been suggested that mothers' investment in offspring could be contingent in that high-risk offspring will either receive more or less investment than low risk offspring based on maternal resources (Beaulieu & Bugental, 2008). Beaulieu and colleagues have tested this in several studies (Beaulieu & Bugental, 2008; Bugental, Beaulieu, & Silbert-Geiger, 2010). Their results in samples including preterm babies and children and women with high or low resource availability suggest that mothers with low resources invest more in low-risk children, whereas those mothers with higher resource levels invest more in high-risk children (as they have sufficient resources to care for other children as well).

Factors Affecting Relatedness

Paternity uncertainty is one of the driving reasons that females invest more in parental care than males. From a genetic perspective, individual males should only invest in an offspring if they can be sure that the offspring is their own. Mammalian females (with internal gestation and fertilization) have always been certain that their offspring are their own. Males do not have such certainty and as a result, should be attuned to signs of paternity and inclined to invest only when such signs are present.

There are a variety of results that suggest that paternity uncertainty does have an impact on human paternal investment. Given the relatively high levels of heritability in many physical traits, the more a child resembles the putative father, the greater the paternity confidence is likely to be. As a result, one might expect that paternal affection and investment will be influenced by paternal perceptions of resemblance. Several empirical studies have demonstrated that perceived father-child similarity is associated with higher degrees of paternal emotional closeness and investment, typically measured as time spent with the child, or involvement in education (Alvergne, Faurie, & Raymond, 2010; Apicella & Marlowe, 2004, 2007; Li & Chang, 2007).

Data also suggest that people pay more attention to a child's paternal resemblance compared to maternal resemblance, despite the fact that the degree of actual resemblance between parents and infants is, in fact, quite low and, if anything, is slightly biased toward mother-child resemblance (Alvergne, Faurie, & Raymond, 2007; Bressan & Grassi, 2004). Greater resemblance to the mother makes sense if paternity confusion is beneficial for offspring (because they might be living with a social rather than biological father). Yet evidence suggests that mothers and maternal relatives are highly inclined to emphasize paternal resemblance in newborns. This is most often interpreted as an attempt to manipulate fathers' perceptions of paternal resemblance, increasing their paternal attachment and investment (Bressan, 2002; Daly & Wilson, 1982; McLain, Setters, Moulton, & Pratt, 2000; Regalski & Gaulin, 1993). For further discussion of paternal investment, see Geary (Chapter 20, this volume).

Stepparenting also obviously affects relatedness in that a stepparent is not related biologically to any stepchildren they may have. In species with biparental care, when one parent dies or disappears and is replaced by a new mate, any preexisting offspring now have a stepparent. As with paternal uncertainty, one would expect mechanisms of parental allocation of investment to be sensitive to whether an offspring is one's biological child, with resources being directed away from stepchildren toward biological children.

Daly and Wilson's (1984, 1988, 2001) studies of discriminative parental solicitude have focused attention on the dynamics of stepparenting in humans. Parental care can be viewed as a continuum with self-sacrifice at one end and acts that inflict costs on the child, including child abuse and homicide, at the other end. Daly and Wilson's study of child abuse in Hamilton, Ontario demonstrated that children living with one genetic parent and one stepparent are about 40 times more likely to be physically abused than children living with both genetic parents. This occurs even when controlling for poverty and socioeconomic status (to control for the higher rate of child abuse in low-income families).

Data on child infanticide tells the same story. The rates of child murder are far higher for stepparents than for genetic parents. The risk is highest for the very young, particularly children under 2 years of age. Daly & Wilson (1988) found that the risk of a preschool-aged child being killed ranged from 40 to 100 times higher for stepchildren than for children living with two genetic parents.

A less extreme example involves amounts of investment, rather than termination. Stepfathers invest fewer monetary resources in their stepchildren. In a study of men living in Albuquerque, New Mexico, Anderson, Kaplan, and Lancaster (1999) reported that genetic children were 5.5 times more likely to receive money for college than stepchildren. On average, genetic children received $15,500 more for college and had 65 percent more of their college expenses paid for than stepchildren. There have also been suggestions that when stepparental investment is seen, it may reflect mating effort on the part of males (intended to make themselves more attractive to their new mate) rather than parental effort (Anderson et al., 1999; Hofferth & Anderson, 2003; Rohwer et al., 1999).

Adoption is another factor that changes relatedness. One needs to distinguish between the adoption of related versus unrelated individuals. With one's own offspring, relatedness is 0.5. The adoption of other kin (niece, cousin's offspring, etc.) would entail a lesser degree of relatedness, but there would still be some genetic common interest. Under these circumstances, one would expect a lesser degree of parental investment than in one's own biological children. From this perspective, one would expect very little to no parental investment in an adopted child as they are not genetically related at all. With stepparent situations, at least one parent is the biological parent; in adopted situations there is no biological parent present.

However, there is little evidence that the adoption of unrelated individuals has ever occurred with any frequency over most of human evolutionary history. Nonhuman primates, who often live in close knit kin groups like humans, tend not to adopt orphaned young (Silk, 1990). Most historical evidence of human adoption and adoption practices in traditional societies has been of genetically related individuals. Those individuals who cannot have their own children have often adopted their siblings' extra children (r = 0.25) (Pennington & Harpending, 1993; Silk, 1980, 1987). In Stack's (1974) study of a Chicago urban Black community, most of the fostered children were with maternal kin—either older sisters, aunts, or grandmothers. There's no reason to expect that we would have a mechanism designed specifically to deal with the adoption of unrelated individuals. It may be that, in our current human environment, strong biological desires and cultural expectations lead some individuals to adopt unrelated offspring. Indeed, the relationship between adopted children and parents typically functions in the same way as that between genetic parent and child, particularly when they are adopted as very young infants.

Parent-Offspring Conflict

At the core of inclusive fitness theory is the idea that our kin are valuable, that we share a commonality of interest. In a genetic sense, what enhances the fitness of one's kin enhances one's own fitness. The more closely related two relatives are, the more common their genetic cause. But the inevitable consequence of social living is that at some point, individuals who interact will experience some conflict. Individuals act so as to increase their own inclusive fitness, even when it has fitness costs to others. Parent-offspring interaction can be highly cooperative but it can also involve significant conflict. There may be agreement about the general goal of offspring fitness, but conflict can occur over amounts of investment in one offspring versus another.

Being closely related does not mean that two individuals' interests are identical. As much as the degree of genetic similarity is a source of unity, the degree of genetic difference is a source of possible conflict. This becomes obvious when individuals are competing for scarce resources (mates, food, or territory). Conflicts can also happen when disagreement occurs over the optimal distribution of resources (to shared offspring, offspring of a previous union or mating effort). Such conflicts can also occur between parent and offspring.

Parent-offspring conflict can arise because some actions that advance the fitness of an offspring can potentially reduce the lifetime success of the parent and vice versa. In general, we would expect individuals to allocate their parental investment among their offspring in ways that optimize their own inclusive fitness. All other things being equal, parents are equally related to all their offspring. However, we would expect offspring to have a somewhat different take on that matter. They are more closely related to themselves than to their siblings. (Trivers, 1974). As a result, one might expect each offspring to want to extract more than their share of parental investment. Conflicts arise over the level of investment considered to be appropriate. This zone of conflict can be predicted from kin selection theory. When the costs to parents are less than the benefits, both parents and offspring benefit from parental investment and there is no conflict. When the cost becomes greater than the benefit but not more than twice the benefit, parents lose but offspring still gain, so there is conflict. When the cost becomes greater than twice the benefit, both lose, so there is no conflict, and parental investment ends. (For reviews of parent-offspring conflict in humans and nonhumans, see Maestripieri, 2002; Salmon & Malcolm, 2011.)

Maternal-Fetal Conflict

Maternal investment begins long before birth. The mother's own resources provide nutrients and a safe environment for the developing child over the 9-month gestational period. Although, at first glance, this would seem a very harmonious relationship with fetus and mother sharing the same goals, the genetic interests of both parties are not identical. Because the fetus is more closely related to itself than either its mother or any future siblings, the process of pregnancy becomes a sensitive balance between the developing fetus' tendency to secure as large a share of maternal resources as possible and the mother's tendency to preserve resources for herself and future offspring. Often this balancing act results in a variety of unpleasant symptoms for the mother and occasionally serious complications. Haig (1993, 1998) has analyzed pregnancy complications from a maternal-fetus conflict perspective, suggesting that such conflicts are responsible for some puzzling aspects of pregnancy and its complications.

Weaning Conflict

Conflict over weaning in mammals (Maestripieri, 2002; Trivers, 1974) is a very clear example of parent-offspring conflict. As Figure 21.1 illustrates, parents are selected to continue to invest in their offspring up to the point when the cost in terms of reduced reproductive success (the more a parent invests in a current offspring, the less they have to invest in future offspring) begins to outweigh the benefits of increased survival for the current offspring. Or, as soon as the costs begin to exceed the benefits (B/C < 1), parents should stop investing in the current offspring and start to work on the next (Trivers, 1974).

Figure 21.1 Analysis of the Costs and Benefits of Parental Investment and the Parent-Offspring Conflict That Results. Adapted from “Parent-Offspring Conflict,” by R. L. Trivers, 1974, American Zoologist, 14, pp. 249–264.

But at this point, the offspring would like investment to continue, being more closely related to itself than to any future siblings, it has been selected to demand investment until the cost–benefit ratio drops below 0.5. After that point, continued demands for investment would lead to a reduction in indirect fitness, since the parent would produce fewer siblings with whom the offspring would share genes. But until that point is reached, offspring should attempt to obtain as much parental investment as possible, enhancing its own reproductive fitness in the process. As a result, weaning conflict tends to involve a gradual shift in parental investment.

Parent-Offspring Conflict Over Mating

Another zone of conflict can occur over offspring mate choice. Parental influence over mate choice has been documented in many cultures (Apostolou, 2007a, 2007b) ranging in degree from minimal influence to complete control. Recent studies suggest that parental and offspring mate preferences are often not in sync with parents exhibiting stronger preferences for SES and family background features, whereas offspring focus more on attractiveness and a sense of humor (Apostolou, 2008a, 2008b, 2011; Buunk, Park, & Dubbs, 2008; Perilloux, Fleischman, & Buss, 2011). It has been suggested that conflict over offspring mate choice is driven less by differences in genetic relatedness and more by parent-offspring conflict over resource distribution in that parents can benefit by having more in-law investment allowing them to redistribute their own investments to other offspring (for a model of such conflict over allocation of resources to daughters see van den Berg, Fawcett, Buunk, & Weissing, 2013).

Attachment

In a sense, attachment type can be seen as the result of the form of parental investment a child receives. Bowlby (1969) characterized attachment as reflecting a child's “internal working model” of the self, others, and relationships, emphasizing the importance of early experience on adult personality and behavior. The central propositions of attachment theory are that: (a) individual differences in the quality of infant-parent attachment relationships are primarily determined by the quality of care provided to the child, and (b) early security shapes later development (Belsky, 1997). It has been suggested that variations in attachment security evolved to serve reproductive fitness goals and that environmentally induced modifications in life history traits tend to be reproductively strategic (Belsky, Steinberg, & Draper, 1991). The argument has been that patterns of attachment evolved as psychological and behavioral vehicles for translating information about prevailing ecological conditions into fitness-enhancing reproductive strategies (Belsky, 2000; Bjorklund & Pellegrini, 2002; Chisholm, 1996; Wiley & Carlin, 1999). This relies on two assumptions: that patterns of attachment are relatively stable from childhood through adolescence and early adulthood, and the relative stability of environmental conditions across the first 20–30 years of human life in the EEA (environment of evolutionary adaptedness).

According to Chisholm's life history model of attachment (1996), the type of attachment seen is a facultative adaptation to the style of parenting. Consistently responsive attentive parenting produces secure attachment because, in the ancestral environment, such parenting was evidence of access to resources and a commitment to provide the necessities of life to that offspring. Nonresponsive or rejecting parenting produces insecure attachment. And in the ancestral environment, such parents would have been unwilling or unable to invest in their offspring. The suggestion has been that insecure-avoidant behavior in offspring represents the facultative adaptation to parental unwillingness to invest, whereas insecure-resistant behavior in offspring is the facultative adaptation to parental inability to invest (Chisholm, 1996).

Attachment is normally classified as secure, insecure-avoidant, and insecure-resistant. If individual differences in attachment organization represent facultative adaptations to conditions of risk and uncertainty that were recurrent in the EEA (Chisholm, 1996), we can examine the nature of styles of attachment in a new light. Secure attachment would develop under ecological conditions that indicated that resources were reasonably abundant and would remain so. This would foster the belief that the world is a relatively safe place, that other people can be trusted, and that relationships last. The result would be the emphasis of parenting over mating.

The psychological and behavioral data on secure individuals is consistent in terms of adult relationships and parenting. Secure men have more positive and supportive interactions with their spouses than insecure men, whereas secure women are more likely to seek emotional support and physical comfort from their male partner when in a stressful situation (Simpson, Rhodes, & Nelligan, 1992). Conflict and negative affect are pronounced in married couples when both are insecure (Cohn, Cowan, Cowan, & Pearson, 1992) but when both partners are secure, negative interactions are rare (Senchak & Leonard, 1992). In general, lower levels of conflict and more skilled ways of managing conflict occur in relationships involving secure individuals. Security facilitates the development of mutually rewarding relationships. Secure individuals report higher levels of satisfaction when dating (Simpson, 1990), married (Kobak & Hazan, 1991), and their romantic relationships are longer lasting (Hazan & Shaver, 1987; Kirkpatrick & Davis, 1994; Shaver & Hazan, 1993). A history of security fosters the development of mutually rewarding and stable pair bonds in the service of promoting high investment parenting (van Ijzendoorn, 1995; Ward & Carlson, 1995). Secure attachment in childhood is a central component of developing a facultative reproductive strategy selected to promote a quality versus quantity orientation toward reproduction.

Belsky et al. (1991) and Chisholm (1996) have suggested that, when the flow of resources is chronically low or unpredictable, it may be (or have been) biologically adaptive to reduce parental investment, to allocate resources not to parenting but to offspring production (Borgerhoff Mulder, 1992). Limited and unpredictable resources would result in a reproductive strategy designed to foster in offspring beliefs and expectations that the world is uncaring, that others can't be trusted, and that relationships are not likely to be mutually rewarding or enduring. As a result, individuals are predicted to pursue interpersonal relationships that are disproportionately self-serving, opportunistic, and exploitative. Under such conditions, individuals will have many partners, and pair bonds will be unstable with many kids and little paternal care, a quantity not quality strategy.

The data suggest that insecure-avoidant individuals are more promiscuous and less committed (Kirkpatrick & Hazan, 1994; Simpson, 1990) and more likely to be involved in a break-up (Feeney & Noller, 1992). As well, avoidant mothers are less responsive (van IJzendoorn, 1995), less supportive and helpful, less concerned, and more remote and controlling (Crowell & Feldman, 1988, 1991).

The insecure-resistant case is a little different. These children tend to exaggerate their need for care and attention (Cassidy & Berlin, 1995) in response to inconsistently responsive care. One suggestion about why this develops has been related to the helpers at the nest phenomenon (Borgerhoff Mulder, 1992; Emlen, Wrenge, & Demong, 1995). Inconsistently responsive parenting seems to produce a dependency in their children that has been suggested to promote the parent's reproductive fitness. Kunce and Shaver (1994) noted that insecure-resistant women are compulsive caregivers, particularly toward younger siblings. Resistant mothers are difficult to separate from their toddlers (Crowell & Feldman, 1988) and doubt their offspring's ability to function away from home (Kobak, Ferenz-Gillies, Everhart, & Seabrook, 1994). These mothers keep their children close while maximizing their own ability to psychologically manipulate the children, perhaps fostering helping at the nest behavior (Belsky, 1997). We might expect that such an attachment style would be seen more in some niches (firstborn females for example) if the mother's ability to care for her offspring is taxed (lack of resources, caregiving help, etc.).

Attachment, whether secure (reliable parental investment) or insecure (the consequences of early stress), might have evolved to function as an assay by which future social relations might be predicted. Data on early menarche, father absence, maternal harshness, and sexual activity (Belsky, Steinberg, Houts, & Halpern-Felsher, 2010; Ellis, Schlomer, Tilley, & Butler, 2012; Graber, Brooks-Gunn, & Warren, 1995; Nettle, Coall, & Dickens, 2010; Surbey, 1998; Wierson, Long, & Forehand, 1993) suggest that this may be the case. When girls grow up in father-absent homes (cues that the local males may be unlikely to stay and invest or that long-term survival prospects are poor), they tend to mature faster and follow a strategy of quantity over quality. Under some circumstances, adaptations are designed to facilitate reproduction early and often. In a similar vein, data suggests that boys that grow up in father-absent homes may exhibit increased promiscuity and criminality as well as a general increase in “macho” behavior (Bereczkei & Csanaky, 1996). Such a strategy of increased aggression might serve under some circumstances to intimidate rivals and attract women interested in protection (Kim, Smith, & Palermiti, 1997).

Sibling Relations

The other side to parent-offspring conflict is how the battle for resources plays itself out among a group of siblings. Natural selection has shaped strategies for sibling competition just as it shaped mechanisms for discriminative parental solicitude. Many factors play a role in the approach individual siblings may take but two are of particular interest: birth order and birth spacing (interbirth interval).

Birth Order

Theoretical models of the evolution of parental inclinations predict that parents will often treat their offspring differently. However, if one assumes that human parents typically have enough resources available to them to raise all of their children to adulthood (as presumably most do in Western societies), this assumption leads to the expectation that human parents may use a decision rule that divides investment equally among all their children. Such a rule is called the equity heuristic (Hertwig, Davis, & Sulloway, 2002).

However, the equity heuristic is not the only model of the allocation of parental investment. There are times when the equal allocation of resources may not provide the optimal result, perhaps because, most of the time, all else is not equal and some offspring may be more valuable fitness vehicles than others.

In addition to enjoying the relative security of parental preference in a pinch (as discussed earlier), firstborn children have always benefited from an early absence of sibling contenders for a share of parental investment (Salmon, Shackelford & Michalski, 2012). Even in the modern West, where parental resources are presumably less stretched than in noncontracepting, premodern societies, firstborn children still receive more parental caretaking and attention in infancy than laterborns (Jacobs & Moss, 1976) and they grow faster, such that despite being smaller at birth, they are larger by 1 year of age (Wingerd, 1970).

There is, however, a counterveiling effect: As parents themselves grow older, the fitness value of an offspring of any given age and phenotype increases relative to the parent's residual reproductive value. Thus, in any species in which expected future reproduction is a declining function of parental age, older parents will have been selected to invest more in offspring, all else being equal, than younger parents. Thanks to menopause, this argument certainly applies to the human female, and dramatic decreases in maternally perpetrated infanticide as a function of maternal age appear to be one reflection of age-related changes in the relative weights that the maternal psyche places on one's infant versus one's future (Bugos & McCarthy, 1984; Daly & Wilson, 1995).

Impact of Birth Order on Personality and Development

Sulloway and others (Salmon, 1999, 2007; Salmon & Daly, 1998; Sulloway, 1996, p. 305) have suggested that the favoring of firstborns (due to their greater reproductive value) and lastborns (due to older parents and lack of younger rival) means that middleborns are the birth order that loses out on average in the parental investment game. Certainly middleborns seem to report lesser levels of financial and emotional support from parents (Janicki & Salmon, 2002; Kennedy, 1989; Salmon & Daly, 1998) than firstborns or lastborns (who tend to be more parentally and familially oriented). As a result, they seem to focus more on developing nonkin reciprocal relationships outside the family unit (Salmon, 2003), and their personality traits seem to be a reflection of that. They are often noted for their skills in getting along with other people and in being excellent negotiators, traits that would have tended to serve them well in trying to find their niche within the family and a network of support outside it (Sulloway, 1999).

Sulloway (1996) has summed up many of the birth order differences with regard to personality, shaped by sibling competition and parental investment, in terms of the five-factor model of personality (which posits five basic personality dimensions: conscientiousness, agreeableness, openness to experience, extraversion, and neuroticism). Sulloway's (1995) meta-analysis of those studies that control for related background variables reveals consistent birth order differences across the five-factor model. The big-five trait of conscientiousness is one that shows consistent birth order differences with firstborns tending to score higher than laterborns. In the area of openness to experience, laterborns tend to score higher on risk-taking (Sulloway & Zweigenhaft, 2010).

Even if parents do not actively favor one child over another (Hertwig et al., 2002), siblings compete with each other for a greater share of parental resources. Sulloway (1995) suggested that they do so by carving out unique niches, or roles, within the family that are influenced by their birth order. Secure in their expectation of parental favoritism (and benefiting from an early absence of competitors for parental investment), firstborns tend to have their choice of niches; motivated to fulfill parental expectations, they typically become supporters of parental values and the status quo. Laterborns cannot compete as effectively in the same roles (being smaller in size and less experienced) so they seek out different niches, other routes to sources of parental (or other) investment. Personality traits that facilitate this include openness to experience and unconventionality, traits that sometimes mark them as rebels (Saraglou & Fiasse, 2003). Michalski and Shackelford (2002) have also suggested that firstborns are more likely to follow long-term mating strategies than laterborn children, with laterborn children desiring a greater variety of sexual partners in the future.

Siblings are not only different in the ways they approach parental investment and cultivating niches, but also in the strategies they use in interacting with each other. Human siblings have dominance hierarchies much like that of other mammals (Sulloway, 2001b). Anyone who watches a litter of puppies can observe the largest using physical strength and the threat of it to get their own way. Firstborn humans are very similar, tending to dominate their younger siblings. Smaller siblings (or laterborn humans) have to resort to alternative strategies, finding ways to get parental assistance or forming bonds with other siblings to unite against their oppressor. What eldest sibling hasn't been occasionally frustrated at having their plans thwarted by a junior sibling who has gone whining or crying to a parent?

Only children (those with no siblings) are an example of what happens without sibling competition. In a sense, they are firstborns who never had a sibling come along after them, never had to compete for parental resources. And, like firstborns, they tend to have a drive for success and respect for parental values. But on many other measures, they fall somewhere between firstborn and laterborns (Sulloway, 2001a). Birth spacing can also affect the correlation between birth order and personality as well as levels of parental investment.

Birth Spacing

The impact of birth order is decreased when the birth interval is so short that the siblings are on almost equal footing or when the interval is so large that they are not competing for the same resources from parents. For example, a middleborn with a sibling 7 years older and another sibling one year younger, may have a personality more representative of a firstborn than a typical middleborn (Sulloway, 1999).

Parents invest in their offspring based on many things, including offspring quality, reproductive value, their own residual reproductive value, and the amount of available resources. There is a cost–benefit analysis going on. And for siblings, their brothers and sisters also entail costs and benefits, which vary in proportion to the birth interval. Substantially older offspring, no longer dependent on parental care, experience minimal costs from additional siblings (hence their typical protectiveness). Close age spacing increases competition for parental investment, promoting greater parent-offspring conflict as well as increased sibling rivalry. As well, the costs represented by a younger sibling are greatest when both are infants, requiring the same high levels of parental investment. In traditional and low- to middle-income societies, short birth intervals (less than 2–3 years) are associated with increased infant mortality (Kozuki et al., 2013; Lindstrom & Berhanu, 2000).

The influence of birth order on sibling strategies should be greatest for offspring who are spaced within 5 years. Under these circumstances, older siblings should tend to highlight their own worth and run down the value of their younger sibs. Younger siblings should respond by trying to minimize direct comparisons with older siblings, diverging in their interests and perhaps searching out nonparental sources of investment as they get older (Salmon & Daly, 1998). For example, in terms of openness to experience, the greatest disparities are among offspring separated by moderate age differences. Those that are more distant or very close are less polarized (Koch, 1956; Sulloway, 1996). The reasons for this seem clear in terms of large birth intervals but less intuitive for close ones, until we consider the issue of benefit, not just the cost of having a sibling. If we look at the relative differences in the likelihood of survival of offspring, those that are close in age are more equal. The cost of having a sibling may be high but the benefit is also high as you are equally likely to survive. For large intervals, the cost is much less as you yourself need less parental investment, though the benefit may be lower as the younger sibling may be less likely to survive simply by virtue of the fact that it is young. At middle age spacings, the adjusted costs of having a younger sibling are elevated in relation to the benefits. As a result, moderate age gaps result in more polarization between siblings.

Conclusions

Basic human relationships and characteristic conflicts show a startling consistency across time and space and it is reasonable to expect that psychological adaptations have evolved to deal with them that are particular to each type of relationship. Evolutionary psychology contributes to our understanding of parent-offspring relations, as well as sibling relations, allowing us to predict and explain the behavior of parents and offspring with regard to social and ecological variables.

A number of factors influence the degree of parental investment. Females invest substantially more than males, the amount of investment given is influenced by the availability of resources and the likelihood of their successful use, and male investment reflects genetic certainty of paternity. Human children require greater investment than other primate offspring and, in particular, fathers must contribute more than they do in many species. Maternal investment begins in utero as do conflicts over levels of investment. Conditions like preeclampsia can be seen as the result of a tug of war between mother and fetus over the amount of fetal growth that is appropriate. Differential investment in offspring is quite common and reflects the factors that affect the costs and benefits of parental investment to parents including: parental age, number of offspring, parental resources, age of offspring, offspring's expected future prospects, paternity certainty, and stepparenthood. Sibling conflict can be seen as an extension of parent-offspring conflict and the degree of conflict is influenced by birth spacing (exacerbated by small intervals) and birth order (in that parents may bias their investment toward a particular birth order).

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