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ChaPter 8 Problems of Kinship Learning Objectives Afer studying this chapter, the reader will be able to: ■ Explain the theory of inclusive ftness. ■ Provide three empirical examples that illustrate the importance of genetic relatedness for helping. ■ describe two studies that suggest that gen...

ChaPter 8 Problems of Kinship Learning Objectives Afer studying this chapter, the reader will be able to: ■ Explain the theory of inclusive ftness. ■ Provide three empirical examples that illustrate the importance of genetic relatedness for helping. ■ describe two studies that suggest that genetic relatives make a diference between life and death. ■ analyze why maternal grandmothers would invest more than paternal grandfathers in their grandkids. ■ Explain, from an evolutionary perspective, why families might form. ■ summarize the three major sources of confict within families. Imagine a world in which everyone loved everyone else equally. There would be no favoritism. You would be just as likely to give your food to a passing stranger as to your children. Your parents would be just as likely to pay for a neighbor’s college education as they would be to pay for yours. And when forced by fate to save only one person’s life when two were drowning, you would be just as likely to save a stranger as you would your brother or sister. Such a world is hard to imagine. Inclusive ftness explains why. From the perspective of inclusive ftness theory, people difer in their genetic relatedness to others. As a general rule, we are related by 50 percent to our parents, children, and siblings. We are related by 25 percent to our grandparents and grandchildren, half brothers and half sisters, and uncles, aunts, nieces, and nephews. We are related by 12.5 percent, on average, to our frst cousins. An individual’s relatives are all vehicles of ftness, but they difer in value. In Chapter 7, we saw that children difer in their value to their parents; in this chapter, we will explore the theory that kin difer in value to us. Theoretically, if everything else is equal, selection will favor adaptations for helping kin in proportion to their genetic relatedness. Selection will favor mechanisms for helping ourselves twice as much as we help a brother, for example. But a brother, in turn, is twice as related to us as a nephew and so would get twice the help. In life, of course, not everything is equal. Holding genetic relatedness constant, for example, one brother struggling to make it as a songwriter might beneft more from our gifts of aid than would another brother who happens to be wealthy. Furthermore, altruism can evolve under conditions of low relatedness or even no relatedness, as we will see in Chapter 9. But if there is one straightforward prediction from inclusive ftness theory, it is this: Selection will often favor the evolution of mechanisms to help close kin more than distant kin and distant kin more than strangers. Theory and Implications of Inclusive Fitness In this section, we frst introduce Hamilton’s rule—the technical formulation of inclusive ftness theory. We will see that favoritism that parents show their own children is a special case of favoritism toward the “vehicles” that contain copies of their genes. We then explore the profound consequences of this formulation for topics such as cooperation, confict, risk taking, inheritance of wealth, and grieving. 8 PRObLEMS OF KiNShiP Hamilton’s Rule You might recall from Chapter 1 the technical concept of inclusive ftness: The inclusive ftness of an organism is not a property of himself, but a property of its actions or efects. Inclusive ftness is calculated from an individual’s own reproductive success plus his efects on the reproductive success of his relatives, each one weighted by the appropriate coefcient of relatedness. (Dawkins, 1982, p. 186) To understand this formulation of inclusive ftness, imagine a gene that causes an individual to behave altruistically toward another person. Altruism, as used here, is defned by two conditions: (1) incurring a cost to the self to (2) provide a beneft to the other person. The question that Hamilton (1964) posed was: Under what conditions would such an altruistic gene evolve and spread throughout the population? Under most conditions, we would expect that altruism would not evolve. Incurring costs to the self will hinder personal reproduction, so selection will generally operate against incurring costs for other people. Hamilton’s insight, however, was that altruism could evolve if the costs to the self were outweighed by the beneft to the recipient of the altruism, multiplied by the probability that the recipient carried a copy of that gene for altruism. Hamilton’s rule, stated more formally, is that natural selection favors mechanisms for altruism when c < rb In this formula, c is the cost to the actor, r is the degree of genetic relatedness between the actor and the recipient (genetic relatedness can be defned as the probability of sharing a particular focal gene with another individual over and above the average population frequency of the gene; see Dawkins, 1982; Grafen, 1991), and b is the beneft to the recipient. Both costs and benefts are measured in reproductive currencies. This formula means that selection will favor an individual to incur costs (being “altruistic”) if the benefts to a .50 kin member are more than twice the costs to the actor; if the benefts to a .25 kin member are more than four times the costs to the actor; or if the benefts to a .125 kin member are more than eight times the costs to the actor. An example will illustrate this point. Imagine that you pass by a river and notice that some of your genetic relatives are drowning in a ferocious current. You could jump in the water to save them, but you would lose your own life. According to Hamilton’s rule, selection will favor decision rules that, on average, result in your jumping into the water to save three of your brothers, but not one. You would be predicted not to sacrifce your own life for just one brother, because that would violate Hamilton’s rule. Using the logic of Hamilton’s rule, evolved decision rules should lead you to sacrifce your own life for fve nieces or nephews, but you would have to save nine frst cousins before you would sacrifce your own life. The key point to remember is not that people’s behavior will necessarily conform to the logic of inclusive ftness. Hamilton’s rule is not a psychological theory. Instead, Hamilton’s rule defnes the conditions under which adaptations for aid to kin can evolve. It defnes the selection pressure to which genes for altruism—indeed any genes—are subject. Any traits that happen to enter the population through mutation that violate Hamilton’s rule will be ruthlessly selected against. Only those genes that code for traits that fulfll Hamilton’s rule can spread throughout the population and hence evolve to become part of the species-typical repertoire. This is sometimes called an evolvability constraint because only genes that meet the conditions of Hamilton’s rule can evolve. Hamilton’s theory of inclusive ftness is the single most important theoretical revision of Darwin’s theory of natural selection in the past century. Before this theory, altruism was puzzling because it appeared to go against the actor’s personal ftness. Why might a ground squirrel give an alarm call when encountering a predator, thus making that squirrel vulnerable to the 219 ChALLENGES OF PARENTiNG AND KiNShiP 220 predator? Why would a woman sacrifce a kidney so that her brother might live? Hamilton’s formulation of inclusive ftness solved these puzzles by showing that altruism far removed from personal reproduction could easily evolve. Theoretical Implications of Hamilton’s Rule The social behaviour of a species evolves in such a way that in each distinct behaviour evoking situation the individual will seem to value his neighbours’ ftness against his own according to the coefcients of relationship appropriate to that situation. (Hamilton, 1964, p. 23) At the most general level, the most important implication of Hamilton’s theory of inclusive ftness is that psychological adaptations are expected to have evolved for diferent types of kin relationships. Nothing in Hamilton’s theory requires that such kinship mechanisms necessarily evolve; afer all, in some species, members don’t even live with their kin, so selection could not fashion specifc kin mechanisms. But the theory yields predictions about the general form of such kin mechanisms if they do evolve. In Chapter 7, we saw that there were many specifc “problems of parenting,” and we reviewed evidence for the evolution of parental mechanisms sensitive to qualities of children such as genetic relatedness and reproductive value. The theory of inclusive ftness renders the parent–child relationship as a special case of kinship relationships. Kin relationships that would have recurred throughout human evolutionary history include sibships, half sibships, aunt–niece, uncle–nephew, grandparent–grandchild, and others. Let’s consider a few of these to get a sense of the sorts of adaptive problems these kin relationships would have posed. Sibships Brothers and sisters pose unique adaptive problems. First, a brother or a sister can be a major social ally—afer all, your siblings are related to you by 50 percent. But sibs, perhaps more than all other relatives, are also major competitors for parental resources. As we saw in Chapter 7, the “interests” of parents and children do not always coincide. One consequence is that siblings historically faced the recurrent adaptive problem of competing with each other for access to parental resources. Therefore, it is not surprising that sibling relationships are ofen riddled with confict and ambivalence (Daly, Salmon, & Wilson, 1997). One study in rural Ethiopia found that as economic conditions improved and parents had more to give, sibling competition and rivalry over parental resources actually increased (Gibson & Lawson, 2011). Sulloway (1996, 2011) proposed that the adaptive problems imposed by parents on children will create diferent “niches” for children, depending on their birth order. Specifcally, because parents often favor the oldest child, the frstborn tends to be relatively more conservative and more likely to support the status quo. Second-borns, however, have little to gain by supporting the existing structure and everything to gain by rebelling against it. Later-borns, especially middle-borns, according to Sulloway, develop a more rebellious personality because they have the least to gain by maintaining the existing order; studies of birth order and personality confrm this prediction (Healey & Ellis, 2007). The youngest, on the other hand, might receive more parental investment than middle children, as parents often let out all the stops to invest in their fnal direct reproductive vehicle. Salmon and Daly (1998) fnd support for these predictions. They discovered that middleborns difer from frst- and last-borns in scoring lower on measures of family solidarity and identity. Middle-borns, for example, are less likely to name a genetic relative as the person to whom they feel closest. They are also less likely to assume the role of family genealogist. 8 PRObLEMS OF KiNShiP Middle-borns, compared to frstborns and last-borns, are less positive in attitudes toward their families and less likely to help a family member who needs help (Salmon, 2003). These and other results (Salmon, 1999) lend some support to Sulloway’s theory that birth order afects the niches a person selects. Firstborns are more likely to feel solidarity with parents and perceive them as dependable, whereas middle-borns appear more likely to invest in bonds outside of the family. Interestingly, middle-born children might receive less total investment from parents even if parents treat all their children equally (Hertwig, Davis, & Sulloway, 2002). This result occurs because frstborns receive all of their parents’ investments early in life before other children are born and last-borns receive all of their parents’ investments after all other children leave the house. Middle-borns, in contrast, must share their parents’ investments, because there is rarely a time when other siblings are not around. Even when parents strive to invest equally in their children, middle-borns end up on the short end of the stick—perhaps accounting for why middle-borns are less identifed with their families (Hertwig et al., 2002). Sibs Versus Half Sibs Another important aspect of kinship is whether a sib is a full or a half sib. Given a common mother, for example, do you and your sibling share a father? This distinction is theoretically important because full sibs are genetically related by 50 percent on average, whereas half sibs are genetically related by only 25 percent on average. In an intriguing study of ground squirrels, Holmes and Sherman (1982) discovered that full sisters were far more likely than half sisters to cooperate in the mutual defense of their young. The distinction between full and half sibs was likely a recurrent selection pressure over the course of human evolutionary history. Mothers in contemporary traditional societies commonly have children by diferent men, either from extramarital afairs or serial marriages (Hill & Hurtado, 1996). Daly et al. (1997) speculate that it “could well be the case that in human prehistory it was a virtual toss-up whether successive children of the same woman were full or half-siblings, and the distinction between (r = .5) and (r = .25) is by no means trivial when the decision to cooperate or to compete is a close call” (Daly et al., 1997, p. 277). Some empirical support exists for the key distinction between full and half sibs. A study conducted in Finland examined more than 4,000 people of two diferent generations (Tanskanen & Danielsbacka, 2014). They found that full sibs had considerably more contact with each other than half sibs, with amount of contact being an indicator of investment. Moreover, Finns had more contact with the nephews and nieces of their full sibs than with their half sibs. These fndings suggest that, among sibs, higher levels of genetic relatedness importantly predict investment, supporting a straightforward prediction from Hamilton’s theory of inclusive ftness. Grandparents and Grandchildren Grandparents are related to their grandchildren by an r of .25. The fact that modern women ofen live well beyond menopause has led to the hypothesis that menopause itself evolved as a means of ceasing direct reproduction in order to invest in children and then grandchildren, in what has become known as the “grandmother hypothesis” (Hill & Hurtado, 1991). Across cultures, post-menopausal women contribute substantially to their grandchildren (Lancaster & King, 1985). If grandparenting has been a recurrent feature of human evolutionary history, adaptations for grandparents’ investment might have evolved. As we will see later in this chapter, there is evidence for this hypothesis. 221 ChALLENGES OF PARENTiNG AND KiNShiP 222 Hypotheses About Universal Aspects of Kinship Daly et al. (1997) outline a set of hypotheses about the universal aspects of the psychology of kinship. First, they suggest that ego-centered kin terminology will be universal. That is, in all societies, all kin will be classifed in reference to a focal individual: “My parents are not the same people as your parents” and “My brothers are not the same as your brothers.” All kin terms, in short, fow from the ego-centered focal individual. Second, all kinship systems will make critical distinctions along the lines of sex. Mothers are distinguished from fathers, sisters are distinguished from brothers. This sex distinction occurs because the sex of a kin member has important reproductive implications. Mothers, for example, have 100 percent certainty in their genetic relatedness to children, whereas fathers do not. Sons might become highly reproductively successful through multiple matings, whereas daughters cannot. The sex of the kin member, in short, is pivotal to the adaptive problems faced, so all kin systems should diferentiate according to sex. Third, generation is also critical. The relationship between parents and children is often asymmetrical. Early in life, for example, children learn much about the world from their parents, who in turn often provide teaching and protection. With advancing age, children become increasingly valuable vehicles for their parents, whereas parents become less and less useful to their children. Therefore, we expect that all kin systems will make distinctions according to generation. Fourth, kin relationships will be universally arrayed on a dimension of closeness, and closeness will be highly linked with genetic relatedness. The emotional (feeling close to someone) and cultural recognitions of closeness, in short, are predicted to correspond to genetic closeness. Fifth, the degree of cooperation and solidarity between kin will be a function of their degree of genetic relatedness. Cooperation and confict should be predictable from the degree of genetic relatedness between kin members; people are predicted to turn to close kin rather than distant kin when it really matters; and whatever conficts of interest exist, they will be mitigated more among close kin than among distant kin. A sixth implication of inclusive ftness theory is that the elder members of an extended kin family will encourage the younger members to behave more altruistically and cooperatively toward collateral kin (i.e., kin who are not direct descendants, such as one’s brothers, sisters, cousins, nephews, and nieces) than is their natural inclination. Imagine an older man who has a son, a sister, and his sister’s son as relatives. From this man’s perspective, his sister’s son (his nephew) is genetically related to him by .25 and so constitutes an important ftness vehicle for him. But from his own son’s perspective, this person is merely a cousin and so is related to him by only .125. Any sacrifce the son makes for his cousin would have to yield eight times the beneft to the son, according to Hamilton’s rule. Thus, helping by the older man’s son toward his sister’s son (the boy’s cousin) will be more benefcial to the ftness of the older man than to his son. Hence, the older man might try to infuence his son to behave more altruistically than would be the son’s natural inclination. A seventh implication of inclusive ftness theory is that one’s position within an extended kin network will be core components of the self-concept. Your beliefs about “who you are” will include kin linkages, such as “son of X,” “daughter of Y,” or “mother of Z.” An eighth implication of inclusive ftness theory is that despite diferences across cultures in the exact kin terms used, people everywhere will be aware who their “real” relatives are. Consider the Yanomamö Indians of Venezuela. They use the kin term abawa to refer to both brothers and cousins. In English, however, we have diferent words, brothers and cousins. Does this terminological confation among the Yanomamö obscure their real kin relationships? Anthropologist Napoleon Chagnon examined this issue by interviewing Yanomamö and showing them photographs of what English speakers would call their brothers and cousins. Although the Yanomamö said abawa when looking at both of the photographs, when asked “which one is your real abawa?” each invariably pointed to his actual blood brother and not to his cousin