The Evolutionary Psychology of Social Behaviour - Kin Relationships PDF

The Evolutionary Psychology 7 of Social Behaviour – Kin Relationships and Conflict Key Concepts inclusive fitness direct and indirect fitness coefficient of relatedness kin altruism parental investment parent–offspring conflict K- and r-selection parental manip...

The Evolutionary Psychology 7 of Social Behaviour – Kin Relationships and Conflict Key Concepts inclusive fitness direct and indirect fitness coefficient of relatedness kin altruism parental investment parent–offspring conflict K- and r-selection parental manipulation grandmother hypothesis non-paternity To many people human behaviour is social behaviour. Linguistic communication is meaningless unless used between at least two people; sex by definition involves more than one person (usually, although not invariably, two); child-rearing and sibling relationships are clearly social and most working practices occur in groups. Humans do engage in some solitary activities – reading a book or taking a bath, for example, are not normally social events. But even by primate standards, we are an extremely socially integrated species. So it’s not surprising that some evolutionary theorists have suggested evolutionary psychology may have the greatest impact on social psychology (Neuberg et al., 2010; Wilson, 2012; Zeigler-Hill et al., 2015). Social behaviour can broadly be divided into pro- and antisocial patterns of response. Both can be found frequently in interactions that involve kin. Social scientists have long sought to explain why such love–hate relationships exist in families. Evolutionary psychologists think they have the answer – to them it’s all about evolved psychological mechanisms that arose to deal with ancient recurrent challenges. Social Psychology and Evolutionary Theory Social psychology is a well-developed area of psychological enquiry. During the late twentieth and early twenty-first centuries, social psychologists developed theories to account for, among other things, obedience to authority, group conformity, in- and out-group stereotyping, intergroup ag- gression, social concept and attitude formation (Hewstone et al., 2015). Moreover, it has been very successful in testing these theories in both the lab and field. So what can the evolutionary approach bring to improve our understanding of human social behaviour? At the least, the evolutionary approach can provide an interesting new perspective, which might help to derive new models and suggest new ways of testing them; at best, it might radically alter the very way that we consider human behaviour. Despite its successes at explaining various aspects of human social behaviour, by taking a largely SSSM stance (see Chapter 1), conventional social psychology has yet to develop a well-accepted general theory which connects its disparate findings. In the words of two American evolutionary psychologists, Jeffry Simpson and Douglas Kenrick, social psychology has developed a series of minitheories but lacks an overarching theory  164 Evolutionary Psychology of Social Behaviour – Kin Relationships that can bring these together (Kenrick and Simpson, 1997). More poetically, in the words of William von Hippel and David Buss: [S]ocial research is more aptly described as a thousand points of light than as a coherent beam illuminating the landscape. (von Hippel and Buss, 2017, 7) By adding the ultimate level of explanation to the fabric of social psychology, the evolu- tionary approach may provide the framework on which the various findings of the field might be wo- ven together. That is, it might provide a metatheory. Evolutionary psychologists consider that this will be achievable through an integration of evolutionary theories which pertain to social behaviour (Archer, 1996; Wilson, 2012; Zeigler-Hill et al., 2015; von Hippel and Buss, 2017). We can identify five such theories (Buss, 1999). The first of these is the theory of sexual selection, which we dealt with in Chapters 3 and 4 and may be traced back to Darwin himself (1871). Of the remaining four theories, three are attributable to Robert Trivers and one to William Hamilton: kin altruism/inclusive fitness (Hamilton, 1964a, 1964b); parental investment (Trivers, 1972); parent–offspring conflict (Trivers, 1974); reciprocal altruism (Trivers, 1971). In this chapter we consider the first three theories in relation to social behaviour (Trivers’ reciprocal altruism is considered in the next chapter). Charity Begins at Home – Inclusive Fitness Theory and Kin Altruism Social psychologists generally define altruism as selfless behaviour conducted on behalf of others without regard for one’s own self-interest (Colquhoun et al., 2020; Hewstone et al., 2015; see Box 7.1). Note that such definitions include intentionality on the part of the altruist. In contrast, evolutionists define altruism purely in terms of the act performed, not the intention behind it. In this way ethologists have no problem in discussing examples of apparent altruism in animals. Table 7.1 lists a number of well-documented acts of apparent altruism in the animal king- dom. In all of these cases, it is now known that the individual who provides a benefit to others does so to its relatives. Note that in Table 7.1, r, which may vary from 0 to 1, equals the proportion of genes shared between two relatives by common descent (full siblings, for example, share 0.5 of their genes and identical twins share 1.0). Table 7.1 Documented acts of apparent altruism in the animal kingdom Species Beneficial act Relationship to altruist and proposed Source reason for altruistic act (example) Ants (many Soldier caste ants do not Soldier (and worker) castes are sterile Wilson (1975) species) breed themselves but defend and are more closely related to their the colony with their lives. sisters (r = 0.75) than they would be Likewise, worker caste ants to offspring (r = 0.50) they produced do not breed but care for their themselves. younger siblings.  Charity Begins at Home 165 Table 7.1 (cont.) Species Beneficial act Relationship to altruist and proposed Source reason for altruistic act (example) Wild dogs of Adult male and female dogs All dogs in pack are closely related and Schaller (1972) Africa bring back meat from kills and thereby feed young relatives and older, regurgitate to other members of infirm ones. the pack. Florida scrub Individual males help feed and Scrub jays feed their younger siblings (r Woolfenden jays defend nestlings which are not = 0.50). It is difficult to eke out a living and Fitzpatrick their own. on the harsh scrubland, so helpers at the (1984) nest at least ensure that some of their younger relatives survive. Dwarf Female dwarf mongooses Dwarf mongooses live in closely related Rood (1986) mongooses suckle the young of other groups and nurse each other’s offspring. females. Naked mole Mature females help to raise Colony members are highly inbred Sherman et al. rats the young of the nest’s ‘queen’ (mean r = 0.81). Younger females (1991) rather than breed themselves. are rendered temporarily sterile by pheromones released by the ‘queen’; they may be playing a ‘waiting game’ whereby they will have a chance to breed when she dies. Figure 7.1 Illustration of naked mole rats (Heterocephalus glaber) inside burrow. 166 Evolutionary Psychology of Social Behaviour – Kin Relationships As we saw in Chapter 2, why acts of self-sacrifice or ‘altruism’ such as those outlined in Table 7.1 occur at all has long been an area of debate among evolutionists. Since the 1960s, however, there has been a growing consensus as to why individuals of social species frequently appear to act altruistically (Colquhoun et al., 2020). Most recorded examples of self-sacrificing behaviour involve giving aid to relatives with which the altruist shares a varying proportion of their genes (note that this is true of all of the examples in Table 7.1). In Hamilton’s terminology we can expect ‘actors’ to show self-sacrificing behaviour for ‘recipients’ (relatives) when the cost to the actor is less than the benefit to the recipient. This has become known as ‘Hamilton’s rule’ and can be reduced to this simple formula: c < rb Here c is the cost to the actor, r is the coefficient of relatedness between the actor and the recipient and b is the benefit to the recipient. Ultimately, these costs and benefits are measured in terms of inclusive fitness. Hamilton introduced the notion of kin altruism as an explanation of self-sacrificing behav- iour, and today we look at animals as inclusive fitness maximisers rather than simply as individual fitness maximisers (see Chapter 2). Modern-day evolutionists consider the genes which are passed on to the next generation via an individual’s own offspring as their direct fitness and those which are passed on via aid to other kin as their indirect fitness. This means that if a human being rears two children and helps other (non-descendant) kin then they are boosting both their direct and indirect fitness. For worker ants and most naked mole rats, however, the only way they may be able to raise their fitness will be indirectly through providing care for younger kin which are not their offspring. Taken together direct and indirect fitness equal inclusive fitness. In this way Hamilton argued that we can predict that animals are likely to provide care for others who share genes with them by common descent and that the amount of care given will increase as does the proportion of these genes shared. But why did Hamilton add ‘by common descent’ to his explanation of kin altruism? Members of a population may share the majority of their genes without being closely related. Siblings may share 99 per cent of their genes with each other and with other members of their species. They will, however, share only 50 per cent on average from their common parentage (Dickins, 2011; Buss, 2019). In this way ‘r’ (the coefficient of relatedness; see Chapter 2) can be thought of as the probability that an allele (gene) shared by two individuals is ‘descended from the same ancestral gene in a recent common relative’ (McFarland, 1999). If an individual shows altruism to those that share the same gene by common descent, rather than those that share the gene simply by being members of the same population, it is the former strategy that evolutionists consider will be favoured by natural selection. John Maynard Smith coined the term kin selection for this part of natural selection that promotes the favouring of relatives. Although Hamilton is rightly credited with developing inclusive fitness theory, it is historically interesting to note that the eminent evolutionary biologist J. B. S. Haldane had come close to developing this theory during the 1930s when he scrawled the idea on the back of a beer mat in a London pub that he would risk his life for at least an identical twin or eight cousins. Although Haldane later went on to briefly outline this idea in the mid 1950s (Haldane, 1955) he did not develop it, and evolutionists had to wait another decade before Ham- ilton’s work changed the course of evolutionary thinking.  Charity Begins at Home 167 Box 7.1 Kindness to Relatives – Is It Altruism? Altruism Stinks of Fallacy (Catatonia, 1998) If most cases of altruism involve acts between relatives so that those involved are helping copies of their own genes to be passed on indirectly, then this raises a rather large question – is such behaviour really altruistic? To most social psychologists altruism is concerned with a willingness to help others at a cost to oneself (Bierhoff, 1996; Hewstone et al., 2015). If we consider self- sacrificing behaviour from a traditional human individual perspective then we can certainly term beneficial acts to others (be they relatives or not) as ‘altruism’. You may recall that this level of explanation is known as proximate causation and that this is the normal level of causation as used by social scientists. Social scientists interested in evolutionary explanations, however, also use ultimate causation, which may be defined as ‘causation on a generational time scale’ (Daly and Wilson, 1983; see also Chapter 1). Thus ultimate causation is concerned with the likelihood of copies of genes being passed on. Clearly, acts that aid copies of shared genes in relatives are ul- timately selfish. Thus, whether we consider an act as altruistic or not depends on whether we are discussing it at a proximate or an ultimate level of causation. A beneficial act towards a relative is altruistic when considered at an individual (i.e. proximate) level, but when using a gene-focused level of explanation (i.e. ultimate) then the very same act may be considered as selfish! Today, evolutionists interested in behaviour tend to look at animals as nepotistic strategists rather than individual strategists. In the words of ethologist John Alcock, ‘Hamilton’s explanation for altruism rests on the premise that the unconscious goal of reproduction, from an evolutionary perspective, is to propagate one’s distinctive alleles’ (Alcock, 2005, 564). Note that Alcock is mak- ing it explicit that ethologists do not consider it necessary for animals to be in any way conscious of their actions towards relatives. It is sufficient merely that they act appropriately. Under natural conditions it is nearly always immature animals which are in greater need of aid than their mature relatives. For this reason we can expect the giving of aid to younger, more vulnerable individuals by their older or less vulnerable relatives to be widespread in the animal kingdom. All of the examples given in Table 7.1 would certainly fit this prediction. Can Kin Altruism Explain Human Acts of Self-Sacrifice? For most social scientists the idea that animals help their kin in the interests of shared genes by com- mon descent is not problematic. However, applying the same reasoning to our own species has led to a number of quite vociferous debates. On one side of the debate a number of social scientists have continually argued that, due to our intelligence and complex culture, our self-sacrificing behaviour is free from genetic influence (Sahlins, 1976; Harris, 1979; Bierhoff and Rohmann, 2004). On the other side, sociobiologists and, more recently, evolutionary psychologists have proposed that any genetically influenced responses which habitually lowered inclusive fitness, such as laying down one’s life for complete strangers, would be likely to be removed from the population long ago (Wil- son, 1975; Brown, 1991; Sigmund and Hauert, 2002). This does not mean that humans continually 168 Evolutionary Psychology of Social Behaviour – Kin Relationships act to increase their inclusive fitness but that we have developed the type of mind which tends to do things that aided inclusive fitness in our past and may frequently do so today (Colquhoun et al., 2020). A major question facing evolutionary psychologists is: how well does Hamilton’s reasoning fit in with current human behaviour? Many of the early studies of kin altruism involved social animals such as those described in Table 7.1. In recent years, however, despite the misgivings of some social scientists, a number of evolutionary psychologists have applied this model to examples of human social behaviour where individual actions may have a bearing on the survival of others. During the first Gulf War of 1990, for example, Shavit et al. (1994) studied altruistic behaviour with regard to air raids in Israel. They found that individuals were more likely to share air raid shelters with relatives and to call up kin to see whether help was needed. In contrast, when it came to lesser forms of aid such as giving advice about preparing for air raids, individuals were more likely to aid friends than relatives. Such a finding suggests that people may rely more on friends for companionship but, when it comes to life-threatening circumstances, charity really does begin at home (Shavit et al., 1994; Badcock, 2000). As well as making use of naturalistic observations psychologists have also begun to use social–psychological rating scales to test Hamilton’s kin selection theory in humans. Burnstein et al. (1994), for example, asked participants to make hypothetical decisions with regard to giving aid either to relatives or to non-relatives under either life-and-death conditions or where a small favour was involved. In a series of such studies, Burnstein et al. manipulated a number of char- acteristics of the hypothetical recipients such as sex, health, wealth and level of kinship. Over all of these studies participants’ decisions followed precisely the predictions made by Hamilton’s inclusive fitness theory (see Figure 7.2). Not only did they favour relatives over non-relatives but also their favouritism towards relatives was stronger under life-and-death conditions and young- er, healthier relatives were particularly favoured. Interestingly in other studies of hypothetical decisions to give aid to friends or relatives Stewart-Williams (2007) found that for low-cost aid, friends were given as much aid as kin but that when this shifted to high-cost aid, kin were fa- voured and the closer the kin relationship, the greater the aid provided (see also Roberts and Dunbar, 2011; Colquhoun et al., 2020). Tendency to help relatives Figure 7.2 Our tendency to provide aid under life- in life-and-death situations and-death situations increases markedly, as does the 3.5 coefficient of relatedness. 3 Tendency to help 2.5 2 1.5 1 0.5 closely unrelated related 0 0.00 0.125 0.25 0.50 r (coefficient of relatedness)  Charity Begins at Home 169 Adoption and Fractions – Sahlins’ Criticisms of Kin Altruism Despite these positive findings some acts of human kindness appear to run counter to Hamilton’s notion of a species of nepotistic strategists. Take adoption. If humans are designed to aid relatives then why is it that we sometimes choose to look after individuals other than our own children and even treat them as we would our own? Adoption practices have been used by critics of the evolution- ary approach to human behaviour as an example of behaviour patterns that appear to run counter to inclusive fitness theory. One such critic, anthropologist Marshal Sahlins, suggested in his influential book The Use and Abuse of Biology (1976) that Hamilton’s notion of kin altruism is fundamentally flawed. Sahlins made two main criticisms. First, he suggested that, because most hunter-gatherer cultures have not invented the fraction, then they are incapable of calculating ‘r’ and hence would be unable to act appropriately towards relatives. And second, he suggested that adoption practices of hunter-gatherer societies do not reflect patterns that kin altruism theory would predict. In particular, Sahlins claimed that the adoption practices of the islanders of the central Pacific (Oceania) are quite arbitrary rather than occurring to aid relatives. How might evolutionists deal with such claims? The first of these criticisms demonstrates a misunderstanding of how evolutionists use the notion of kin altruism (Buss, 2019). Sahlins is a distinguished social anthropologist. Unfortunately, however, his argument showed a poor understanding of the proposed relationship between natural selection and behaviour. It is not necessary for an organism to have insight into either how or why it does anything. It is merely sufficient that it generally behaves in ways that are likely to increase its inclusive fitness – and nothing more. Humans do not require an understanding of fractions in order to give aid to their relatives. As is the case for behaviour in general, the argument that people frequently favour rela- tives is one that is considered to work via heuristics (rules of thumb; see Chapter 9) rather than via a detailed analysis of how and why certain acts should be performed. A logical extension of Sahlins’ argument would have dam-building beavers understanding the principles of civil engineering and spiders understanding trigonometry in order to construct their webs! Richard Dawkins makes much the same point in his withering attack on The Use and Abuse of Biology in the second edition of his book The Selfish Gene (Dawkins, 1989). Today this argument is known as ‘Sahlins’ fallacy’ (Work- man, 2014). While Sahlins’ first criticism may be dismissed quite easily as a fallacy, his second must be taken more seriously. In Oceania around 30 per cent of children are adopted. If such adoptions really do occur arbitrarily then this would throw serious doubt on the use of kin altruism as an explanation for adoption in these societies. One anthropologist who has been more sympathetic to the evolution- ary approach, Joan Silk, decided to undertake a formal analysis of the relationship between adopters and adoptees. Using samples from 11 different Oceanic cultures, Silk uncovered support for the kin altruism argument, demonstrating that the majority of adoptees were cared for by the genetic equivalents of aunts, uncles and cousins (Silk, 1980; 1990; Silk and House, 2016; see Figure 7.3). In contrast to Sahlins’ claim, in Oceania the coefficient of relatedness typically reported between adopters and adoptees actually provides support for a kin altruism hypothesis. Adoption in the West Today – All You Need Is Love In Western society today, where the state controls adoption, most examples of this practice occur between unrelated individuals. Does this suggest that kin altruism has been jettisoned from this 170 Evolutionary Psychology of Social Behaviour – Kin Relationships 60 Figure 7.3 Coefficient of relatedness between adopted children and their adopters, based on 11 Percentage of adoptions Oceanic societies studied by Silk (1980; 1990). 40 20 0 0.50 0.25 0.125 0.06 Unknown or < 0.06 Coefficient of relatedness between adopter and adopted child process in the industrialised countries? In a sense it does. In terms of giving aid to non-relatives and expecting no reward from such adoptees then we must accept that this constitutes altruism and that such behaviour is currently unrelated to inclusive fitness. Such a finding, however, should not be taken as evidence that our adoption practices are completely unrelated to the evolutionary pressures that our ancestors faced. You may recall that one of the central tenets of evolutionary psychology is that, rather than expecting all current behavioural practices to increase inclusive fitness, selective processes have led to the development of a mind which in the ancient past would generally have done so. During our own ancient past (the environment of evolutionary adaptedness or EEA) it may have made adaptive sense to adopt young orphaned individuals since, as members of your small tribe, they would most probably have been relatives of some degree (Silk, 1980; Silk and House, 2016; Alcock, 2009). This means that today we may be left with the legacy of such urges even though they may no longer be appropriate (in an evolutionary sense). Some evolutionary psychologists have argued that the sort of mind which tends to feel sympathetic towards helpless youngsters, and develop a desire to give aid to them, is one which has arisen from a history of kin-selected altruism (Rubenstein and Alcock, 2018). We saw in Chapter 3 how an evolutionary approach to childrearing might help to explain the existence of romantic love between men and women. Steven Pinker (1997) has suggested that kin altruism may also help to explain love of family members. The essence of love is feeling pleasure in another’s well-being and pain in its harm. These feelings motivate acts that benefit the loved one, like nurturing, feeding and protecting. (400) Strong feelings of pleasure derived from nurturing younger, more helpless, individuals might be with us today because they generally aided our inclusive fitness in the ancient past in much the same way that a romantic attachment would have. Perhaps Lennon and McCartney were right when they said ‘all you need is love’ – but perhaps not for quite the reason they intended. Today a number of evolutionary psychologists consider that kin altruism may have played a major role in moulding the social behaviour of many animal species including our own. It is important to realise, however, that animals do not run around searching out more and more distant relatives in order to give them smaller and smaller slices of their investment pie. In most cases parents put effort into raising offspring because these are not only the ones with which they share a large proportion of their genes but also the most likely to be at the most appropriate stage in their life cycle.  Charity Begins at Home 171 Box 7.2 How Do Animals Recognise Kin? If we and other species do channel resources towards our relatives then this surely suggests that many species must be able to recognise just who their relatives are?* How are they able to do this? As far as our own species is concerned it is clear that all cultures are fascinated to know who they are related to and to what degree (Brown, 1991; Palmer and Coe, 2020). Language certainly allows humans to pass on such information. But how might other species recognise kin? Since the advent of the modern-day evolutionary/functional approach during the 1970s ethologists have developed and tested a number of proposed mechanisms for kin recognition. Scottish ethologist Peter Slater (1994) suggests there are four possible mechanisms that might allow for kin recognition: 1. Context-based discrimination. Here recognition is based on location. Many rodents and birds, for example, feed offspring that live in a particular nest or burrow. 2. Discrimination based on familiarity. Here ‘signatures’ (sensory cues) are used for iden- tification which are thought to be learned during development. American toad tadpoles, for example, will associate with other tadpoles that developed in the same clump of spawn and are therefore likely to be siblings. 3. Phenotype matching. In this case, unlike the above category, it is not necessary for individ- uals to have shared experiences but rather that an individual learns about its own or its close relatives’ characteristics and then uses such information to determine whether a novel individ- ual also has a similar signature. Experimental evidence suggests that mice are certainly able to discriminate between unfamiliar kin and unfamiliar non-kin. 4. Genetic systems. This category covers two related proposed mechanisms of recognition. First, genes may be used as kinship markers which produce signals that aid animals in their decisions concerning potential mates. In particular, these may include genes that are involved in the pro- duction of the molecules on the surface of body cells (the major histocompatibility complex or MHC). Experimental evidence suggests that mice prefer to mate with individuals that differ in their MHC from themselves. In so doing they may be more likely to produce offspring with particularly healthy immune systems. Second, there may be specific recognition genes – an idea originally proposed by Richard Dawkins. The idea here is that if an animal had a gene for altruism and if that same gene could also signal its presence via, in Dawkins’ hypothetical example, produc- ing say a ‘green beard’, then two related individuals might recognise such a gene from a common ancestor and act altruistically towards each other. It should be realised that the notion of recognition of genes is currently highly speculative and that such genes would really have to do three things: code for a phenotypic signal; code for recognition of such a signal; code for the correct social response (altruism). Although most genes are probably pleiotropic this is asking a lot from a gene. * Note that, in ad- dition to allowing animals to ‘know’ who to channel aid towards, a second advantage to kin rec- ognition is that it might aid in decisions over who to mate with. Breeding with close relatives is likely to lead to a number of genetic defects (although breeding with more distant relatives might actually lead to certain advantages such as increasing the value of ‘r’ in an animal’s offspring). In this way current theories concerning animal behaviour strongly suggest that kin recognition would confer at least two clear advantages to an individual. 172 Evolutionary Psychology of Social Behaviour – Kin Relationships Figure 7.4 Florida scrub jay – a species which is known to have ‘helpers at the nest’. In the 50 years since Hamilton proposed kin selection theory a large number of lab and field studies have demonstrated that social animals really do aid kin. In addition to the examples presented at the beginning of this chapter there are now a large number of well-documented cases of animals providing aid for relatives who are not their offspring. It is now known, for example, that, in addition to Florida scrub jays, there are around 200 other species of birds that have helpers at the nest (Figure 7.4). Despite myriad well-documented observations of aid to non-descendant kin, such examples pale into insignificance when compared with the extent of parental care found throughout the animal kingdom. Although, as mammals, we tend to think of parental investment as providing milk and pro- tection for offspring, the exact form of aid given varies quite extensively between species. How this varies depends on a number of factors such as the evolutionary history of the species and current ecological pressures. Parental Investment and Family Life Robert Trivers was well aware of the selective pressure on parental care. He also realised that, al- though there are many well-documented cases of organisms giving aid to relatives other than their own offspring, direct parental care is far more widespread throughout the animal kingdom. As we saw in Chapter 4, Trivers called the time and effort that individual parents put into rearing each off- spring ‘Parental investment’. Put formally, Trivers (1972) defined parental investment as  Parental Investment and Family Life 173 any investment by the parent in an individual offspring that increases the offspring’s chance of surviving at the cost of the parent’s ability to invest in other offspring. Whereas parental investment should increase the survival chances of a given offspring, in Trivers’ model this fitness benefit needs to be weighed against the cost of investing in other offspring. How Much Should Parents Invest? How much investment should we expect parents to provide for each of their offspring? There are a number of factors that need to be considered here. First, we need to consider the number of offspring typically produced by an organism’s species. Clearly the larger the number of offspring produced the smaller the investment that can be given to each individual. Oysters produce millions of micro- scopic offspring but provide each with almost nothing. Female bonobos, in contrast, produce only one offspring around every four to five years, but nurture that individual for several years. These are two quite different strategies. The first involves producing enormous numbers, the vast majority of which will certainly perish, in the hope that one or two will, by chance, make it to reproductive age. The second involves an enormous effort being put into a very small number of offspring in antici- pation that each will have a good chance of reaching sexual maturity. Ecologists call the production of vast numbers of offspring at little cost per individual r-selection and the production of very few at great cost per individual K-selection (Drickamer et al., 2002; Rubenstein and Alcock, 2018). You might recall that, for obvious reasons these two strategies are also known as fast (r) and slow (K) strategies (Pelham, 2019). In reality most organisms fall somewhere between oysters and bonobos so that fast/r to slow/K-selection is really a continuum (see Figure 7.5). Whether an organism favours the r end (produce them as frequently and cheaply as possible) or the K end (lavish great effort on each one) of the continuum depends on the ecological pressures and body plan of its species. K-selected species are adapted to stable environments and tend to live longer and have a larger body size. Since stable environments are believed to lead to strong intraspecific competition (see Chapter 2), such selection pressures favour quality over quantity in such species. K, incidentally, refers to the carrying capacity of the environment (the number of organisms that the environ- ment can sustain). In contrast, r-selected species are generally found where, due to unpredictable conditions, it may pay a parent to produce large numbers of offspring which develop very rapidly whenever conditions are conducive. In this case r is derived from the reproductive rate of the population. The strategy of birds and mammals is generally one of a great deal of parental care, that is, very much towards the slow (K) end of the spectrum. But even here we find a fair degree of var- iation. Although rodent pups are suckled by their mother they are produced in large numbers and weaned at quite a young age. The long-lived, large-brained primate species of monkeys and apes, however, have the longest period of dependency in the animal kingdom. As we have seen, our closest relatives the chimpanzees and bonobos may be cared for by their mothers for a third of their lives, during which they will learn much about the social behaviour of the troop from them and from their peers (Strier, 2016).  Oyster Tuna Frog Rabbit Lion Bonobo 500 million 6,000 200 120 3 0.2 r K Figure 7.5 r–K continuum of reproductive strategies. Here we see a range of examples from extreme egg output to extreme parental care (figures represent number of offspring produced per year, not number surviving).  Parental Investment and Family Life 175 Box 7.3 Parental Investment in Spiders – the Ultimate Sacrifice Figure 7.6 Amaurobius fenestralis. There is a family of funnel weaver spiders known collectively as the Amaurobiidae. There are five species of such spiders living in northern Europe. The Amaurobiidae are unremarkable to look at, each being around one centimetre in length. They live in holes in trees and in crevices in artificial structures. There is, however, something about their reproductive behaviour that you may find both fascinating and gruesome. Having mated in their second year of life, females of the Amaurobiidae lay their eggs in a tightly spun feeding chamber and shortly thereafter enter into a state of torpor. The spiderlings initially gain sustenance from the egg yolk that their moth- er has kindly provided for them. But this soon runs out and the voracious spiderlings leave the feeding chamber in search of more food. All they find is her torpid body. So they eat their mother. An Amaurobiidae spider really does give her all for her offspring. But how can such an excessive degree of sacrifice ever have evolved? Prior to Hamilton’s development of inclusive fitness the- ory, such extreme self-sacrifice would have been explained by many as an act which ‘promotes the good of the species’. Today most ethologists ask what is in it for the genes rather than for the species. The Amaurobiidae spiders may be an extreme example of kin-selected altruism. Most parents don’t let their offspring eat them. But this bizarre example serves to demonstrate just how far nepotistic behaviour can go given the right circumstances. In the environment of the Amau- robiidae females the chances of meeting a male are quite slim and it may make adaptive sense to breed just once and make the ultimate sacrifice for your offspring. Humans, of course, have a primate heritage. This means that, even before the evolution of Homo sapiens, our ancestors had internal fertilisation, a lengthy period of immaturity and extensive parental investment (Smith, 1987; Buss, 2019). In all human societies studied women invest more in their offspring than do their partners. As we saw in Chapter 3, evolutionists explain this difference 176 Evolutionary Psychology of Social Behaviour – Kin Relationships partly in terms of greater certainty of maternity than paternity – a sex difference that is true for all species with female internal fertilisation – and partly in terms of simply which sex is left holding the baby. Therefore two further factors which help to determine just how much an individual is likely to invest in each offspring are the evolutionary history of its species and its own sex. This means that if you are the female of a long-lived, internally fertilised species in which there is a lengthy period of development during which a great deal of learning takes place, then you are likely to invest greatly in each of your offspring. It is hardly surprising that humans, and in particular women, put so much effort into raising offspring. Parental Investment and Life History Theory Although it is well established that humans are very much on the slow/K end of the distribution, there are always limits to how much they are prepared or able to invest in any given offspring. How much investment specific human offspring receive depends, in part, on their perceived viability, how challenging the environment is and the level of resources available to the parents to lavish on that individual. According to life history theory, parents are more likely to adopt a ‘fast strategy’ if a child is born into a difficult environment where the parents are low on resources (Stearns, 1992). This, apparently heartless, proposal that humans may provide less for unhealthy offspring might be taken to suggest humans are capable of treating some of their offspring like the ‘runt of the litter’. There are two things we should bear in mind, however. First, we must not fall into the trap of the naturalistic fallacy – that is, what is morally good must therefore be naturally good. And second, ac- cording to evolutionary psychologists, humans have context dependent adaptations (not hard-wired non-contextual ones). Bugental and co-workers considered this notion that parents might adopt a ‘fast strategy’ when they have offspring regarded as unhealthy in relation to wealth and resources. They found that when wealthy parents had children with medical and psychological problems they actually increased their level of investment, whereas for poor parents the reverse was true (Bugental et al., 2013; Pelham, 2019). In the case of the former, wealthy parents spent more on, for example, medical interventions on such children. In contrast, for the latter, impoverished parents provided the least parental care to the least healthy offspring. Recently, in a study of 150 nations it was found that in those countries where parasite load is high and so is infant mortality there is a general tendency by parents to invest less in each offspring and to encourage child labour (Pelham, 2019). Moreover, in such nations teenage pregnancy is relatively high. Such a finding would lend support to the notion that parents can shift from slow to fast strategists in line with life history theory. These studies sug- gest that, under very challenging circumstances, parental investment is more similar to other species than we’d like to think. We should remember that such circumstances would have been the norm for most families during the Pleistocene. Fortunately, for most families around the world today, parents go to extreme lengths to ensure their offspring survive and thrive. Box 7.4 Are Forager Fathers More Attentive than ‘Modern-Day’ Fathers? As we saw in Chapter 2, male parental investment is generally pretty low for mammals as a class of animals. Human males, however, tend to buck this trend. Men from all cultures studied invest in their children by providing food, protection and through teaching (Pelham, 2019).  Parental Investment and Family Life 177 Box 7.4 (cont.) Interestingly, how much direct time fathers spend with their offspring varies quite considera- bly between cultures. One study, which looked at 186 different cultures, uncovered a surprising trend. Although cross-culturally all men provide for their offspring, fathers in hunter-gatherer so- cieties devote significantly more time and effort in childrearing than in other societies (including modern industrialised ones; Marlowe, 2000). This may sound surprising but is probably related to the fact that men in societies which have developed agriculture (and industry later on) spend much of their time on the farm/at the office/factory/sports ground than those in forager societies. Bearing in mind that, for the vast majority of the Pleistocene we were all hunter-gatherers, this might suggest the modern pattern of men spending relatively little of their time with their chil- dren is a recent phenomenon. Alternatively, it might also be argued that, by spending more time away at work, men also contribute to the survival of their children. Figure 7.7 Young father with son from the Erbore (Arbore) tribe, Ethiopia, Africa. Grandparental Investment – the Grandmother Hypothesis Of course parents are not the only family members who expend time and effort in helping to ensure children survive and thrive. Grandparents, across all societies, invest in their grandchildren (Pelham, 2019). Perhaps surprisingly, also across all cultures, grandmothers invest more than do grandfathers. 178 Evolutionary Psychology of Social Behaviour – Kin Relationships Moreover, maternal grandmothers invest more than paternal grandmothers. We might ask how did this curious arrangement arise? Evolutionary psychologists believe they have the answer. It’s all to do with the evolution of the menopause and paternal confidence. It’s worth considering each before putting them together. Beginning with the menopause first – humans are one of only three species where females have a lengthy period of post-reproductive life (the other two are all species of toothed whales; Photopoulou et al., 2017). The fact that many women now spend their last 40 years without reproducing might appear to be both counter-intuitive from an evolutionary perspective and at odds with life history theory. This apparent conundrum disappears, however, when we consider that inclu- sive fitness theory factors in indirect reproductive success. So, just as an aunt or uncle can boost their inclusive fitness by investing in nephews and nieces, so too can a woman who shifts her investment to her grandchildren. To evolutionists this has become known as the grandmother hypothesis (Hawkes, 2016; Ray, 2013). But why should women shift to this strategy at a certain age? Why not simply re- produce until the end of their days? Given that pregnancy and childbirth are so arduous for human fe- males and, given that a large proportion of women living into ancient old age is a recent phenomenon, it is likely that those hominin females who shifted from maternal investment to grandmaternal invest- ment ultimately passed on more copies of their genes than those who continued to reproduce into old age. In other words, there is little point in having babies if you are unlikely to be around to support them for long. Note this means that under the appropriate circumstances, stopping having babies can be a better evolutionary strategy than continuing to do so. Producing a baby at 55 and dying at 56 would not have been a good way to boost your inclusive fitness. Hence at some point in our ancestry the menopause was selected for and women began to develop an increasingly long post-reproductive lifespan where they could provide vital aid to their adult offspring and their grandchildren. With regard to paternal confidence, whereas a female knows that each offspring she pro- duces is hers, her male partner can never be certain it is his. In other words only males can be cuckolded. The precise rate of non-paternity is difficult to determine since it does not pay women to admit to this, but estimates for industrialised societies suggest the figure is around 3 per cent (Anderson, 2006). Three per cent is considered large enough to allow for selective pressure to occur. This uncertainty can be extrapolated back down the male line so that, while a mother’s mother has to be related to any grandchildren produced, a father’s father has two steps of uncertainty. It is this level of uncertainty in the paternal line that is believed to lead to a lowering of investment by grandfathers (and in particular paternal grandfathers) when compared with grandmothers. By the same logic, we can also predict that paternal grandmothers will invest less than maternal grandmothers since they have one step of uncertainty. This means we can predict that the amount of investment will decline along the path of: Maternal grandmother > maternal grandfather > paternal grandmother > paternal grandfather So much for the theory. The question is, does the empirical evidence support the grandmother hy- pothesis? In fact the evidence is quite strongly supportive. Finnish biologist Mirkka Lahdenperä and her team examined a large database of the births, deaths and marriages of 500 Finnish women living in various fishing and farming communities between 1702 and 1823 (Lahdenperä et al., 2004). Through meticulous examination of these records they uncovered three findings which support the grandmother hypothesis: 1. Having a living mother enabled a woman to begin having her own children at an average age of 25.5 compared to 28 for women who had lost their mother.  Parental Investment and Family Life 179 2. Having a living mother shortened the interval between births for their daughter. 3. Having a living mother under the age of 60 increases the survival rate of her daughter’s children by 12 per cent. Bearing in mind that in Finland during this period only around 50 per cent of children survived to adulthood, each of these benefits provided by grandmothers would most likely have given their daughters a selective advantage. This, of course, is informative concerning the importance of grandmothers’ investment, but it tells us nothing about the notion of grandmothers investing more than grandfathers due to uncertainty of paternity confidence. In order to do this we need to have indications that investment does follow the descending order of MGM>MGF>PGM>PGF (see above). In fact another large- scale Finnish study examined just this question. Making use of a data set of more than 20,000 cross-generational individuals (gathered as part of a European-wide health in retirement survey) Danielsbacka and co-workers of the University of Helsinki found the amount of time grandparents spent looking after their grandchildren follows precisely this predicted order (Danielsbacka et al., 2011; see Figure 7.8). You might, at this point, be thinking, yes but women in general provide more care for children than men and surely this needs to be factored in. This would be entirely true, and while it might partially explain the MGM>MGF and even the PGM>PGF, it does not however account for the MGF>PGM. That is a grandfather investing more time and effort into the grandchildren than a grandmother. That is the acid test which supports the notion that the level of investment is attenuated by the potential for non-paternity. In ending this section on the grandmother hypothesis, it’s worth noting that it is based both on kin selection theory (grandparents provide aid to kin other than direct offspring) and on parental investment theory (individuals attenuate their level of investment depending on level of relatedness and on certainty of that relatedness). 0.25 MGM MGF 0.20 Look after grandchild PGM PGF 0.15 0.10 0.05 0.00 Unadjusted Adjusted (n=21,817) (n=20,769) Figure 7.8 A comparison of help provided by four different categories of grandparent to their grandchildren. Note that the amount of help given follows the pattern of maternal grandmother (MGM) > maternal grandfather (MGF) > paternal grandmother (PGM) > paternal grandfather (PGF), supporting the notion that paternal uncertainty leads to a lower level of investment in the male line. Adjusted figures take into account factors such as health problems and distance from grandchildren and led to the removal of around 5 per cent of the sample. Note the pattern of response is retained. 180 Evolutionary Psychology of Social Behaviour – Kin Relationships The Family – the Result of Parental Investment? Most social scientists explain the existence of families in terms of historical and economic factors, but evolutionary psychologists explain it as the result of two main dispositions: dispositions to mate and dispositions to favour kin (Smith, 1987; Palmer and Coe, 2020). Canadian evolutionary psy- chologist Martin Smith suggests that human families exist in order to enhance the inclusive fitness of their members. Following a theme initiated by Richard Alexander (1974; 1980), Smith goes so far as to suggest that humans are ‘genetically disposed to form families that display several common features’ (Smith, 1987, 231). The problem with this claim is that although human families exist as a universal phenomenon they do vary in structure quite considerably. Some families are built around a polygynous marriage with one father and several mothers, others are built around a monogamous couple, and, in a few cultures, there may even be two fathers and one mother – a polyandrous mar- riage system. Smith’s response to this claim of variability is to argue that, cross-culturally, similar- ities are greater than differences. Such similarities include greater parental investment by women than men, differential investment in offspring depending on their projected reproductive potential (Bugental et al., 2013) and a tendency for women to marry men with the greatest resources. Where differences do occur they are seen as the outcome of an interaction between innate dispositions and differing social pressures. But in all human societies mothers invest heavily in their offspring and, as we have seen, most fathers do also. For humans such investment is not simply a matter of feeding. It also involves a great deal of protection and, importantly, teaching (Stearns, 1992). One way that Alexander (1974) suggests parents might be able to increase their inclusive fitness is by teaching appropriate moral codes to their offspring. In particular, he has suggested that, as a consequence of Trivers’ theory of parent–offspring conflict, parents will tend to teach their children to behave be- nevolently towards each other. Alexander calls this parental manipulation, since it is the parents’ genes that will benefit from such cooperative behaviour. Box 7.5 The Cinderella Effect – the Downside to Parental Investment? Controversially, Canadian evolutionary psychologists Martin Daly and Margo Wilson have sug- gested that there may be a downside to parental investment theory when considering the treat- ment of stepchildren. Making use of epidemiological data from Canada, the United States and the United Kingdom, Daly and Wilson have spent more than 30 years investigating the ‘Cin- derella effect’, that is, the notion that step-parents lavish less care and attention on stepchildren than on their own biological offspring (Pelham, 2019). While they are clear in their findings that most stepchildren are not mistreated by their step-parents they have uncovered evidence that the risk of maltreatment is distinctly higher from such ‘non-biological’ parents. Drawing on Trivers’ parental investment theory, Daly and Wilson suggest that parental care is regulated by discrim- inative parental solicitude built around psychological adaptations to allocate resources to their genetic offspring. This means parents are less likely to guide resources to their stepchildren than their genetic ones and more likely to neglect or abuse the former than the latter. Examples of their findings include the fact that infanticide by stepfathers is 120 times higher for step than for genetic offspring (Daly and Wilson, 1998). Moreover, a child under the age of three living in a family with one step-parent is seven times more likely to be abused than one living in a family  Parental Investment and Family Life 181 Box 7.5 (cont.) with two biological parents. To Daly and Wilson the greater likelihood of such maltreatment of non-genetic offspring may be a remnant of the male primate adaptive strategy of killing the offspring of other males in order to increase the possibility of fertilising a female. The Cinder- ella effect has been questioned by critic of evolutionary psychology David Buller (2005a) who argues that abuse by genetic parents is likely to be underreported to the point where Daly and Wilson’s findings can be overturned. Unfortunately for Buller this and a number of other disput- able assumptions weaken his criticism (Daly and Wilson, 2007) and a number of independent studies appear to support the findings of the Canadian evolutionists (Anderson, 1999; Tooley et al., 2006). Of course this difference in levels of investment in genetic and non-genetic offspring does not prove that this is an adaptation (or even a remnant of a primate adaptation). But it does appear to be a cross-cultural phenomenon making a purely cultural explanation less likely (Daly and Wilson, 1998; 2005; 2007; Pelham, 2019). Parental Manipulation – Parents Help to Mould the Social and Moral Behaviour of Their Offspring In the West we typically reward our children when they are polite, hardworking and law abiding. Smith and Alexander would argue that this is the best way for children to gain lucrative employ- ment and a high-quality partner. From an evolutionary perspective Smith views ‘these end points of socialization as intermediate steps in achieving the real goal of parenting: the maximization of inclusive fitness through the maximal production of grand-children’ (Smith, 1987, 235). In other societies children might be socialised to take on quite different moral codes. In the Yanomamö of South America, for example, tribal warfare is common and 44 per cent of men have killed another person (see Chapter 8). Killing is seen as a sign of manhood and those who have killed have on aver- age two and a half times as many wives and three times as many children as non-killers. Pacifists are not common in Yanomamö society – pacifists don’t get wives (Chagnon, 1988; 1992; 1997; 2012). It is not surprising to find that in this culture boys are socialised to meet conflict with an aggressive response. This is not to say that the Yanomamö have a totally different, heartless code of morality or that males in the West are not capable of acting violently. In all societies studied males demon- strate higher levels of aggressive behaviour than females (Daly and Wilson, 1994; Archer, 2019). Biologists attribute this, at an ultimate level, to competition for females and at a proximate level to physiological factors such as raised levels of testosterone relative to females (Daly and Wilson, 1994; Gorelik and Shackelford, 2012; Liddle et al., 2012). The point is that, under the social and economic conditions of the Yanomamö, it may be a good strategy to bring up your sons to be com- petent in the use of violence (a good strategy, that is, if you want grandchildren). In this way aspects of what is socially acceptable in an individual may be the outcome of an interaction between genetic dispositions and parental manipulation to fit in with societal norms. Whether children are brought up to be polite and law abiding or tough and uncompromising, in either event evolutionary theory suggests that parents should attempt to manipulate their offspring to value their siblings as much as they value themselves (Palmer and Coe, 2020). This does not always bode well for a peaceful family atmosphere. 182 Evolutionary Psychology of Social Behaviour – Kin Relationships Parent–Offspring Conflict ‘Blood is thicker than water’ is a phrase commonly used when family members want to pressurise an individual into favouring relatives over friends. Certainly, the extent to which family members will pull together under conditions of adversity can be astonishing and impressive (Palmer and Coe, 2020). Evolutionists consider that they have explained, at an ultimate level, why this state of affairs arose using inclusive fitness theory. However, anyone who has ever lived in a family will recognise that this picture of domestic bliss is rarely a constant one. Personal experience tells us that how harmonious a family is varies greatly from one family to the next. But rare indeed is the family that never quarrels. If Trivers, Alexander and Smith are right that the very existence of the family arises out of increased opportunities for kin altruism to occur, then why is family conflict so common? Social psychologists traditionally explain conflict between parent and offspring as the mal- adaptive effects of poor role models and as the outcome of a lack of conflict resolution strategies (Straus, 1971; Straus et al., 1980; Deaux and Wrightsman, 1983). In short, conflict is seen as a dysfunctional state of affairs. Evolutionary psychologists, however, see some conflict as a natural outcome of inclusive fitness theory. This is because at the very heart of Trivers’ theory of parental investment there exists a contradiction. The same theory that predicts benevolence based on the coefficient of relatedness, paradoxically, also predicts periods of serious conflict between family members. Shortly after he suggested that parental care should be viewed as an investment in copies of an organism’s genes, Trivers realised that natural selection operates differently on the two genera- tions involved. When the parent of a K-selected species, say an ape, gives birth to a single offspring then clearly it pays both her and her offspring for her to invest heavily in it. Since they share 50 per cent of their genes by common descent (r = 0.5) we can expect the mother to suckle her newborn offspring and generally provide a great deal of care for her. The problem arises when we reach the point of weaning: the point at which a female is ready to breed again. As the infant grows she be- comes better able to feed and fend for herself. Eventually there will come a time when it would pay the mother ape to produce another offspring and transfer her investment to him. For the mother this occurs at the point when the cost in terms of fitness exceeds the benefit. Because the first offspring shares 50 per cent of her genes with her newborn brother (assuming they have the same father) she should not be indifferent to his welfare. However, in inclusive fitness terminology, we can think of her as sharing 100 per cent of her genes with herself. This means that it will benefit her to have her mother continue to invest in herself right up until the point when the benefit to the new sibling is at least twice the cost to herself. We can see that the point when it would pay the mother to shift investment to the new offspring comes at a much earlier time for the mother than it does for the older offspring. Hence we can predict parent–offspring conflict to happen regularly at certain times in development (Triv- ers, 1972; 1974). Moreover, in addition to conflict with the mother we can also predict that there will be rivalry between siblings during this period and, to a lesser extent, beyond (except in the case of identical twins, siblings always ‘share more genes with themselves’ than with each other). Using Trivers’ way of thinking about development of social behaviour it might be argued that, although it may pay siblings to provide aid for each other when compared to non-relatives, invest- ment by the parents in one offspring can always be seen as investment taken away from another. Rivalry may be most acute when a newborn child appears and draws investment away from the  Parent–Offspring Conflict 183 Figure 7.9 Redrawn Trivers’ model of parent-offspring conflict. Cost to mother/Benefit to offspring 3 The graph demonstrates the ratio of the benefit to the offspring and the cost to the mother in relation to offspring age. 2 1 Conflict Young Old Age of offspring older offspring – but throughout life it may pay offspring to gain a little more investment than their siblings. Not perhaps to the extent that they would generally see their siblings expire – they do after all share 50 per cent of their genes – but certainly they are unlikely to favour them over themselves. Figure 7.9 demonstrates Trivers’ model of when parent–offspring conflict should take place. In the first section of the graph (bottom left) you will see that when an offspring is young, the ratio of the cost to mother to the benefit to offspring is low (less than 1). Remember that the cost to the mother is a measurement of her not investing in another offspring. However, as the offspring ages the cost to the mother increases faster than the benefit to the current offspring. Between 1 and 2 it makes sense for the mother to invest in another offspring, while it still pays the current offspring to reap the benefit. This is the period of predicted parent–offspring conflict. Beyond 2, when the cost to the mother is more than twice the benefit to the offspring then it pays both to allow her to invest in another offspring. Conflict at Times Other than Weaning This model of conflict over weaning may appear to be rather esoteric. The question is, does the predicted discord occur in the natural world? And does it do so at the predicted time? As with Hamilton’s theory of kin selection, when Trivers proposed his parent–offspring model it made sense on paper but the acid test is: does it work like this in the real world? In recent years, evi- dence has accumulated that this is what happens in the real animal world. Conflicts over weaning have been well known to livestock farmers for centuries, but until Trivers proposed his model it had not occurred to anyone why in ultimate terms this conflict should be so intense at this stage. Recent observations suggest that Trivers’ model works quite well. Evolutionary psychologists Robin Dunbar and Louise Barrett of the University of Liverpool have studied this conflict and see it occurring in a wide range of species. Tantrums are used by a wide variety of infant animals including pelicans, starlings, baboons, rhesus macaques, zebra and chimpanzees as an attempt to increase maternal investment (Barrett and Dunbar, 1994; Barrett et al., 2002; Maestripieri, 2004; Goodman et al., 2012). And the conflict appears to reach a peak at the time of weaning as the mother increasingly rejects attempts from offspring to be fed from her and begins to redistribute her efforts towards breeding once more (Hinde, 1977; Barrett et al., 2002; Goodman et al., 2012). 184 Evolutionary Psychology of Social Behaviour – Kin Relationships The time course for this rejection fits in well with Trivers’ prediction. To date, however, this area has not been addressed in any detail for human mother–infant relationships. In contrast to the lack of studies around the time of weaning, there is now good evidence that the intense conflict that was predicted by Trivers may also occur both much earlier and much later on than this for our species (see below). Conflict at Puberty – Who Should Reproduce? Evolutionary theory predicts that parents should want to see their children grow up to make them grandparents. However, a mother might not be keen on seeing her daughter make her a grandmoth- er too soon. This may often be the case because an immature girl may not make an ideal parent. It might even be the case due to vanity on the part of the older woman. But there is another, more gene-centred, argument as to why a mother might not want to become a grandmother too soon. Evolutionary anthropologist Mark Flinn has suggested an extension of Trivers’ theory of parent– offspring conflict which may occur at the time when a daughter first reaches puberty (Flinn, 1989; 2011; Quinlan et al., 2003). The problem arises essentially for the same reason that it occurs at the point of weaning – in terms of boosting inclusive fitness, there may be a cost/benefit asymmetry between mother and daughter. The scenario goes like this. Imagine that you are a thirtysomething mother with a teenage daughter living at home. She may now be of reproductive age. But so are you. Given limited resources, who would you rather has a baby – yourself or your daughter? Using inclusive fitness theory you should choose yourself because every offspring you produce will have 50 per cent of your genes by common descent whereas every offspring that your daughter produces will have only 25 per cent of your genes by common descent. At this stage in life it does not make sense to shift strategy to the role of grandmother. Of course, once a woman is beyond reproductive age herself then the conflict should vanish since it is in the interests of both generations for the daughter to reproduce. Once again, as in the theory over weaning conflict discussed above, this model may sound rather theoretical and not necessarily a ‘real-world’ scenario. There is, however, at least circumstan- tial evidence that supports this hypothesis. Flinn has reported that in Trinidad agonistic encounters are significantly more frequent between a mother and her teenage daughter if the older woman is still of reproductive age. Moreover, Flinn has also found that no girl of reproductive age in this so- ciety became pregnant while living with her mother until the mother’s last-born child was at least 4 years old. Such a finding suggests that there may be a reproductive suppression mechanism, which is in some way controlled by the mother – perhaps via pheromones. If this is the case, then we might question why the daughter has not fought back and evolved a way of ignoring such a means of suppression. The answer may be that it might not pay her to do so. The reason is that there is an asymmetry built into the equation. Although the mother has a coefficient of relatedness, or ‘r’ (see above) of only 0.25 with her grandchildren, the daughter has an r of 0.5 with her siblings (assuming the same father). This means that while her mother is still of reproductive age it may not matter to her whether it is she or her mother who reproduces. In fact, given the inexperience and lack of personal resources during the teenage years, it might often be a better strategy for the daughter to delay her personal reproduction and play more of a ‘helpers-at-the-nest’ role until both she and her mother are older.  Parent–Offspring Conflict 185 Box 7.6 Conflict in the Womb – an Arms Race of Raging Hormones Since Robert Trivers originally suggested that parents and offspring should not always live in perfect harmony the conflict that he predicted has cropped up in some surprising places. In 1993 evolutionary geneticist David Haig even suggested that this disharmony might occur prior to the birth of the infant. Extending Trivers’ argument back to this prenatal stage of development, Haig has argued that, up to a point, we should view the foetus as a parasite and the mother as host where resources are concerned. Up to a point, that is, because clearly it is in the interest of both parties for the foetus to be born. However, the mother would like the foetus to appear without taking too much in the way of nutrients out of her so that she will be able to reproduce again as soon as possible. Haig has documented two main ways in which the foetus and mother compete for resources. First, foetal cells invade the artery that supplies blood from the mother to the placenta, destroying the muscle cells there, which would otherwise allow the mother to control constriction of this artery. This then allows the foetus, via release of its own hormones, to divert more of the mother’s blood to itself. A knock-on effect of this is a rise in blood pressure as the foetus grows and demands more blood – in some cases leading to the potentially life-threatening condition of pre-eclampsia. Second, there is a battle over blood sugar between mother and foetus during the latter stages of pregnancy. In the three months prior to parturition the foetus directs the placenta to secrete increasing quantities of the hormone human placental lactogen (hPL) into the mother’s blood. This counteracts the effects of insulin that the mother is producing in larger and larger quantities in order to direct blood sugar into her own cells. In a sense, there is an arms race over the sugar supply with both sides secreting hormones in order to counteract the sugar-grabbing hormones of the other side (Haig, 1993; 2002; 2014). Interestingly (and just to complicate matters), there is now evidence that growth of the embryo and placenta are under control of paternal rather than maternal genes – a process known as genomic imprinting. So, like a general who oversees the battlefield without taking part directly in the action, the father may also be involved in this arms race. Although the foetus appears to be winning the arms race for resources in a number of ways, it does not have it all its own way. Haig has accumulated evidence that a large proportion of embryos (30–75 per cent) may be spontaneously aborted in the first two weeks of development and that this process occurs when the mother’s body, picking up on chemical cues, decides that it is of low quality (Haig, 1998; 2002). The proposed mechanism for elimination of the embryo is via changes in levels of the female sex hormone progesterone. If a pregnancy is to progress, then high levels of progesterone are required. Progesterone is produced by the egg follicle or corpus luteum in the ovary after the release of an egg. Normally, progesterone production in the corpus luteum declines after two weeks as the follicle begins to break down. If there is a growing embryo in the uterus it is necessary for it to release a hormone at this point – human chorionic gonadotrophin (hCG), which has the effect of maintaining the corpus luteum and hence progesterone production. Haig has suggested that it is those embryos not able to produce sufficient quantities of hCG that are eliminated from the mother’s body since they do not stand up to her quality control procedure. Perhaps the mother, via her hormones, may have ultimate control during the early stages, but as the foetus grows, like most parasites, it slowly begins to gain a controlling influence. 186 Evolutionary Psychology of Social Behaviour – Kin Relationships This means that reproductive suppression by the mother might partially suit both parties. Note that from this argument we can make at least two predictions. First, we can suggest that household agonistic encounters are more likely to be initiated by the mother than by the teenage daughter, and second we can predict that there should be fewer of these encounters involving teenage sons than daughters. As an overall prediction we might also suggest that mothers will gen- erally be more involved in family conflicts than fathers. This last prediction might be complicated, however, by questions of uncertainty of paternity in some cases. Such hypotheses remain largely to be tested. Did Families Evolve to Maximise Inclusive Fitness? In the eyes of evolutionists such as Trivers, Alexander, Smith, Dunbar, Barrett, Daly, Wilson and Flinn, families exist ultimately to boost the inclusive fitness of their members. Family units may increase opportunities for the creation and care of children. Moreover, they increase nepotistic op- portunities and allow parents to manipulate their offspring into conforming to the type of moral behaviour which, given the societal norms, is likely to lead to the production of grandchildren – at the appropriate time. But does this knowledge inform us about human social behaviour? A knowledge of the- ories of kin selection, parental investment and parent–offspring conflict suggests that from details of family size, sex and age of offspring we will be able to predict when parents are likely to have acute periods of conflict with their offspring and when sibling rivalry is most likely to be a particular problem. When asking questions concerning the relationship between pro- and antisocial behaviour within families with regard to boosting inclusive fitness, however, we should ensure that we are clear whose inclusive fitness we are discussing. Perhaps, in human families, some conflict is almost inevitable. Evolutionary psychologists are currently arguing that humans have evolved psychological mechanisms that help us to understand and predict family interactions. Much of our social life, however, involves interactions with non-relatives and many of the decisions that we have to make in- volve them. Can inclusive fitness theory also be of benefit here? We turn to this and other questions with regard to social behaviour between non-relatives in the next chapter. Summary Evolutionary psychologists have used the concepts of inclusive fitness theory, evolved psycholog- ical mechanisms and kin altruism to help explain social behaviour in ourselves and other species. Inclusive fitness is an estimate of the number of genes that individuals pass on both directly via their own offspring and indirectly via their effects on the survival of other kin. Kin altruism (also known as nepotism) is the term used for self-sacrificing acts towards kin. The tendency to provide aid to relatives appears to be related to the proportion of genes shared by common descent (i.e. the coefficient of relatedness – ‘r’). Although aid to other relatives is well documented in the animal kingdom, most examples of aid consist of parents providing care for  Questions 187 their own offspring. Today evolutionists explain many acts of social behaviour in animals in terms of nepotistic strategies. Parental investment consists of the amount of time and effort that an individual puts into rearing each of its offspring. In some species a large number of offspring are produced but very few re- sources are provided for each. Such a strategy is known as ‘r-selection’. In others a great deal of effort is spent on each of a small number of offspring, a strategy called ‘K-selection’. Within the context of human life history theory, these are also known as ‘fast’ and ‘slow’ selection. Today evolutionists see r to K as a continuum that ranges from little investment to a very great deal. Humans and other primates are at the extreme end of K-selection. The grandmother hypothesis is the notion that the menopause came about because, by shifting their investment from offspring to grandoffspring, a woman can increase her inclusive fitness. Due to the possibility of non-paternity, paternal grandparents are predicted to provide a lower level of investment than maternal grandparents. As a direct consequence of parental investment theory, siblings are predicted to be in conflict with their parents at certain times in their lives. Such parent–offspring conflict is predicted because par- ents may want to divide resources more equally among their offspring than each offspring would ideally like. This conflict may be most acute at three particular points in the development of the family unit. First, there may be conflict between the foetus and the mother during gestation as both compete for limited resources. Second, the mother may want to wean the offspring at a time earlier than the offspring would prefer in order to shift investment towards future offspring. Finally, there may be conflict between a mother and her teenage daughter over who should reproduce if the mother is still of reproductive age. Questions 1. Mark Flinn has predicted that there will quite often be conflict between a woman who is still fertile and her teenage daughter. Based on the material in this chapter, can you think of other reasons for conflict between offspring and parents during this time in life? 2. According to Alcock (2009), ‘bridewealth’ (where a man is expected to donate resources such as cattle or money to the family of his fiancée) occurs in 66 per cent of societies studied by anthro- pologists, and yet in only 3 per cent of societies is a ‘dowry’ (where the bride’s family must pay the groom’s family) the norm. Why might this be the case? What features of a society might help to determine whether bridewealth or dowry payment becomes the norm? 3. The female funnel weaver spider Amaurobis fenestralis lays its life down for its offspring. How might this behaviour be controlled, and how might it have evolved? 4. Under the ‘Cinderella effect’ it was suggested that the greater likelihood of maltreatment of non-genetic offspring may be a remnant of the male primate adaptive strategy of killing the off- spring of other males in order to increase the possibility of fertilising a female (see Box 7.5). How might we gather evidence to support or refute this proposal? Can you think of any problems with this proposal? 188 Evolutionary Psychology of Social Behaviour – Kin Relationships Further Reading Hewstone, M., Stroebe, W. and Jonas, K. (eds.). (2015). Introduction to Social Psychology. Oxford: Blackwell. General multi-author text which covers all of the core topics of social psychology. Trivers, R. L. (1985). Social Evolution. Menlo Park, CA: Benjamin/Cummings. A classic work on the relationship between social behaviour and evolution by one of the major figures in the development of sociobiology and subsequently evolutionary psychology. Zeigler-Hill, V., Welling, L. and Shackelford, T. (eds.). (2015). Evolutionary Perspectives on Social Psychology. New York: Springer. High-powered multi-author text that calls for greater integration of evolutionary principles into social psychology.