The Evolution of Human Mate Choice PDF

4 The Evolution of Human Mate Choice Key Concepts sexual strategies theory mate guarding male provisioning hypothesis last common ancestor male parental investment cryptic oestrus sexual dimorphism polygyny polyandry reproductive value sperm competition...

4 The Evolution of Human Mate Choice Key Concepts sexual strategies theory mate guarding male provisioning hypothesis last common ancestor male parental investment cryptic oestrus sexual dimorphism polygyny polyandry reproductive value sperm competition sexy sons Coolidge effect What does a man look for in a woman? What does a woman look for in a man? To put it more crude- ly, what criteria do people use when looking for a sexual partner? In Chapter 3 we considered sexual selection theory, that is, how morphological and behavioural differences may have come about by enhancing reproductive success. In order to have reproductive success humans need to make deci- sions about mates. Received wisdom suggests that men and women differ in their predilections for partners. Women want generous, emotionally stable and dependable men, and men – well, men want sexy women. The question is, does the evidence fit with this common-sense view and, if so, where did these differences come from? In this chapter we use sexual selection theory and evidence from other primates to explore the notion of mate choice, its roots and its consequences. Sexual Strategy Theory: Testing the Claims of Evolutionary Psychologists Two of the main claims made by evolutionary psychologists are that there is a human nature and that ecological and social pressures of the past were responsible for the evolution of that nature (Pinker, 2002). If this is the case then it might be argued that mate preferences today reflect the decisions of our ancestors. Over the last 30 years David Buss and his co-researcher David Schmitt have been de- veloping a theoretical perspective known as sexual strategies theory to help explain human mating psychology. To Buss and Schmitt humans have evolved a collection of psychological adaptations to respond to ancient recurrent mating challenges (Buss and Schmitt, 1993; 2019). Sexual strategies the- ory is based around the notion that people who made poor mate choices in the ancient past were less likely to pass on their genes than those who made good choices. Sexual selection should, therefore, promote ‘good’ mating choice strategies. According to Buss and Schmitt there are two broad adaptive challenges; making fitness-enhancing mate choices and outcompeting rivals to fulfil these choices. This raises the question, given that it is impossible to study the evolution of behavioural patterns directly, just how do evolutionary psychologists explore the relationship between sexual selection, mate choice and human behaviour? One way is to compare ourselves with our primate relatives that shared a common ancestor with us. Comparing different species in order to add to our knowledge of anatomy, physiology or behaviour is called the comparative method. When behaviour  Origins of Human Mate Choice: The Social Behaviour of Our Relatives 81 patterns differ between closely related species then it might be argued that these differences can be traced back to differing ecological pressures (Clutton-Brock and Harvey, 1977; Davies, Krebs and West, 2012; Ryan, 2018). Where behavioural patterns are similar it may suggest that the responses are ancient and might be traced back to a common ancestor (Wrangham, 1987). We will make use of the comparative method to examine human mating behaviour. A second way in which we can study the evolution of human mate choice is by examining the degree to which such responses are common to separate human cultures. If they are the product of evolution then we might expect to see more similarities than differences. Over the last 30 years evolutionary psychologists have compared mate choice preferences in a variety of cultures with particular regard to the extent to which sex differences in mate choice are universal. We can think of these studies as examinations of the consequences of sexual selection on humans. We will consider these consequences after first considering the possible evolutionary roots of these responses by contemplating the behaviour of our relatives. Origins of Human Mate Choice: The Social Behaviour of Our Relatives Given that we share a recent evolutionary past and hence a large proportion of our genes with other primates, perhaps an understanding of their social and sexual behaviour will provide us with a clue as to how our ancestors lived. It might also give us some insight into the evolution of human sexual strategies. A number of species of primates have been used as models for human behaviour includ- ing common chimpanzees (and their relatives the bonobos), gorillas and baboons. There are two species of chimpanzee and these vary in behaviour both between species and between troops within a species: the common chimpanzee Pan troglodytes, of which there are several sub-species and their cousins the bonobos or pygmy chimps (Pan paniscus). The two species of gorilla – eastern and west- ern – each have two sub-species giving us Gorilla beringer graueri and Gorilla beringer beringer (both eastern sub-species) and Gorilla gorilla diehli and Gorilla gorilla gorilla, respectively (both western sub-species). Baboons comprise five species, all under the genus Papio. The social and reproductive behaviour of each of these primate genera is considered briefly below. Millions of Years Ago Figure 4.1 Diagram of primate 35 30 25 20 15 10 5 0 evolutionary tree, showing all of the primates mentioned in Humans this chapter. s) Chimpanzees m an Bonobos Hu & Gorillas p es (A ds nioi Orang–Utans mi Ho Old World Primates Old World Monkeys Baboons Ne wW Macaques orl dP rim ate s Capuchins Muriquis 82 The Evolution of Human Mate Choice Chimpanzees Pan troglodytes Chimpanzees share over 98 per cent of their genes with humans and current thinking suggests we shared a common ancestor with them some time between 6 and 7 million years ago (Dunbar, 2004; Strier, 2016; Gruber and Clay, 2016). Chimps, like us, are highly social, primates that form groups of between 20 and 100 individuals. They have a fission–fusion society, which means that they fre- quently divide up into smaller groups of perhaps ten or fewer individuals while foraging and then recombine when a large food source is discovered (such as a tree that has just come into fruit). When this happens individual chimps often make ‘pant-hoot’ calls that bring others from afar. Males make up the permanent core of their society, forming a linear dominance hierarchy while females frequently leave to join another troop upon reaching sexual maturity (Goodall, 1986; Strier, 2016). The top-ranking male maintains his rank both through aggressive displays and through forming coalitions with other high-ranking males (often, but not exclusively, brothers). As the males have a strict dominance hierarchy most disputes are resolved by threats rather than physical attacks but these do occur at times. In the 1970s Jane Goodall documented a four-year ‘territorial war’ between two troops that only ended when one troop killed all of the mature males in the other troop. Male chimps have also been known to kill infants – an act that is believed by some to occur in order to bring a female back into oestrus more rapidly and ensure that any new offspring are likely to be those of the infanticidal individual (behaviour that is not uncommon among primates; Strier, 2016). In some troops it is not unheard of for females to kill another female’s offspring – behaviour which is rare in other primate species (Pusey and Schroepfer-Walker, 2013). Mating patterns vary between troops but it is generally the case that dominant males do not have exclusive rights to receptive females, unlike their cousins the gorillas (although this can happen Figure 4.2 An alpha male chimpanzee (Pan troglodytes) grooms the eyebrow of another member of the group. Kibale forest, South Uganda.  Origins of Human Mate Choice: The Social Behaviour of Our Relatives 83 in some troops). When in oestrus, a female becomes distinctly attractive to the mature males, a number of whom will then mate with her. Oestrus is the time in a female’s sexual cycle when mating is most likely to lead to fertilisation and is signalled via sexual swelling and reddening of the large areas around the vagina and anus (the perineum). Although oestrous females will generally mate with a variety of males, it is the more dominant members, and in particular the alpha male, who usually mate with her most frequently especially around ovulation (thereby increasing the chances of fathering offspring). For female chimps this period is relatively brief lasting only a matter of days. Despite being largely frugivorous (fruit-eating), chimps do sometimes engage in group hunting of small mammals – an activity which is largely, but not exclusively, the preserve of males (Gruber and Clay, 2016). On occasions when a male has made a kill he may offer some meat to other males that helped in the hunt but he may also offer some to a fertile female (Kuroda, 1984; Goodall, 1986; Stanford, 1998; 1999). Although it is a contentious area, some authorities have suggested that the giving of meat to fertile females increases the dominant male’s chances of mating with her and while some anthropologists dispute this meat-for-sex hypothesis (Gilby et al., 2010), recent studies do provide some support for it (Gomes and Boesch, 2009). Hence, since the more dominant males are most likely to be the successful hunters, rank might indirectly provide reproductive benefits (Stanford, 1995; Gomes and Boesch, 2009; see also Mitani and Watts, 2001). If rank order and hunting prowess are useful indicators of good genes for females and if females are able to choose which males they mate with, then we might ask: why do they mate with a number of males? Why not simply mate with a single dominant male? Perhaps this ‘sharing of sexual favours’ is a female strategy to reduce the possibility of infanticide and allow for sperm competition (Birkhead, 2000). Bonobo (Pygmy Chimpanzees) Pan paniscus Bonobos differ in their genetic material from humans by approximately 1.3 per cent, but they differ from common chimps by only 0.4 per cent (their genome was finally sequenced in June 2012; Prüfer et al., 2012). It has been suggested that, since they live on either side of the Congo river (bonobos on the south side and common chimps to the north), the separate species evolved when an ancient Pan population crossed the river during a time of drought around 1.5 million years ago (Takemoto et al., 2015). Despite its name, the pygmy chimpanzee, or bonobo, is not actually smaller than the com- mon chimp but is a little slimmer and more gracile (fine-featured). Like common chimps, bonobos live in fission–fusion societies with the main group size generally varying from around 50 to 120 in- dividuals (group size is slightly larger than common chimps’ possibly due to the greater availability of food sources in the great bend of the Congo river where they live in relatively high densities). Al- though they are more arboreal than common chimps, they also spend a fair amount of time foraging on the ground where they are largely frugivorous. While being more arboreal, surprisingly bonobos also appear to be better able to walk bipedally than common chimpanzees, given their longer thigh bones and more centrally placed foramen magnum (Myers Thompson, 2002; see Chapter 2). Again like common chimps they will, on occasions, kill and eat small vertebrates including small primates (Surbeck and Hohmann, 2008). In this case, however, although more common for males, both sexes are known to engage in hunting activity. In direct contrast to common chimps, bonobo society is based around the adult females who form coalitions. Although an individual male is larger than a female, it is this female coalition that allows females to maintain control of the social group (Strier, 2016; Gruber and Clay, 2016). Moreover, males remain with their mothers for many years and even in adulthood they frequently 84 The Evolution of Human Mate Choice Figure 4.3 A group of bonobos (Pan paniscus) engaged in mutual grooming. chimps, they do form dominance hierarchies where having a high-ranking mother can boost her son’s chances of reproductive success (Surbeck et al., 2010). Grooming and food sharing occur frequently between males and females, which is not generally the case for common chimps (outside of possible ‘meat-for-sex’ arrangements in some troops). Again there are records of female bonobos preferentially mating with males that have shared food with them (Kuroda, 1984). Given how frequently bonobos have sex, however, this is difficult to confirm. Also, unlike common chimps, bonobos have ranges that overlap with other groups. When two groups meet they may even combine for a while before separating and going their separate ways. This contrasts markedly with recorded antagonistic encounters between different common chimp groups. Bonobos have a promiscuous mating system which, in addition to heterosexual pairings, also involves male–male and female–female sexual activity. Additionally, immature bonobos also take part in such sexual activity. Sexual activity in general is very common in this ape and appears to be used as a means to communicate reassurance, both helping to maintain group cohesion and dispel tension. It has been suggested that the fact that bonobos are less aggressive than their chimp cousins may be due both to the use of sex as a form of conciliation and the fact that groups are led by female coalitions (Nishida and Hiraiwa-Hasegawa, 1987; Strier, 2016; Gruber and Clay, 2016; see below). Despite this arrangement, when it comes to oestrus females, once again, like the common chimpanzee, the higher up the dominance hierarchy he is, the more likely a male is to be successful in the production of offspring (Surbeck et al., 2010). One difference between common chimps and bonobos lies in the length of time that females of the latter species maintain their sexual swelling. For bonobos this typically last for around 20 days and this may help to explain why the males are  Origins of Human Mate Choice: The Social Behaviour of Our Relatives 85 attracted to the females during much of their cycle (Ryu et al., 2015). It has been suggested that this enhancement of female attractiveness to males helps to reduce levels of aggression towards them (Furuichi, 2011). This, in turn, might help to explain overall lowered levels of aggression in this species when compared to common chimps (Furuichi and Thompson, 2008). Gorillas Gorilla beringer and Gorilla gorilla Gorillas shared a common ancestor with humans and chimpanzees around 10 million years ago and currently share close to 98 per cent of their genetic code with us (Scally et al., 2012). Gorillas are the largest of the primate order with mature males weighing up to 230 kilograms. As with most primates, gorillas are highly social animals but unlike the two species of chimps they form unimale groups based around a mature silverback male and perhaps three mature females. Additionally there are likely to be one or two black-backed sub-adult males (8–12 years old) and a number of youngsters (less than 8 years old). The silverback protects the group from predators and threatens any other mature males that he encounters with the archetypal chest-beating display. Since he mates with each mature female, he will almost certainly be the father of any offspring produced in the group. The mature females form a linear dominance hierarchy with the first one to join the male having precedence. A high-ranking female spends more time close to the silverback as do her off- spring – a situation that decreases the chances of predation since females are approximately half the size of males (Stewart and Harcourt, 1987). Grooming occurs throughout the group as is the case for many primate species. Eastern gorillas tend to form larger groups than western ones and may reach up to 30–40 individuals. The slightly smaller western gorillas live in groups varying from 2 to 20 individuals (see Figure 4.4). Home ranges can be as large as 30 square kilometres, but unlike Figure 4.4 Family of Eastern Lowland Gorillas in the jungles of Congo. 86 The Evolution of Human Mate Choice common chimps they do not defend these as territories (in fact groups commonly overlap without dispute; Morgan et al., 2017). Sexually mature males and females leave the group – a situation that may well have evolved to avoid inbreeding. Both eastern and western gorillas are regarded as generalist frugivours consuming a very wide range of herbaceous ground vegetation (Morgan et al., 2017). Additionally they consume in- vertebrates such as ants, termites and various grubs and larvae, but unlike chimps and bonobos they do not kill and eat mammals. Interestingly, gorillas of the critically endangered western ‘cross-river’ sub-species Gorilla gorilla diehli (fewer than 200 remaining) have been observed throwing sticks and grass at humans that were tracking them (Wittiger and Sunderland-Groves, 2007). While such behaviour is well documented for chimpanzees this was the first time this was observed for gorillas. Baboons Papio Although baboons share fewer genes with us than chimps and gorillas (around 94 per cent), a num- ber of experts have claimed that they provide a better model for human social behaviour because they may well have shared similar ecological pressures. When the great African forests shrank be- tween 5 and 10 million years ago it is likely that the only primates to have left the trees for the newly expanding open grasslands were our ancestors and those of the baboon family. In this way baboons may be the only other savannah-adapted primates. In fact today various species of baboons are found in the open woodland and savannah of sub-Saharan Africa where early hominin fossils were also found. The classification of baboons has changed in recent years and might change yet again in the future but currently five separate species of Papio are recognised. These are: P. hamadryas (ham- adryas baboon), P. ursinus (chacma baboon), P. papio (red baboon), P. anubis (olive baboon) and P. cynocephalus (yellow baboon). Given that Papio hamadryas appears to differ physically (with males having a large white mane) and to some degree behaviourally, from the other species, some authorities refer to the hamadryas baboon and all of the rest as ‘savannah baboons’ (see below). Baboon troops vary in size from around 15 to over 150 individuals but in the case of the hamadryas baboon they can reach up to 750 (in which case there are various levels of sub-group). They typically inhabit a home range of several square kilometres. Their diet is largely vegetarian, but, as with chimpanzees, baboons will also kill and eat smaller mammals such as vervet monkeys. In fact, after humans, baboons are the most carnivorous of all primates. Once again hunting is large- ly (but not exclusively) a male preserve and, although baboons are not renowned for sharing their kills, in the case of the anubis baboon at least there are documented accounts of individuals donat- ing meat to other troop members (Strum, 1987; Strum and Mitchell, 1987). When this does occur, as with chimps, it is claimed that the oestrous females are most commonly the favoured recipients (Strum, 1987; Strum and Mitchell, 1987). Baboon society is matrilineal, that is, social relationships are based around female mem- bers of the group with relationships being sustained over three generations (Silk et al., 2018). For all non-hamadryas species males generally leave the troop on reaching maturity. There are two types of mating systems in Papio. For the hamadryas baboon each troop is made up of a number of unimale units where one male has the attention of between 2 and 11 females (i.e. it is like the gorilla, a ‘har- em’ system). In the case of all other baboon species, males compete for access to females and there is an overall dominance hierarchy for males in the troop. In these non-hamadryas species males and females form parallel dominance hierarchies with dominant males having first access to oestrous females. Research suggests that individual ‘rank’ depends as much on the status of an individual’s family within the troop as it does on individual merit. Hence having high-status kin pushes an  Reconstructing Human Behavioural Evolution 87 Figure 4.5 Male and female African hamadryas baboons (Papio hamadryas) holding each other close. Note the white ‘mane’ of the male and the general degree of sexual dimorphism. individual up the rank order (Bergman et al., 2003). As with chimps (and indeed with non-human primates in general), females in oestrus can easily be spotted by the red swollen perineum area and once again this signal appears to be attractive to mature males. Dominant male baboons of a number of species attempt to limit the access of other males by forming consort relationships with females in oestrus. This involves following a female around and mating with her intermittently during her fertile period, while simultaneously chasing off other potential subordinate suitors. For obvious reasons such male behaviour is known as mate guarding. While males can show a great deal of aggression towards other males they do show some care for infants including protection from other individuals, carrying and grooming behaviour (An- derson, 1992; Silk et al., 2018). In some cases these males may be the infant’s father but in others it is thought that such caring behaviour towards an infant might make them more attractive partners for the female who might then mate with them. This demonstrates that, despite being much smaller than the male, female baboons, as in so many other species, exhibit a degree of choice in whom they mate with. Reconstructing Human Behavioural Evolution Are Humans Different? Having looked briefly at the social organisation and reproductive behaviour of chimps, bonobos, gorillas and baboons, can we now use their behaviour to help understand how sexual selection might have acted on our early ancestors? There are three related areas that we can consider – diet, social behaviour and reproductive strategies. According to some researchers, diet (or rather a change in 88 The Evolution of Human Mate Choice diet) may help us to understand much about these other features in our species. The arguments that might be developed from such field observations are rather convoluted and require that we consider all three together. Meat-Eating Ancestors Most experts today consider that, for more than 3 million years, our ancestors lived in small troops of hunter-gatherers on the open savannah (McHenry, 2009; Leakey and Lewin, 1992). As with chimps, bonobos, gorillas and baboons, they were likely to have exploited a range of plant foods. Additionally, however, there is clear evidence that meat has been of long-standing importance in our ancestral diet (Wrangham, 2009). Both our teeth and our gut show tell-tale signs of this. Although chimps, bonobos and baboons do eat some meat, their typically primate gut is clearly the product of an almost exclusively vegetarian ancestry. They have, for example, a relatively lengthy large intestine, which breaks down vegetable material, and a short small intestine, which breaks down protein-rich foods such as meat. In contrast, our relatively long small intestine and the cutting edg- es of our incisors, canines and premolars all suggest that meat played a prominent role during our savannah-dwelling ancestry. In fact, unlike chimps, there are a number of nutrients, in particular vitamin B12, which we are no longer able to synthesise from a plant-based diet. Today these vital nutrients are only available to us through animal products. Also, quite conclusively, fossilised ani- mal bones from over 2 million years ago show clear signs of having been butchered by the tools of our early ancestors (Leakey and Lewin, 1992; Wrangham, 2009). Moreover, primatologist Richard Wrangham has suggested that our ancestors (Homo erectus) began to cook meat as long ago as 1.8 million years – releasing energy-rich and more easily digested nutrients (cooking denatures proteins and helps to kill off pathogens; Wrangham, 2009; Wrangham and Carmody, 2010). According to Wrangham, this led to a reduction in the size of our teeth compared to the great apes (and may have facilitated an increase in brain size; Martin, 1983; see below). Whether cooked or raw, throughout humanity today the amount of meat in the diet is considerably higher than in any of the 200-odd other species of primate (Tooby and DeVore, 1987; Wrangham, 2009). The Male Provisioning Hypothesis While we differ somewhat from our primate relatives in our dietary needs it is obvious to anyone who has ever looked at monkeys and apes that there are other, more dramatic, ways in which we differ. One area where humans differ from chimps and baboons lies in the lack of an oestrous swelling in females. As we will see, some experts consider that this difference is one of a number of human features which may be directly related to the point when meat became an important con- stituent in our diet. Some evolutionists have suggested that meat eating might have led to an early division of labour along gender lines when it came to food production. Evolutionary psychologists John Tooby and Irv DeVore (1987), both of whom came from a background in anthropology, have attempted to reconstruct human behavioural evolution. The conclusions that they reached are most conducive to what has been called the male provisioning hypothesis (see Box 4.1 and Buss, 1999; Mattison, 2017). Given our largely vegetarian primate past, we might ask why did our ancestors leave the trees and begin to incorporate meat into their diets? There are debates as to whether our ancestors and those of baboons left the rainforests in order to exploit the open grasslands or whether we were elbowed out of the diminishing rainforests by other primates that had superior arboreal adaptations  Reconstructing Human Behavioural Evolution 89 (see e.g. Dunbar, 2004; Strier, 2016; Leakey and Lewin, 1992). Whether we jumped or were pushed it really makes no difference, in either event, both groups clearly became adapted to the challenges and the opportunities provided by the open savannah. One suggestion is that when our early hu- man-like ancestors, or hominins, left the shelter of the rainforests and began to live on the open savannah they, unlike baboons, made use of their primate features to take serious advantage of the animal life that lived there (Tooby and DeVore, 1987; Wrangham, 2009). Given that our common ancestor with the chimpanzees (known as the last common ancestor, or LCA) was likely to have been an occasional meat eater, as are common chimps and bonobos today, then perhaps early hom- inins put such arboreal primate features to use in hunting. These features include good stereoscopic visual acuity, highly manipulative hands and a large brain. All of these characteristics might have been put to good use both to track prey and to aim missiles at it. This change to open savannah hunting may have had important consequences for both sex- es. As we noted earlier, male chimps of at least some troops are known to exchange meat for sex on occasions (and possibly baboons and bonobos). Interestingly, males of modern-day hunter-gatherer societies who are prolific hunters are the ones who are seen as the most attractive by the women of such bands. In fact in those societies, which are polygynous (i.e. where males have more than one mate; see below), the best hunters have the most wives and are more likely to have extra-marital affairs (Hill and Kaplan, 1988; Smith, 2004). If women preferred males that were most able to pro- vide them with meat, then we could see immediately that, over evolutionary time, sexual selection would increase hunting prowess in males since females would be choosing their mates on such a basis (see Box 4.1). Box 4.1 Male Provisioning Hypothesis – the Roots of the Human Pair Bond? Anthropologists have long wondered why, in contrast to the general mammalian pattern, humans form enduring pair bonds and engage in biparental care (Workman, 2016). In 1981 anthropolo- gist C. Owen Lovejoy proposed that this development was linked to what became known as the male provisioning hypothesis (MPH). Lovejoy proposed that hominins formed enduring pair bonds to increase the food supply to infants (which by primate standards were born relatively immature; see Box 4.2). In this scenario meat, and other food sources, were used by men to pro- vision their mates and their joint offspring (Mattison, 2017). The male provisioning hypothesis was derived from an earlier hypothesis known as the hunting hypothesis (HH) which places hunting centre stage with regard to a number of hu- man adaptations and is most closely associated with Sherwood Washburn. Under HH a shift to male-centred hunting increased calorific intake and this, in turn, led, in addition to the evolution of the pair bond, to a number of other adaptations including bipedal locomotion, complex tool use and to an increase in brain size (Washburn, 1968). It was the larger brain that necessitated the birth of an immature offspring and a greater degree of dependency of both mother and child on the male hunter (see Box 4.2). It is worth noting that this hunting hypothesis for human evolution was later challenged by an alternative, female based, gathering hypothesis (GH). The gathering hypothesis suggested that the evolutionary pressure that led to these human developments came from the problems presented to women who went out to gather food and who benefited from the mutual exchange of vegetable items (being bipedal, for example, allowed them to carry infants 90 The Evolution of Human Mate Choice Box 4.1 (cont.) and babies when foraging; Tanner and Zihlman, 1976; Tanner, 1981). Of course such hypotheses are not mutually exclusive and it may be that by combining them we might be able to refine our understanding of the relationship between these adaptations and the human pair bond. So how, we might ask, does the MPH differ from the HH? The main difference is that the MPH does not attempt to explain such a wide range of adaptations. Perhaps the HH was too grandiose in attempting to explain so much (the paleoarchaeological record demonstrates that bipedalism, big-game hunting and bipedal locomotion were separated by millions of years; Klein, 2009; Mat- tison, 2017). The other difference is that MPH requires less skill than HH as provisioning also involves scavenging and other forms of less dangerous foraging. Male provisioning is important cross-culturally as in forager societies men contribute up to 65 per cent of calories and 85 per cent of protein intake (Kaplan et al., 2000). It may also help us to understand the roots of the romantic pair bond (Marlowe, 2007; Miller, 2009; Videan and McGrew, 2002). Today, however, it is generally considered that male provisioning constitutes one of a number of important routes to boosting childhood investment. Other forms include grandparental investment and investment by other close relatives (Mattison, 2017). Why Only Men? Sexual selection may have acted to improve hunting prowess in men, perhaps by increasing upper-body strength. It may also have improved the manufacture and use of weapons and tools in males and might even have favoured the formation of male coalitions in order to exploit large prey through cooperative hunting (Washburn, 1968; Washburn and Lancaster, 1968; Hill, 2002). If meat had become an important part of the human diet, however, might we not also expect women to have become good hunters? Up to a point they may have been. Women also have large brains, hands that can grip with precision and good stereoscopic vision. Moreover, it is not unheard of for women in modern forager societies to engage in hunting occasionally (Estioko-Griffin and Griffin, 1981; Waguespack, 2005). Also, as we have seen, female common chimps and bonobos are known to engage in hunting on occasions, so this is by no means male-limited behaviour for primates. But the problem for women as hunters is that control over the reproductive cycle is a recent human invention. For our female ancestors, certainly as judged by their role in the majority of present-day forager societies, hunting was a less likely option since for most of their (usually short) adult lives they would either be lactating or pregnant. Of course being pregnant or nursing a baby would not be a great encumbrance when gathering plant food. For this reason it has been argued that this hunting– gathering sexual division of labour is likely to be ancient (Tooby and DeVore, 1987; Waguespack, 2005). This is not to say that women never engaged in hunting or that men never gathered plant food. The argument is about how each sex is likely to have foraged habitually. How, then, is this related to the oestrous swelling (or lack of it) in women? A number of researchers have argued that a shift towards hunting by men may have had a knock-on effect on the oestrus cycle of women (see e.g. Badcock, 1991). From this point on the argument becomes somewhat convoluted and decidedly speculative. Unlike plant foods, meat is a highly economic food source. In addition to its high calorific content, it contains all of the amino acids necessary for the human diet and the essential fatty acids required for brain development (Martin, 1983). Clearly  Reconstructing Human Behavioural Evolution 91 women would benefit just as much from meat as men. However, as we have seen, women bear a child-rearing burden that would not be entirely compatible with big-game hunting. We saw earlier that some male primates exchange meat for increased access to females. Perhaps while male homi- nins took advantage of the game available on the savannah, female hominins took advantage of the males. Perhaps by forming a long-term sexual relationship with a good hunter, a woman and her offspring would benefit from the food provided in return. Remember that in forager societies it is the best hunters that are the most successful in attracting women and are arguably those most likely to pass on copies of their genes (Hill and Kaplan, 1988; Smith, 2004). In this way, meat may be linked to the formation of strong pair bonds between men and women, with a male helping to provide for his partner and for their offspring. Note that according to this argument the long-term pair bonds formed would have benefit- ed both male and female partners. Such an argument would have to be weighed against the benefit of desertion and possible missed mating opportunities elsewhere for both sexes. Also infanticide is not uncommon in forager societies when a woman takes a new husband (Dunbar, 2004). In other words there are both costs and benefits to forming long-term pair bonds (see long-term mate choice preferences later). Cryptic Oestrus As we have seen, along with primates in general, chimpanzee and baboon males are particularly attentive to females who show an oestrous swelling on the rump – a feature that is lacking in hu- man females. Richard Alexander and Katherine Noonan (1979) have suggested that by concealing their period of oestrus (which they call ‘cryptic oestrus’) women may have made men attentive to them continually, since only in this way would males be able to ensure that they father a partner’s offspring. You may, however, have spotted a problem with this argument. If human females conceal oestrus why would males be attracted to them? British sociologist Christopher Badcock (1991) may have an answer to this dilemma. He suggests that rather than never appearing to be in oestrus, wom- en appear to be permanently in that state. Box 4.2 Bipedalism and Pair-Bonding – Why Do Men Help Out? Arguments concerning pair bond formation in humans raise one big question. If female apes can rear offspring without help, why should female hominins (and arguably present-day women) rely on male aid in rearing their children? Or to put it another way, why do human males bother to help out when their primate relatives generally get away with doing little or nothing? One idea, which was originally proposed by Stephen Jay Gould (1977), is also based around the evolution of bipedalism. Gould has suggested that this change to habitual bipedalism led to a narrowing of the pelvic girdle and that this, in turn, meant that a large-brained baby could only be passed through the pelvis at a highly immature stage (given our lifespan, human babies should be born at 21 months rather than 9). So human infants are delivered in a relatively immature and helpless state. This means that they require almost constant attention for a considerable period com- pared with any other animal. Females dealt with this problem, so the argument goes, by forming long-lasting pair bonds with male partners who were prepared to help provide for the offspring. Technically this is called high male parental investment (or MPI; see Chapter 7). 92 The Evolution of Human Mate Choice However, if women don’t even show a regular oestrous swelling, then how is Badcock able to make such a claim? Interestingly, in the gelada monkey, females provide another ovulation signal – they have swollen breasts at this stage in their cycle. By all accounts this swelling is attractive to males. Human females, in contrast to all other primates, have permanently swollen breasts during their fertile years. In this way human females may be providing a false oestrus signal and are therefore permanently sexually attractive to men (note that this, arguably, parallels the perineal sexual swelling that bonobos exhibit well beyond the period of oestrus; Gruber and Clay, 2016). A woman who was presumably able to keep a man around who would help to provide for her and her offspring by forming a long-term sexual relationship would be at a selective advantage over one who was unable to do so. Thus the development of oestrus signals (which we might call dishonest oestrus) outside of genuine oestrus may have come about via sexual selection. If this is the case then we can think of a woman’s shape as being the equivalent of a peacock’s tail, evolving to suit the preferences of the opposite sex. Note that in this case we are dealing with male choice. Males in general are not normally particularly choosy about sex, but once they begin to invest in offspring, that is provide parental care, then they may become choosy about long-term commitments (Buss, 2011; 2016). This constant female sexual attraction may well be a unique human feature and it may have led, in turn, to a unique long-lasting pair bond. Clearly, given as we have seen our primate relatives do not generally engage in monogamy (with the possible exception of serial monogamy in the hamadryas baboon), the changes that led to the formation of a long-lasting pair bond would have required a degree of psychological re-plumb- ing in both sexes. Like a chick that becomes imprinted on its hen, it would be necessary that these partnerships involved a desire for the couple to spend much of their time together. This pair bond that evolved might therefore be called sexual imprinting or, in colloquial language, ‘love’ (see Chap- ter 11). Thus, in addition to explaining the sexual division of labour, greater upper-body strength in men and dishonest oestrus in women, the male provisioning hypothesis may even explain the very reason we fall in love. We can see that some evolutionists have used the behavioural and anatomical features of our primate relatives to make suggestions about how sexual selection might have had an effect on our own criteria for choosing a mate and other aspects of our evolution. Science or Speculation? Some readers might have found the arguments based around the provisioning/‘meat for sex’ hypoth- esis somewhat speculative. Indeed, the arguments presented here are by no means cut and dried. At present the provisioning hypothesis does appear to explain a number of features that set us apart from other primates. In the future, however, new evidence or other arguments could arise which might weaken it. We should also bear in mind that in most forager societies, plant-based foods provide a fair proportion of the calories consumed – which means that females also provide provi- sions to males (as well as to offspring). Perhaps today the male provisioning hypothesis can be seen as a part of a mutual provisioning explanation whereby both partners benefit from increased rates of offspring survival (Mattison, 2017). Furthermore, we must be careful when using behavioural evidence in comparative studies since, as we have stressed, direct observation is not an option in evolution (Wrangham, 1987; Potts, 1996). When dealing with purely anatomical features, however, we can be a little more confident in making use of the comparative method since physical features are ultimately there to provide support for behaviour and may be quantified more accurately than behaviour. Modern men are larger than women and do have greater upper-body strength. Women  Reconstructing Human Behavioural Evolution 93 certainly did not develop an oestrous swelling following divergence from the great apes unlike many of our primate relatives. There is a modern sexual division of labour along hunting and gathering lines in extant forager societies and long-term male–female pair bonds, where males invest highly in offspring, are a cross-cultural phenomenon. How precisely these human features are causally re- lated through sexual selection, however, is an area of debate and speculation. These areas of greater certainty are largely physical and quantifiable features. And, as we will see in Figure 4.9, studies of comparative anatomy have led some evolutionists to some surprising conclusions. Table 4.1 Mating system categories Category Description Examples Distribution Monogamy individuals mate with one partner dwarf antelopes; humans 90% of bird species, but rare in mammals Polygamy individuals mate with more than polygamy can be sub-divided see below one partner; there are two forms into polygyny and polyandry; see of polygamy below Polygyny a form of polygamy where bighorn rams; gorillas; the most common individual males mate with more elephant seals; wood frogs; mating system found in than one female humans mammals Polyandry a form of polygamy where African jacanas; red-necked very rare; found in individual females mate with phalaropes; Galapagos some bird species more than one male hawk; humans Figure 4.6 African jacana (Actophilornis africana) walking over sea rose leaves. Okavango River, Botswana, Africa. 94 The Evolution of Human Mate Choice Sexual Dimorphism and Mating Systems We have seen that sexual selection can lead to differences both in behaviour and in physical form between the sexes and some would argue that this is also true for our own species. We have also seen how diet may have an impact on sex differences. As was noted above, frequently this means that males of a species are bigger than females. Clearly, the difference in size between males and females is not an all-or-nothing phenomenon. Human males are about 20 per cent bigger than their female counterparts; in gorillas males are almost twice the size of females, but in gibbons the sexes are virtually the same size. Current theory suggests that the greater the increase in male size rela- tive to females the greater the competition between males for access to groups of females. Species in which the largest and strongest males monopolise groups of females are said to be polygynous. Hence polygyny is a form of polygamy where one male has access to a number of females but each female is normally limited to one male (the reverse situation where one female monopolises a num- ber of males is called polyandry and is very rare; see Table 4.1). In monogamous species where a lasting pair bond is formed (such as in gibbons) sexual dimorphism is likely to be low since, once paired up, males are no longer in competition for further mates. Examples of mating systems are provided in Table 4.1. Mating systems vary greatly between human cultures (you will notice that humans are represented in all of the categories in Table 4.1). Because humans vary so much between cultures in their mating strategies some social scientists have claimed that our reproductive behaviour is cultur- ally determined and hence unrelated to our evolutionary past (e.g. Mead, 1949; 1961; Butler, 2004). This is a question that evolutionary psychology is beginning to address and to which we now turn. Human Mating Strategies We began this chapter by asking what men and women look for in a potential partner. Under the male provisioning hypothesis we suggested that, following a shift towards meat in the diet and a sex- ual division of labour, our species began to form long-term pair bonds, with males investing heavily in their offspring compared with their primate relatives. Although there is evidence to support this theory, today many experts consider that male provisioning is a component of what we consider to be mutual provisioning practices (Mattison, 2017). As we stated earlier, evolutionary explanations for current human practices are highly speculative and are currently characterised by a great deal more theory than data. One person, however, who has sought to redress the balance is David Buss of the University of Texas who has spent the last 30 years conducting research into human sexual strategies. As you will see, when it comes to mate choice criteria, he has drawn some controversial conclusions. Buss’ original work on mate choice criteria was startlingly different from previous studies. It was not startlingly different because he asked essentially new questions or because he applied radically new theories to explain his findings. Duplicating studies going back at least to the 1930s, Buss and his collaborators presented men and women with a list of 18 characteristics that they might find desirable in a serious long-term partner. These include financial prospects, social status, attractiveness, ambition and sense of humour. The task of respondents is to rate each of these at- tributes on a scale that ranges from 0 (irrelevant) to 3 (indispensable). Where Buss’ work differed radically from previous studies is in the sheer size and range of the operation. Whereas previous studies had used samples of dozens or a few hundred, in their largest study Buss and his co-work- ers sampled around 10,000 people and they included subjects from every habitable continent. The range of different cultures chosen (37 in this case) is at least as important as the actual numbers  Long-Term Mate Choice Preferences 95 Table 4.2 Human mean mate preference scores in 9474 people from 37 different cultures Characteristic Male rating Female rating Degree of sex Degree of cross- for females for males difference cultural variation Love 2.81 2.87 low low Dependability 2.50 2.69 low high Emotional stability/maturity 2.47 2.68 moderate moderate Pleasing disposition 2.44 2.52 low high Good health 2.31 2.28 low high Education/intelligence 2.27 2.45 moderate low Sociability 2.15 2.30 low moderate Desire for home and children 2.09 2.2 low high Refinement/neatness 2.03 1.98 low high Good looks 1.91 1.46 high moderate Ambition and industriousness 1.85 2.15 high high Good cook and housekeeper 1.80 1.28 high high Good financial prospect 1.51 1.76 high low Similar education 1.50 1.84 moderate high Favourable social status 1.16 1.46 moderate moderate Chastity 1.06 0.75 low high Similar religious background 0.98 1.21 low high Similar political background 0.92 1.03 low low Note: Rating scores potentially vary from 0.00 (irrelevant or unimportant) to 3.00 (indispensable). Source: Adapted from Buss et al. (1990). sampled. Uncovering differences between the relative ratings of various attributes both within and between the sexes in a single culture is clearly of interest. It tells you about that culture. However, by using only one culture we are unable to determine to what extent such a pattern is culture-specific. In large-scale cross-cultural studies universal similarities might be taken as evidence of a spe- cies-specific response. And species-specific responses are important to evolutionary psychologists since they may be taken as evidence that they are related to evolutionary process (Brown, 1991; Pinker, 2002; see also Chapters 1 and 14). So what did Buss and his co-workers uncover? The find- ings can broadly be divided into long- and short-term mating preferences and are summarised in Table 4.2. (Before examining this table it is worth noting that, although this study was conducted some 30 years ago, the pattern of sex differences revealed in mate choice criteria has been replicated in more recent times, suggesting Buss’ findings are robust; Lippa, 2008; 2009, Walter et al., 2020; see also Archer, 2019.) Long-Term Mate Choice Preferences How might we interpret these findings? In exploring this table we can see that there are a number of areas in which the sexes differ in their responses. In other areas males and females appear to want the same things. At this point it may be useful to tease out and explore some of the figures under a number of topic headings in relation to the proposals that evolutionary psychologists have put forward. 96 The Evolution of Human Mate Choice Preferences for Financial Resources, Industriousness and Social Status The mean scores suggest that women rate social status, industriousness and financial prospects high- ly in potential male partners and that men regard such features as of lesser importance in women. Moreover, it is well established that men of particularly high occupational status are able to attract and marry particularly attractive women (Dunn and Searle, 2010; Elder, 1969; Taylor and Glenn, 1976; Daly and Wilson, 1983). Evolutionary psychologists such as Buss (1999; Buss and Schmitt, 2019) claim that this supports the notion that, since ancestral females invested so highly in their offspring, they would have benefited greatly from choosing mates that were able to provide for them and their offspring. Clearly, resources may be assessed directly (in today’s society this generally means indications of financial well-being) or they may be inferred from social status and level of industriousness. Although women in all cultures favour these characteristics more highly than men do, the degree of difference between the sexes varies between cultures. Women in India, Iran and Nigeria, for example, value financial prospects more highly than do those in South Africa and Holland. This cultural difference is more difficult to explain than the sex difference. Perhaps differences between cultures with regard to access that women have to education and other means of independence from men might help to explain this observed variability. This very point has been suggested as a criticism of Buss’ findings by social constructionist Alice Eagly and her co-workers. Eagly has uncovered evidence that the preference for males with resources is negatively correlated with the degree to which the women of a particular culture have access to financial resources (Eagly and Wood, 1999). In other words, in some cultures women have to have preference for males with resources since they are unable to gain them themselves. In others, such as the Scandinavian coun- tries, women are financially less dependent on men. It is certainly the case that under the Swedish social welfare system, for example, where childcare and a lengthy period of maternity leave are both paid for by the state, effectively taxpayers provide women with opportunities to become financially independent single parents (Buss, 2016). At the time Eagly saw this correlational finding between state support and lowered interest in male status as suggesting that social roles are more important than evolved dispositions for mate characteristics. As evolutionists Robert Kurzban and Martie Haselton have pointed out, however, evolutionary psychologists (including Buss himself) generally see human mate behavioural adaptations as structured to ‘respond contingently to local social and ecological factors’ (Kurzban and Haselton, 2006). This is a point that Eagly now accepts, and in fact she now regards herself as a social-evolutionary interactionist (Eagly, pers. comm.; see also Eagly and Wood, 2011). Preference for Good Looks While both sexes demonstrate a clear preference for physically attractive partners, cross-culturally males rate this more highly in a partner than females. Furthermore, and surprisingly to many people, what males find attractive in females is pretty universal. They like large eyes, good teeth, lustrous hair, full lips, a small jaw and a low waist-to-hip ratio (i.e. the hour-glass shape) (Ford and Beach, 1951; Johnston and Franklin, 1993; Singh and Luis, 1995; Singh and Singh, 2011; Cunningham et al., 1995; Buss, 2016). Some cultural anthropologists find it hard to believe that men from differ- ent cultures find similar features attractive in women. In fact, although there is cultural variability in a number of features such as overall weight, colour of hair and height (Swami and Salem, 2011;  Long-Term Mate Choice Preferences 97 Cunningham et al., 1995; Ford and Beach, 1951), the characteristics that are claimed to be universal have one important feature in common – they correspond to youthfulness. We might ask why should men favour these features and why should physical attractive- ness be universally rated more highly by men than by women? In the words of David Buss, ‘To be reproductively successful, ancestral men had to marry women with the capacity to bear children’ (Buss, 1999, 133). According to Robert Trivers, men in the past who found fertile women attractive would be likely to pass this preference on when in competition with men who found infertile women attractive. Trivers’ theory of parental investment predicts just this difference. But men face a prob- lem that women do not have when it comes to choosing a fertile partner – women have a limited period of fertility. Whereas a man may be fertile from early teens right into old age, a woman is fertile from perhaps her mid teens only until her late forties. A number of studies have now uncov- ered a clear relationship between signals of general fertility in women and what men find sexually attractive (Malinowski, 1929; Cross and Cross, 1971; Singh, 1993; Buss, 1995; Sugiyama, 2005). Such signals are related to high levels of circulating sex hormones necessary for fertility: oestrogen and progesterone. And these, in turn, are correlated with clear skin, full lips, lustrous hair and a low waist-to-hip ratio. They are also correlated with youthfulness. So, the argument goes, men who made mate choices on the basis of these youthfully attractive features left more surviving offspring than men who didn’t. Interestingly, while the age gap between the sexes does vary between cultures, in each of Buss’ 37 cultures men preferred younger women and women preferred older men (exam- ples of this difference are provided in Figure 4.7). The relationship does not, however, work in reverse since, given a man’s lengthy period of fertility, there would not have been the same pressure on ancestral women to seek out signals of youthfulness. In fact, as we saw above, since women have a clear preference for good financial resources and a high social status in men, then it may pay them to seek older partners who are more likely to have climbed the slippery pole of success. This notion is supported by a number of studies +8 +7 Women +6 Men +5 +4 Older than self Age difference preferred +3 +2 +1 0 –1 –2 –3 –4 Younger than self –5 –6 –7 –8 Zambia Colombia Poland Italy United States N = 119 N = 139 N = 240 N = 101 N = 1,491 Country Figure 4.7 Preferred number of years younger or older for a potential spouse across five cultures. 98 The Evolution of Human Mate Choice of age preference in lonely-hearts columns in a variety of newspapers (see e.g. Kenrick and Keefe, 1992; Waynforth and Dunbar, 1995; Pawlowski and Dunbar, 1999). Such studies demonstrate not only that men generally look for women who are younger than themselves and that women general- ly prefer older men, but, interestingly, as men grow older they prefer women who are increasingly younger than themselves. As they age, women, in contrast, still expect to date older men. Again such a proclivity is certainly consistent with an evolutionary explanation for mate choice. The fact that men look for increasingly younger women (relative to themselves) as they grow older may well be a mating strategy that is unique to humans. Other primates such as chim- panzees and orang-utans appear to favour older more experienced females (Stumpf et al., 2008). Human male mating preferences may have resulted from an almost unique feature of women – that is, they can expect a lengthy period of life beyond their fertile years. Why women become infertile in their mid years is a matter for speculation. One possibility is that, given how arduous childbirth became following the evolution of bipedalism, it may have paid women to shift their investment to their grandchildren beyond a certain age. This means that, unlike other primates, men have a two-fold problem in making mate choices. First, they have to determine whether the potential partner is in oestrus and second, they have to decide whether she is within the correct age range to potentially bear offspring. Arguably, ancestral males who chose females of childbearing age who had the longest period of fertility ahead of them would have an advantage over males who were unable to do so. Evolutionists call the number of children that a person of a given age and sex is likely to have in the future reproductive value. It is important to distinguish between female reproductive value and fertility. Fertility refers to the likelihood of a female producing an offspring from a given mating whereas reproductive value is the potential for future offspring production. This may appear a pe- dantic distinction to stress, but for males making a long-term commitment, it may be of great impor- tance. A woman of 30 might be as fertile as a girl of 16, but the 16-year-old is likely to produce more offspring in the future than the 30-year-old. This difference may also be relevant when considering short-term mating opportunities (see below). Before leaving ‘good looks’ behind we should note that, although women do not rate phys- ical attractiveness as highly as men, they do, as Table 4.2 shows, consider it to be of some impor- tance. However, there is a fair degree of cultural variability in the importance that women place on physical attractiveness in men. How might evolutionary psychologists explain this? In contrast to some areas where cultural variability for features is not yet well understood, a simple explana- tion has been proposed for this case. It appears that women place greater emphasis on physical attractiveness in parts of the world where parasites are most common (Gangestad and Buss, 1993; Park and Schaller, 2009). Since physically attractive cues such as symmetrical faces and bodies are believed to be good indicators of parasite resistance (Hamilton and Zuk, 1982; Park and Schaller, 2009; see also Chapter 3), then it may make sense for women in these parasite-ridden cultures to be more choosy about partners on the basis of physical attractiveness. Preference for Love and Dependability Romantic love was long thought by social scientists to be a recent invention of Western culture (Symons, 1979; Jankowiak and Fischer, 1992). If evolutionary psychology has achieved anything, it is the destruction of this myth. Love may not make the world go round, it may not be easy to define scientifically, but it is certainly something that everybody can experience (see e.g. Jankowiak and Fischer, 1992; Toates, 2014). We would be very surprised if any reader of this book has never been  Long-Term Mate Choice Preferences 99 in love. In Buss’ study, both sexes report love as an essential requirement for long-term partnership (figures for both sexes are very close to the maximum ‘3’). Wanting to engage in a long-term rela- tionship with someone that we feel we are in love with is such an obvious state of affairs that you might feel it needs no explanation. However, forming an enduring pair bond is a very rare state of affairs for a mammalian species. What purpose might love serve? As we discussed above, it is clear that women generally favour resources and men favour youthful, attractive looks. Gaining a partner who demonstrates such characteristics is only the first step towards successful reproduction, however. If a man has a surplus of resources but deserts a woman immediately after sex or a woman is very beautiful but has multiple sexual partners, then in neither case will their partner be satisfied with the outcome. In other words, for long-term relation- ships to work, both partners require signals of commitment. Signals of love may provide this com- mitment. Signals such as promises of undying fidelity and dependability are also rated very highly by both sexes. Signals such as buying gifts for a partner, or listening to their woes. In other words, channelling time and effort into a relationship is what people in love expect from their partners. If the ultimate function of falling in love is to produce offspring in which both parents will invest, then the sexual imprinting that we call love may be a feature which sexual selection has led to in our species (see male provisioning hypothesis earlier). Preference for Chastity Chastity, in Buss’ study, is defined as ‘no previous experience in sexual intercourse’ (Buss, 1989, 19). Chastity appears quite low down the list for both sexes. Crucially, however, it is substantially more important for men seeking a long-term partner than for women. Why should this be the case? In all placental mammals the embryo develops inside the female uterus. This means that, whereas an offspring that a female gives birth to must be hers, it may not necessarily be that of her long-term partner. In humans there is an extra problem for males. As we saw earlier, in the case of chimps, bonobos and baboons (and indeed of mammals in general) females give off visual and olfactory signals when they are ovulating. Ancestral human females, in contrast, began, at some point, to dis- guise their ovulatory signals. This means that, whereas in other primates a male may be able to pay close attention to a female during oestrus and possibly engage in mate guarding, for ancestral men who had to deal with other matters, such as hunting, there was an increased potential for cuckoldry. Some evolutionists have even suggested that this increased possibility of cuckoldry may have been the very reason that marriage was invented since it acted to increase a male’s certainty of paternity (Alexander and Noonan, 1979; Buss, 2016). This male-centred view contrasts with the widely held view that marriage developed primarily to ensure joint investment in offspring. By rating no prior sexual experience as a relatively important criterion for long-term mate selection, males may be attempting to reduce the possibility of cuckoldry. In contrast, since females know that any offspring they deliver must be their own we can expect chastity in a partner to be of lesser importance to them. This fits in well with what we would expect from a knowledge of sexual selection theory. However, if males have such a great fear of investing in another male’s offspring then there is a problem with the figures that are revealed by Buss et al.’s study and you may have spotted it. Although males rate this characteristic more highly than females, why is it so low down the list of priorities? Moreover, we might also ask, why, if this is an inherited feature, does its im- portance vary so much between different cultures – so much so that it is the most variable charac- teristic between cultures in the entire study? In China, for example, chastity is considered to be of great importance for both sexes, whereas for Palestinian Arabs and Zambians it is expected for the 100 The Evolution of Human Mate Choice Indispensable 3.0 Males Females 2.5 2.0 1.5 1.0.5 Unimportant 0 China Palestinian Arabs Zambia USA Sweden N = 500 N = 109 N = 119 N = 1,491 N = 172 NS p <.0001 p <.001 p <.0001 NS Figure 4.8 Importance placed on chastity (no prior sexual intercourse) in a potential spouse. women but not for the man. In the United States it is of lesser importance to both sexes (but still more important for males than for females), and in Sweden chastity is of little importance to both parties (see Figure 4.8). Some anthropologists explain differences in chastity and in sexual permissiveness in gen- eral in terms of random differences between cultures, which may suggest that it is free from evolu- tionary influences (Mead, 1949). This is part of the Standard Social Science Model (SSSM; Tooby and Cosmides, 1997; see also Chapter 1) and is often referred to as the arbitrary culture theory (Alcock, 2009). It is true that cultures do vary in how sexually permissive they are, but such features of human life also vary within cultures over time. Since the ‘sexual revolution’ of the 1960s and the invention of freely available and reliable contraception, being a virgin bride has become less common in the West. And it might be argued that since women have been able to engage in sex with comparative freedom from its reproductive consequences, their attitudes and behaviour have become more similar to those of men (Ruse, 1987; Crooks and Baur, 2013). Interestingly, in the Scandinavian countries where premarital sex is least frowned upon, the provision of welfare systems with regard to supporting unmarried mothers is the most generous. This is particularly true of Sweden where women enjoy greater economic independence from men than virtually anywhere else in the world. Perhaps economic independence is a precursor of sexual independence (Posner, 1992; Elman, 1996). Extending this line of reasoning, we might suggest that it will be in those cultures where women are financially dependent on men that we can expect to see chastity rated as very important for them. This would certainly be the case for China, Iran and India where women earn substantially less than in the West. It does not, however, explain why chastity is so important for men in some cultures (such as China). The findings with regard to chastity suggest that one cultural shift, such as a change in economic status for women, might have a knock-on effect on other cultural variables such as what is morally acceptable. Such cross-cultural findings might also suggest that some areas are difficult to explain purely in terms of evolutionary principles. We return to this important point at the end of the chapter.  Long-Term Mate Choice Preferences 101 Emotional Stability and Pleasing Disposition Although evolutionary psychologists are apt to concentrate on the differences between the sexes, you will note from Table 4.2 how similar men and women are in many of the things that they want. Apart from a partner who is in love with them, both sexes want someone who is emotionally stable and of a pleasing nature. Moreover, after ‘love’ and ‘dependability’, these personal qualities are the most highly rated by both sexes. In a word, we all want a partner who is going to show kindness to- wards us. Forming a long-term relationship with a shifty, unreliable person of appalling disposition would not have boded well for our ancestors. Cues that suggest kindness, however, would certainly be regarded as conducive to mutual investment in offspring (although our ancestors may not have conceived of it in quite this way). In summary, most of what people want from a long-term partner is surprisingly similar be- tween the sexes. The areas where the greatest divergence appears, however, are exactly those Trivers’ theory of parental investment predicted (Trivers, 1972). That is, men do appear to place greater em- phasis on physical attractiveness and women place a premium on signals of wealth and status. Both sexes require indicators of commitment and therefore, in a subject that treats relationships in such a cold and calculated light, it seems reassuring to discover that both men and women value love as of primary importance. At least that is the picture when considering long-term relationships. When we consider less permanent affairs things begin to look less romantically rosy. Short-Term Mate Choice Preferences So far we have considered the preference that each sex has when looking for a partner with whom a lengthy commitment is intended. However, all societies have their fair share of short-term sexual rela- tionships (whether sanctioned or not), so it is necessary to consider these also. Evolutionists interested in human behaviour have long considered whether humans are psychologically adapted to commit to long-term relationships (whether monogamous or polygamous) or whether it is in our nature to at- tempt to have multiple short-term partners (Badcock, 1991; Wright, 1994; Campbell, 2008; Buss and Schmitt, 2019). Of course, such claims assume that there is a genetic component to our mating strat- egies and that they comply with Buss and Schmitt’s sexual strategy theory. When considering behav- iour, just how big an impact evolution has had on our psychological wiring is an area of great debate. In contrast, when considering physiological and anatomical peculiarities, few would disagree that such features are largely genetically determined. And it is studies of the reproductive physiology and anatomy of males that have provided some surprising insights into the sexual strategies of both sexes. Size of Testes and Mating Strategy As we saw earlier, differences in the level of sexual dimorphism between various species may pro- vide clues both to the mating strategy and to levels of inter-male competitiveness of a species. An- other area where males vary between species and which may provide further clues as to differences in sexual behaviour is in the size of their testes. Once again we can make use of the comparative method to see how sexual selection may have acted on primate species differentially. Gorillas, in contrast to their enormous body size, have relatively small testes (0.018 per cent of body weight), whereas chimpanzees have extremely large ones (0.26 per cent of body weight). The reason for this difference becomes clear when we realise that the amount of sperm produced increases with the size of the testes. In chimpanzee society, where sexual dimorphism is quite low, high-ranking 102 The Evolution of Human Mate Choice chimpanzee gorilla human Indicates relative largest size of female genitals; indicates relative size of penis in erection; indicates approximate relative size of testes. Large circles indicate approximate relative body size of sexes. Figure 4.9 The relative body and testis size of apes and humans. males do not monopolise a group of females but instead attempt to pair off with individual females during their period of oestrus. As we have seen, female chimpanzees show a large oestrous swelling during this fertile period of their cycle and may mate with a number of males at such a time. Such multiple mating behaviour in female chimps means that their reproductive tract will often contain sperm from more than one male. This female mating strategy has led the males to engage in sperm competition so that those able to produce the largest quantities of sperm are most likely to insem- inate a female and thereby pass on their genes (Short, 1979). Hence sexual selection has driven chimpanzees’ testes to their current enormous size. In contrast to chimpanzees, as we saw earlier, silverback gorillas that are able to defend a harem from the attentions of other males produce rela- tively small quantities of sperm since their sperm will not have to compete with that of other males. This explains why their testes are so small (see Figure 4.9). Thus in males a large body size relative to females indicates polygyny (multiple mating partners for males) whereas large testes compared to closely related species indicates polygamy (multiple partners for both sexes). The finding of a relationship between testes and body size in relation to mating strategy raises the question – what sort of mating strategy do these measurements suggest in our own species? It turns out that we are intermediate on both measures. In terms of body size, humans are a little more dimorphic than chimps and decidedly less so than gorillas. In terms of testis size humans, at 0.079 of body weight, are four times larger in volume than gorillas, relative to body size, but only a third the size of chimpanzees. By these measures then, in our ancient past, human males may well have competed for females partly via strength and size and partly via sperm competition. In order for sperm competition to occur, some human females must have routinely mated with more than one male within a matter of days. This means that we may be able to infer something about the mating strategy of both sexes by studying the anatomical detail of one sex. The relative size of male testes suggests that human females were less promiscuous than chimps but more promiscuous than gorillas. Pre- and Extra-Marital Activities The current workings of human testes suggest that, although all modern human societies recognise and encourage the formation of long-term pair bonds of various sorts, our ancestors of both sexes engaged in extra-marital activities (Baker, 2006; Shackelford et al., 2005). Such a finding poses two  Long-Term Mate Choice Preferences 103 big questions. Why might short-term relationships have been of benefit to each sex? And what do men and women look for in a short-term partner – is it the same as in a long-term partner, or some- thing quite different? In other words, we need to consider the costs and the benefits of engaging in casual sexual relationships. Costs and Benefits of Short-Term Relationships to Men and Women One of the most revealing studies of the difference between men and women in their reactions to casual sex (at least in the West) was also one of the simplest. Clark and Hatfield (1989) simply had attractive male and female students walk around a university campus and ask students of the opposite sex whether they would sleep with them. The responses of men and women could hardly have been more different. While 75 per cent of males agreed to sleep with a woman they had met only minutes before, every one of the females approached declined the offer! This study provides striking confir- mation of what most of us would have guessed. Men saw an invitation for casual sex with an attractive unknown woman both as a compliment and an opportunity. Women saw it as a threat and an insult. We have to bear in mind that, as with the original Buss study, the Clark and Hatfield study is also over 30 years old. Given just how much social mores (not to mention sexual politics) have changed during this period we might ask to what extent have the sexes modified their attitudes to casual sex since then? Have males and females converged in their responses to an invitation to have sex with an attractive stranger since then? In fact the study has been repeated on a number of occa- sions (with various modifications) since 1989 and in the vast majority of these the researchers were able either to replicate or partially replicate these findings (Buss, 2016; Schmidt, 2017; Walter et al., in press). One study, using a modified design, found something a little different however. German researchers Andreas Baranowski and Heiko Hecht adapted their design to account for the fact that fe- male participants might not consent to sex due, in part, to their greater vulnerability when approached by a stranger. In one of their experiments both men and women were given an elaborate cover story of how safe specific individuals of the opposite sex were and then shown a series of photographs and asked whether they would consent to a date or sex with each stranger (this is legal in Germany). They were able to choose as many as they liked for either or both. Under this safer condition the female consent rate rose to that of the males (for both dating and consenting to sex with). Interestingly, how- ever, men wanted to meet significantly more potential partners than women (see Figure 4.10). Male Figure 4.10 Results from Baranowski and Hecht (2015) demonstrating that 5 Female men chose to meet significantly more 3.67 (0.35) 3.57 (0.39) women compared to women choosing Number of selected partners 2.80 (0.37) men, both to date and to agree to have 4 2.73 (0.38) sex with. Error bars represent ±1 SEM. 3 2 1 0 Date Sex Condition 104 The Evolution of Human Mate Choice Baranowski and Hecht’s experiment suggests that, while men are more likely to consent to casual sex with a stranger under a wide range of social contexts, when they feel safe and when they are allowed to choose who the stranger is, women may be just as likely to do likewise. Sexual strat- egy theory, while suggesting women are less likely than men to agree to casual sex in general, also suggests the chances of them doing so increases when a man is assessed to have highly desirable traits (such as good genes assessed by their physical appearance). Perhaps the main difference be- tween the sexes, when it comes to casual sex, is that men are less choosy and less context dependent rather than less coy per se (Zuk and Simmons, 2018). David Buss, David Schmitt and Donald Symons all consider that this difference in behav- iour with regard to casual sex reflects differing psychological adaptations in men and women. They argue that men have evolved to be licentious and to find casual sex particularly appealing in order to increase their chances of producing extra offspring at little cost (Symons, 1979; 1989; Buss, 2016; Archer, 2019). Women, in contrast, they suggest, have evolved to be more choosy about casual sex because, as Trivers realised, they have more to lose if they make a poor choice. Box 4.3 Altering Sperm Production The finding of a relationship between testis size and human promiscuity is also supported by the finding that a man’s sperm count rises quite dramatically when he has been away from his partner for some time (Baker and Bellis, 1989; 1995; 2014; Shackelford et al., 2005). In fact in, contrast to couples who spend 100 per cent of their time together, where men have an average of 389 million sperm per ejaculate, for those who spend only 5 per cent of their time together this figure rises to 712 million sperm per ejaculate (Buss, 2016). The rationale for this steep rise in sperm production is that when ancestral males spent some time away from their mates, upon their return they increased their sperm production in order to compete with other males that might have had sex with their partners. So in addition to producing relatively large amounts of sperm, human males are also able to alter the amount they produce depending on how long they have been away from their partners. Again such a male adaptation is difficult to explain without resorting to sperm competition being driven by ancestral females having more than one partner over quite short periods of time. Before we begin to feel let down by the lack of fidelity that this suggests for our female ancestors we should also bear in mind that it takes two to recombine their genes: both sexes are equally tainted by such findings! As Buss (2016, 134) puts it: Ancestral women must have sought casual sex for its benefits in some contexts at some times at least, because if there had been no willing women, men could not possibly have pursued their own interest in short-term sex. How Many Partners? Although historically most studies of sexual relationships have focused on married couples, there is now supportive evidence for the views of Symons and Buss. Cross-culturally, women desire fewer lifetime sexual partners than men. One particular study conducted by Buss and his colleague David Schmitt (1993) revealed that, during a period of one year, women would ideally like to have  Long-Term Mate Choice Preferences 105 one sexual partner whereas men would prefer to have around six. During a period of three years women would like to have two whereas men would ideally like to have around ten (see Figure 4.11). One reason for this difference might be the greater costs and lower benefits that such be- haviour holds for women when compared with their male counterparts. Unlike a man, a woman is unlikely to increase the number of offspring she has by increasing the number of partners she has. Moreover, a single woman who engages in short-term sexual relationships runs the risk of desertion (leaving them ‘holding the baby’) and a married woman who engages in extramarital sex runs the risk both of her husband’s wrath and of his desertion. Furthermore, cross-culturally, promiscuous women are regarded as less ‘marriageable’ than monogamous or chaste ones (Buss, 2016). By being openly promiscuous a woman might reduce her chances of gaining a high-quality long-term partner. A 20 15 Number of partners Male 10 Female 5 0 1 mo 6 mos 1 yr 2 yrs 3 yrs 4 yrs 5 yrs 10 yrs 20 yrs 30 yrs life Time B 3 2 Likelihood of intercourse 1 Male Female 0 –1 –2 –3 5 yrs 2 yrs 1 yr 6 mo 3 mo 1 mo 1 wk 1 day 1 eve 1hr Length of relationship Figure 4.11 Number of sexual partners desired by males and females over various periods of time. 106 The Evolution of Human Mate Choice In a very real sense, then, the benefits that men gain from short-term relationships might also be seen as the costs that women incur from such liaisons. That is, a reduction in the level of investment in the offspring by males at the expense of females. In the words of Buss and Schmitt: Because women and men differ dramatically in the minimum obligatory investment to produce a single offspring (nine months pregnancy for women; one act of sex for men), selection has favored a sex-differentiated suite of adaptations regarding short-term mating strategies and the circumstances in which they are implemented. (Buss and Schmitt, 2019, 81–2) However, given that human testes reveal an evolutionary history of multiple partners for both sexes we must ask – what is in it for women? What Is in Casual Sex for Women? Until recently it was widely considered that, while extra-pair copulations might benefit males, the best strategy for a female must surely be to find a partner who would stick by her, and to stand by him. In recent years this view has changed. It has changed partly because of the find- ings with regard to human sperm production, but also due to two further developments. First, it has been suggested that, under certain conditions, women might also benefit from multiple pre- marital or extra-pair post-marital matings (Buss, 2016) and, second, there is growing evidence that cross-culturally women are by no means as monogamous as was once considered (Wrang- ham, 1993; Buss, 2011; 2016; Zuk and Simmons, 2018). Indeed in the United States, for example, confidential surveys suggest that somewhere between 20 and 50 per cent of married women have engaged in an extra-marital affair (Kinsey et al., 1953; Glass and Wright, 1992). Also in England, via genetic testing, Baker and Bellis (1995; 2014) have found that about one in five children could not be the offspring of their supposed fathers. Given the findings that women appear to be less monogamous than was once thought, we might ask what possible benefits might they gain from brief sexual encounters. Evolutionary psychologists have suggested that women may gain both material and ge- netic benefits for their offspring from casual sexual relationships. By having more than one sexual partner a woman may derive benefits such as food and other gifts. Recall that it has been argued that female chimps and baboons may sometimes exchange sex for food – a process which biologists call resource extraction. Indeed, anthropologists have found that in forager societies such as the Trobriand Islanders where casual sex is permitted or, at least, not heavily frowned upon, women generally act as mistresses only as long as their lovers bring them gifts (Malinowski, 1929). Further- more, Buss has found that women rate having an extravagant lifestyle and generosity more highly in a temporary lover than in a permanent partner (Buss and Schmitt, 1993; 2019). These findings suggest that there may be material benefits to women in casual sexual relationships. Buss (2016, 135) even suggests that, These psychological preferences reveal that the immediate extraction of resources is a key adaptive benefit that women secure through affairs. In terms of genetic benefits, there are a number of ways in which a woman may benefit from casual sex, whether she is married or not. A woman can increase the variability of her offspring  Long-Term Mate Choice Preferences 107 by mating with more than one man (Smith, 1984). Furthermore, by taking a secret lover who is physically superior to, or of higher status than, her husband, a woman can potentially improve the status of the male offspring she produces. In support of this, Baker and Bellis (1989) have found that, when a married woman has an affair, her lover is almost invariably of higher status than her husband. Some evolutionists have even suggested that when women cuckold their partners they do so with more attractive men than their husbands (Meston and Buss, 2009; Simpson et al., 1993; Wright, 1994; Buss, 2016). This may be of benefit if such women pass on these attractive features to their sons who would then have improved chances of attracting women. For obvious reasons this notion has become known as the sexy sons hypothesis (Fisher, 1958; Gangestad and Simpson, 1990). Do Women Have a Self-Knowledge of Their Market Value? A final factor, which, it has been suggested, might affect a woman’s likelihood of becoming promis- cuous, is her perceived level of attractiveness to males. Some evolutionists have proposed that, as a girl grows up, the feedback she receives from males provides her with a self-knowledge of her level of attractiveness – her ‘market value’ (Trivers, 1972). This self-knowledge, Robert Trivers specu- lates, may then affect her reproductive strategy. Females who perceive themselves as less attractive might be more likely to engage in casual sexual relationships since they are less likely to gain a long-term high-quality partner (Harris, 1998). For those who receive feedback suggesting that they are particularly attractive, however, it may make more sense to hold out for a high-status male from which they are likely to gain a high level of investment (Trivers, 1972; Wright, 1994). This is a con- troversial notion but one for which there is some empirical support. Walsh (1993) has found that the less attractive a woman feels she is, the more partners she is likely to have. Moreover there is clear evidence that the most beautiful women in a given society do tend to marry the most powerful men (Kanazawa and Kovar, 2004; Symons, 1979). Clearly, the sexual strategy that a woman adopts reflects a number of complex interacting factors – and we haven’t even considered the importance of inherent personality variables (see Chapter 13). Men, in contrast, as has been suggested, are less context dependent in their proclivities for casual partners (Buss, 2016). The potential reproductive benefits that men gain from casual sex- ual relationships should not blind us to the fact that they also might incur costs and need to weigh up their options. A man, who has a relationship with another man’s wife, risks injury or even murder at the hands of her husband. Daly and Wilson (1988), for example, have found that a large proportion of male/male homicides are the result of sexual jealousy. Furthermore, in a number of societies it is permissible for a man to kill another man who has slept with his wife (and to kill the wife). Even when a man engages in a short-term sexual relationship with an unmarried woman he might not be safe from violence. In a number of societies he might be attacked and beaten by her brothers and father (Daly and Wilson, 1988). Finally, by