Why Us and Not Them? A Review of Sarah Hrdy’s Mothers and Others

“Thou hast no sense. You French people love your own children; but we love all the children of our tribe.” ~Naskapi tribesman

Sarah Hrdy has done the field of evolutionary psychology an inestimable service by writing Mothers and Others: The Evolutionary Origins of Mutual Understanding (2009). First and foremost, Hrdy has contributed to the advancement of a burgeoning field by bringing together a large and diverse assortment of cutting-edge empirical work in one volume. Second, Hrdy’s deep familiarity with recent work in primatology and cross-cultural anthropology provides a helpful constraint on evolutionary speculations about the human “Environment of Evolutionary Adaptedness” (EEA). Insofar as Hrdy’s book can be seen as a plea for sociobiology to go beyond weakly substantiated speculation about our ancestral way of life, it deserves attention by anyone interested in the origins of human cognition.

The first chapter kicks off with a provocative thought experiment. Hrdy points out that we take it for granted just how well humans get along when stuffed on an airplane with three hundred cranky strangers.  But imagine the same airplane crammed shoulder-to-shoulder with chimpanzees. The inevitable bloody mayhem stands in stark contrast to the overt politeness usually governing human strangers. Hrdy now asks the central question of the book: Why us and not them?  That is, why do humans get along so well with each other but chimps don’t?  Hrdy says “The goal of this book will be to explain the early origins of the mutual understanding, giving impulses, mind reading, and other hypersocial tendencies that make [riding airplanes] possible” (p. 4).

Hrdy catalogues several traditional answers where the difference between humans and chimps has to do with our big brainy intelligence. After arguing the fossil record paints a different story,  Hrdy chastises such overly “intellectualist” stories for putting the cart before the horse and instead favors a hypothesis recently argued by Michael Tomasello (Herrmann et al., 2007; 1999): “The crucial difference between human cognition and that of other species is the ability to participate with others in collaborative activities with shared goals and intentions” (p. 9). Why are humans so “ultra-social” compared to chimps? It obviously won’t do to point to the existence of special human capacities like our ready disposition for empathy because that only pushes the question back further: why do humans have such capacities but chimps don’t? Given that natural selection is blind to possible future benefits, there must have been some “initial payoff” for developing such social-cognitive competencies. Clearly we will need to give an “ultimate” evolutionary explanation for the origin of our uniquely human social-cognitive competencies that outlines a plausible fitness payoff.

In the second chapter, Hrdy reviews contemporary evolutionary accounts of our social-cognitive capacities and finds them lacking. A popular theory is to claim that increased intersubjectivity would have been adaptive for a  group of social primates, particularly with respect to “intergroup conflict” (Choi & Bowles, 2007). However, Hrdy asks “How much sense would it have made for our Pleistocene ancestors eking out a living in the woodland and savannas of tropical Africa to fight with neighboring groups rather than just moving?” (p.19)[1] Moreover, Hrdy is skeptical of these hypotheses for a more general reason. She asks,

If intersubjectivity was so useful for maintaining cohesive social groups, defending one’s in-group from violent neighbors, or wiping out competitors, why didn’t other social primates (those ‘demonic neighbor-stalking chimpanzees in particular) evolve such gifts as well? (p. 37)

Hrdy applies a similar logic to other evolutionary accounts such as the Machiavellian Intelligence hypothesis or the hypothesis that humans are special because we have mirror neurons. For Hrdy, most hypotheses on the table either fail to answer the question, “Why us and not them?” or they rely on empirically false claims about the abilities of chimpanzees (e.g. the false claim that chimp infants cannot imitate or follow eye-gaze, see p. 58). Hrdy’s ultimate diagnosis of all these false starts is that they mistakenly used the chimpanzee model as a basis for theorizing about our human ancestors. Hrdy’s prescription is to turn to recent developments in primatology and cross-cultural anthropology to study how human childcare in extant hunter-gatherer societies works, and from there find an appropriate primate model to make inferences about the EEA.

The third chapter in a nutshell is Hrdy’s defense of the old expression “It takes a village to raise a child”. Indeed, Hrdy’s overall answer to the central motif of the book is that the  selection pressure for human competence in social cognition arose due to novel rearing conditions approximately 1.8 million years ago where youngsters depended on more people than just their parents for care. Hrydy proposes that these “alloparents” like sisters, aunts, grandmothers, and even extended exchange networks involving non-paternal males were the crucial link in the evolutionary story. If a child’s survival in the EEA would have been affected by the availability of alloparental care, then there could have been selection pressure for infants to develop the mental resources to decode the mental states of others in order to illicit extra-parental help. As Hrdy puts it, “the need for alloparental succor transformed the selection pressures that shaped our species, and in doing so altered the way infants developed and then the way humans evolved” (p. 67). Although she acknowledges a possible role for intergroup competition in shaping our prosocial attitudes (p. 20), Hrdy believes many researchers have overlooked the crucial importance of child-rearing and have not sufficiently thought about the difficulty of ensuring the survival of helpless, slow-maturing children in the wild.

To support her hypothesis, Hrdy turns to primatology data (some of which she collected herself) to examine patterns of mother-child care in Great Apes. The most salient finding is that Great Ape mothers under no circumstances ever hand over the infants to another caretaker (even sisters eager to practice their parenting skills). This stands in sharp contrast to modern hunter-gatherer societies where “mothers trust others and allow them to take their infants shortly after birth” (p. 78), a form of child-rearing known as “cooperative breeding”. According to Hrdy, humans are not the only cooperative breeders, a distinction also shared by a family of New World monkeys called the callitrichids, of whom the marmosets are a representative example.[2]  Although humans are cognitively similar to chimps in many ways, it is these “dumber” yet socially sophisticated New World monkeys that may provide the best primate model for reconstructing the EEA in virtue of their shared emotional proclivity for prosociality and cooperative breeding (Not to mention the sociality seen in bonobos, who are genetically equa-distant from humans as chimps).

In the fourth chapter, Hrdy takes up the popular 20th century framework of “attachment theory” and updates it in light of recent developments in the study of alloparenting in human societies. John Bowlby, the founder of attachment theory, famously modeled the mother-child attachment relationship on the iconic notion that the mother and child were inseparable, just like chimp mother-infant relationships. Reviewing both old and new data, Hrdy concludes that modern research on attachment formation overwhelmingly suggests that the development of healthy social attachments depends crucially on forming bonds with nonparental caretakers. Indeed, “infants nurtured by multiple caretakers grow up not only feeling secure but with better-developed and more enhanced capacities to view the world from multiple perspectives” (p. 132). And in essence, since children with reliable alloparental care would have had access to more calories and care-taking resources, they would have survived better than those who didn’t, thus generating an adaptive selection pressure for the development in infants of the mind-reading capacities necessary to solicit help from others.

The fifth chapter takes on standard evolutionary theories of parental investment and considers the potential role of fathers in successful childrearing. A standard story might be that because females rely on the assistance and resources of fathers to raise their children, the practice of pair-bonded monogamy arose due to a tacit “sex contract” between males and females. In essence, the contract states that in return for parental investment, the females will “exclusively” offer the male sexual access. But Hrdy wonders what happens when the “loving father” is not around to help? Could alloparents step in? In light of paleoanthropological data concerning the relative infrequency of male hunters scoring meat in the Pleistocene, as well as numerous anthropological data describing the strong egalitarianism of hunter-gatherer tribes (sharing the spoils of the hunt with nonkin during large communal feasts), Hrdy says “It’s clear that the most successful hunter would often get no more for his family than the most hapless did” (p. 149).  Here Hrdy approvingly cites Kristen Hawkes’  “show-off hypothesis” (1991) where the benefit of successful male hunting was cashed out more in terms of prestige and reputation rather than pure caloric load distributed to kin. Accordingly, it is the hard work of female gatherers that probably made the biggest daily caloric impact on the survival of children. And if such females could form matrilinear coalitions for cooperative breeding, an inflexible sex contract to secure caretaking resources would have been unnecessary,  leaving room for more flexible parenting (and mating) strategies, including ones where the males don’t offer care exclusively to their kin (which is not to say fathers would place equal weight on nonkin, see p. 157)

As Hrdy puts it, “At the heart of the [sex contract] model lay a pact between a hunter who provided for his mate and a mate who repaid him with sexual fidelity so the provider could be certain that children he invested in carried at least half of his genes” (p. 147). Hrdy doesn’t deny the existence of sexual jealously and male concerns about paternity, but Hrdy’s moral is that “A fixation with genetic paternity obscures the full range of emotions and motives that influence nurturing tendencies in men, and may also obscure their impacts on child survival” (p. 159). Such strategic flexibility might explain the existence of otherwise puzzling cultural diversity concerning male sexual proprietariness, including so-called “partible-paternity” societies like the Eskimos, Montagnais-Naskapi, Central American people like the Siriono, and many tribes in Amazonian South America (p. 153).

Hrdy spends the rest of the book bringing more empirical data to the table and elaborating on the theory of alloparenting, including further analyzing the conditions that favor alloparenting in other species (chapter six), the features of babies that makes them so alluring (“sensory traps”) to adult caretakers (chapter seven), the importance of grandmothers in hunter-gatherer societies (“the most reliably beneficial of all alloparents” (p. 260)) and how this might have facilitated matrilocal (or “matri-patrilocal”) rather than strictly patrilocal residence patterns in the EEA (chapter eight), and finally,  a consideration of various life history traits such as long childhood and as well as some broad and speculative thoughts about how the rise of agrarian civilization affected female sexual autonomy (chapter nine).

To appreciate the significance of Hrdy’s scholarship, it helps to review a standard methodological procedure for doing evolutionary psychology. First, you identity an adaptive problem facing our ancestors in the EEA e.g. the problem of finding a good mate. Second, you develop a computational model that is capable of solving this problem e.g. gather evidence about proxies of fitness such as facial symmetry. Third, you hypothesize plausible neurological mechanisms that could realize the computational solution. Last, you run experimental tests looking for confirmation that the hypothesized mechanisms actually exist. Crucially, this methodology will only produce plausible results if you can realistically set up the initial adaptive problem. That is, if your assumptions about the problems encountered in the EEA are mistaken, then the rest of your explanation will inherit the mistake and you will end up proposing solutions to a problem that never existed. Accordingly, the success of evolutionary psychology as a discipline critically depends on using all of the scant evidence available to make realistic assumptions about the EEA.

Some standard assumptions about the EEA are unassailably right e.g. female pregnancy. But other standard assumptions about possible parenting investment strategies are more questionable. For example, I already mentioned the standard “sex contract” account whereby females “agree” to stop sleeping around with other men in order to secure their fatherly resources. This tense arrangement supposedly benefits both parties. The men receive assurance that they won’t waste resources on some other man’s baby, and the women receive assurance that they will have enough resources from a committed male to raise their baby. This story is supposed to take us all the way from the Pleistocene to contemporary cultural patterns of serial monogamy (albeit with occasional but limited cheating). However, several recent books  (Barash & Lipton, 2001; Ryan & Jethá, 2010)[3] have challenged the standard sex contract story on several dimensions (particularly the assumption that females actually are sexually monogamous). The most relevant dimension for our purposes is skepticism about the following assumption:  the strategy of a father providing care to anyone outside his direct kin network is not evolutionarily stable due to the pressure of competing “selfish” fathers who only provide care to their kin. In a critical review of the latter book, Ellsworth (2011) attempts to undermine the alternative narrative by approvingly citing Thornhill and Gangestad (2008) in claiming the “primary selective pressures favoring such female estrus adaptations were pair-bonding and dependence on male provisioning” (p. 332, emphasis added). However, if Hrdy’s emphasis on the importance of alloparental care for decreasing childhood mortality rates has any validity, then the standard sex contract story needs to be updated to allow for the possibility of more flexible and opportunistic female mating strategies. If alloparental care was available from non-fathers, then mothers would not have depended entirely on male provisioning and could have more room for strategic maneuvering through matrilineal coalitions and extra-pair mating  (Greiling & Buss, 2000).

While many details are needed to flesh out her narrative, Hrdy manages to synthesize a remarkably diverse catalogue of evidence from a variety of academic fields to paint a picture of the human species that tentatively answers the question: Why us and not them? The field of evolutionary psychology has long been accused of telling groundless “Just so stories” that miss the complexity of human life, but Hrdy’s book is a persuasive testament to the sweeping power of informed evolutionary explanation. Hrdy weaves decades of interdisciplinary research into a compelling and charmingly human story, one that challenges the necessity of overly Machiavellian or “demonic” metaphors[4] when describing the whole of our prosocial life, particularly when it comes to understanding the emotions that regulate the critical mother-child relationship. If nothing else, Mothers and Others paints a tantalizing portrait of what 21st evolutionary psychology might look like, and for that, Hrdy should be commended.


Barash, D., & Lipton, J. (2001). The Myth of Monogamy: Fidelity and Infidelity in Animals and People. New York: W.H. Freeman and Company.

Choi, J. K., & Bowles, S. (2007). The coevolution of parochial altruism and war. science, 318(5850), 636-640.

Ellsworth, R. (2011). The Human That Never Evolved. Evolutionary Psychology, 9(3), 325-355.

Greiling, H., & Buss, D. M. (2000). Women’s sexual strategies: The hidden dimension of extra-pair mating. Personality and Individual Differences, 28(5), 929-963.

Hawkes, K. (1991). Showing off: tests of an hypothesis about men’s foraging goals. Ethology and Sociobiology, 12(1), 29-54.

Herrmann, E., Call, J., Hernandez-Lloreda, M. V., Hare, B., & Tomasello, M. (2007). Humans Have Evolved Specialized Skills of Social Cognition: The Cultural Intelligence Hypothesis. Science, 317(5843), 1360-1366.

Hrdy, S. (2009). Mothers and Others: The Evolutionary Origins of Mutual Understanding. Cambridge, MA: Harvard University Press.

Ryan, C., & Jethá, C. (2010). Sex at dawn: The prehistoric origins of modern sexuality. New York: Harper.

Thornhill, R., & Gangestad, S. W. (2008). The evolutionary biology of human female sexuality. New York: Oxford University Press.

Tomasello, M. (1999). The Cultural Origins of Human Cognition. Cambridge, MA: Harvard University Press.

Wrangham, R., & Peterson, D. (1996). Demonic Males: Apes and the Origins of Human Violence. Boston: Houghton Mifflin.


[1] Hrdy claims that despite abundant evidence for intergroup conflict within the past 10-15,000 years, “there is no evidence of warfare in the Pleistocene” (p. 19). Rather, homicidal violence among hunter-gatherers “tend to involve individuals who know each other rather than warfare between adjacent groups” (ibid.).

[2] Hrdy also points out tha “cooperative breeding occurs in a taxonomically diverse array of anthropod, avian, and mammalian species, including some 9 percent of roughly 10,000 species of birds and at least 3 percent of all mammals” (p. 177).

[3] The latter book has a more aggressive and less scholarly tone than the former, but both are challenging similar elements of the standard monogamous sex-contract narrative.


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