Martin Daly
Few topics demonstrate psychological science's desperate need for evolutionary theory as starkly as parenting and kinship. Lacking a Darwinian perspective, mainstream psychologists had no idea how to approach these central domains of human existence, and simply neglected them. If you can somehow collect a valid, representative sample of human social interactions, I guarantee you that close genetic relatives will be prominent in your data, but no social science has paid less attention to “blood ties” than social psychology. And although parental care consumes an immense proportion of human time, energy, and attention, psychologists have had little more to say about the psychology of parenting.
Google “parenting” plus “psychology” and what you'll find is pep talks encouraging you to be more child-centric and advice about how to prepare your kids for success. (Should you be more of a “tiger mother”?) How parents actually feel and behave surfaces mainly in taxonomies of parenting styles, which are discussed only with respect to their alleged impacts on child development. Well, that's pop psychology, but the academic literature, except for that which is explicitly evolutionary, yields scarcely more. Consult a general motivation textbook or treatise, for example, and you may find, at best, a page or two on how to induce the “maternal state” in a virgin rat, or whether maternal “drive” results from pressure in the mammary glands. And yet the theory required to generate a rich set of well-founded and well-supported hypotheses about the sources of variability in women's and men's parental efforts and inclinations has long been available. As the six ensuing chapters, comprising Part IV of this handbook, all demonstrate, the theory that psychologists have so desperately needed was and is Darwin's. All motivational mechanisms, including those modulating parental efforts, are products of natural selection, and can, therefore, be understood as strategic means to the end of genetic posterity.
As regards kinship, genealogical relatedness has been central to evolutionary biology's analysis of social phenomena since Hamilton (1964), whose seminal theory of inclusive fitness is introduced in Chapter 19 by Hames and informs all six chapters in Part IV. Because Hamilton's theory identifies what it is that any species' social psychology has evolved to accomplish, it is generally recognized by biologists as the essential framework for analyzing social evolution (West & Gardner 2013). But you don't need math to recognize the centrality of kinship in human affairs. Anthropologists just had to watch people and listen to them. According to the eminent British social anthropologist Edmund Leach, “Human beings, wherever we meet them, display an almost obsessional interest in matters of sex and kinship” (Leach 1966, p. 41).
Ask any sample of people who they feel closest to, care the most about, would sacrifice the most to help, and the bulk of the nominees will be either close genetic relatives or the romantic partners of your respondents. It is, therefore, scandalous that experimental social psychology remains overwhelmingly a science of stranger interactions, a state of affairs that I blame mainly on an addiction to the convenient “pool” of captive undergraduate research participants. As I write, the most recent complete volume of the Journal of Personality & Social Psychology, considered by many to be the field's top journal, is Volume 106, January–June, 2014; the 58 primary research reports therein include just six that are about social interaction with anyone other than strangers. Only one treats genealogical kin as a meaningful social and mental category.
Fortunately, the evolutionary psychologists who have tackled these neglected topics have had a lot of help from evolutionary biologists, and from anthropologists, many of whom are contributors to this handbook. Attending to and collaborating with anthropologists is a psychologist's best defense against making the ethnocentric error of imagining that the familiar practices of her own culture provide a direct window onto human nature. So it is a heartening sign of synthesis in the community of scientists who study “evolution and human behavior” that four of the five chapters in this Handbook's Part I (Foundations of Evolutionary Psychology) have been authored or coauthored by anthropologists. And here, in Part IV, we have three new chapters, all by anthropologists, in addition to three chapters that constitute updated versions of contributions to this Handbook's first edition, one of which is again by anthropologists.
In Chapter 19, Raymond Hames, an evolutionary anthropologist with extensive field experience among tribal horticulturalists in Amazonia, provides a brisk review of Hamilton's inclusive fitness theory, and of some of the many ways in which it has begun to elucidate human affairs.
The most striking peculiarity of human family life, in comparison to the behavior of other living hominids, is the parental participation of men, so in Chapter 20, David Geary provides a thorough and thoughtful review of the ideas and evidence bearing on why paternal investment evolved in our lineage, and why it nevertheless remains spotty. This review updates Geary's treatment of the same topic in this Handbook's first edition, incorporating new evidence on the endocrinology of paternal behavior, on fathers' impacts on child outcomes, and on how boys' childhood experiences affect their subsequent behavior as fathers. Almost half of the 120 references cited in this revised chapter are new.
Like Geary, Catherine Salmon has updated her treatment of parental investment and parent-offspring conflict in Chapter 21. Since the seminal paper by Trivers (1974), formal models of parent-offspring conflict have proliferated, but Salmon is able to convey the essential ideas with admirable clarity while eschewing mathematics. One novel topic is the initially puzzling phenomenon of systematic disagreement between young adults and their parents about the offspring's mate-selection criteria, and Salmon briefly introduces the new thinking and findings.
Chapter 22, by anthropologist Ruth Mace, is a new addition to the Handbook. Mace endeavors to integrate many important topics of relevance to human social evolution and cross-cultural diversity, including the peculiarities of our species' evolved life history, with its prolonged prereproductive phase and its even stranger postreproductive phase; how different modes of subsistence, property ownership, and heritable wealth affect and are affected by family structure and family relations; and why fertility has declined in a context of relative abundance (the demographic transition), in seeming defiance of a Darwinian imperative to maximize reproduction.
Chapter 23, by anthropologists Coren Apicella and Alyssa Crittenden, is another new addition to the handbook, zeroing in on what we know about parenting and kinship among hunter-gatherers. Evolutionary psychologists have long stressed the importance of hunter-gatherer lifeways as the crucial social and material environment of evolutionary adaptedness (EEA) in which our species' attributes evolved. This view has sometimes been criticized as unduly essentialistic on the grounds that hunter-gatherer societies are diverse, that contemporary hunter-gatherers have been affected by their agricultural neighbors, and that the hallmark of human success has been flexibility of social practices. Apicella and Crittenden are fully cognizant of these complications, but make a convincing case that hunter-gatherer studies do indeed provide crucial evidence bearing on hypotheses about the nature of human sociality and about how and why it evolved.
Chapter 24, by anthropologists Mark Flinn and Carol Ward, is another update, with the central topic being the peculiarities of the human family, human development, and social endocrinology. In reviewing the latest information and ideas on these topics, Flinn and Ward maintain a strong comparative focus, and thereby make it evident that a number of aspects of human family life and sociality more generally are true evolutionary novelties.
While celebrating this interdisciplinary synthesis, we mustn't forget that convincing anthropocentric social scientists that they need a Hamiltonian overview is a long, uphill struggle, and one that is far from over, as will be apparent to anyone who peruses recent issues of the many journals that include the word family in their titles. Anthropology had a head start over psychology in its attention to kinship, but influential figures like Marshall Sahlins and David Schneider naively insisted that cross-cultural diversity proves that human kinship has no biological basis, and the bankrupt biology-culture dichotomy still bedevils the extensive, stagnant backwaters of cultural anthropology. We're all in this together.
In my opinion, there is one substantive challenge to the proposition that Hamilton's inclusive fitness theory has given us the essential metatheory for social evolution, and hence for evolutionary social psychology. None of the six chapters in Part IV mention it, so I feel I should. No, it's not any of the multilevel selection models that have tried to give between-group selection new life as an explanatory principle. Even their proponents admit that multilevel models are simply alternative accounting practices that are interchangeable with inclusive fitness accounting, and I am not aware of any cases in which they have yet been shown to yield more fruitful insights. The substantive challenge to which I refer is one “from below” rather than “from above”: the issue of intragenomic conflicts.
Imagine a woman with a novel mutation on one of her two X chromosomes. There is a 50% chance that her X (like any of her autosomal genes) will be transmitted to each child, regardless of its sex, but what of her grandchildren? Her daughter's children of either sex have a 25% chance of inheriting the mutant, but if her son has a daughter, that granddaughter will definitely get her father's maternal-origin X (the only one he has) and thus has a 50% chance of carrying the mutant, whereas a son's son has a 0% chance. Now imagine that the mutation's phenotypic effect is to bias investment toward sons' daughters at the expense of sons' sons. Any resultant gain in the son's daughter's fitness, however small, at the expense of a loss in the son's son's fitness, however large, would be sufficient to give the mutant X a selective advantage! This situation, dubbed “sexually antagonistic zygotic drive” (SAZD) by Rice, Gavrilets, and Friberg (2010) is not simply hypothetical (Friberg, Stewart, & Rice, 2011).
One might imagine that SAZD couldn't possibly be important in creatures like ourselves, given both the competing interests of the rest of the genome and the fitness interests of other relevant persons besides paternal grandmothers. But if costs to male children were offset by benefits to female children, selection for suppressors would be weak until the sex ratio became seriously unbalanced. Interestingly, there are data that have been interpreted as indicative of SAZD in humans (Fox et al., 2010), and the case is open.
SAZD is an intriguing idea, but its relevance in human social evolution, if any, is still up in the air. However, we already have plenty of convincing evidence that genes whose transmission dynamics make their fitness interests distinct from the inclusive fitness interests of their organism have important phenotypic effects on intrafamilial interactions, especially imprinted genes (Crespi, 2011; Haig, 2002, 2009). During the sociobiological revolution that began in the 1960s, as it became increasingly clear that one had to think about the “fitness interests” of genes in order to understand social evolution, it was Hamilton's concept of inclusive fitness that allowed us to retain our focus on the organism as an actor with an integrated agenda. But the many forms of intragenomic conflict oblige us to acknowledge that the apparent integrity of the individual is to some degree illusory (Burt & Trivers, 2008).