In reading this book, one gets the sense that Peter Richerson and Robert Boyd could have written many volumes on the role of culture in human evolution. To their credit, they have confined themselves to a succinct and eminently readable 257 pages, in which they make a compelling argument: neither human behavior nor human evolution can be understood without taking culture into account. The book draws together an impressive array of data and theory to generate a coherent picture of the coevolutionary dance between human genes and culture and to provide a framework for future research, suggesting that changes in both genes and culture can be studied using population-level Darwinian models of evolutionary change.

In the first chapter, Richerson and Boyd identify the two main points of the book. The first is this: in order to understand human behavior, we must understand culture. There are significant patterns of geographical variation in human behavior at the level of social groups (tribes, nations, ethnic groups, etc.), and neither genes, nor environment, nor a combination of the two is sufficient to explain them. Culture—which the authors define as socially acquired information that is “capable of affecting individuals’ behavior” (5)—is crucial. Genetic explanations fail because genetic differences rarely demarcate social groups. Based on studies of cross-cultural adoption and rapid cultural change, Richerson and Boyd argue that genes cannot explain most cultural differences. Environmental explanations also fail to suffice, since people who live in very similar physical environments often differ substantially in lifeways. Richerson and Boyd cite the Nuer and Dinka tribes of southern Sudan and the German and ‘Yankee’ farmers of southern Illinois as just two examples of this phenomenon. Although genes and environment play important causal roles in human behavior, culture is a key difference maker at the level of populations. What's more, culture can create biological feedbacks. Richerson and Boyd argue that the cultural transmission of beliefs and norms facilitated a biological evolutionary trajectory for human beings that would have been otherwise impossible—which leads to the second main point of the book, that “culture is part of biology” (4).

This second claim requires unpacking. It is not meant to imply that culture can be reduced to biology. Rather, Richerson and Boyd suggest that biology ought to be expanded to encompass culture, because culture and biology are deeply connected, and in humans, neither can be understood independently of the other. Indeed, one of the central achievements of the book is to show how culture, environment, and biology (in the traditional, unamended sense) intricately intertwine, and the account of human evolution that Richerson and Boyd sketch includes central roles for all three. Biology, for example, provides the foundations for culture: cumulative cultural evolution has certain cognitive prerequisites such as the capacity to learn by imitation, and the basic cognitive building blocks for imitation must have been in place before culture took off in protohumans. However the environment, hypothesize the authors, also played a critical role. The ‘deterioration’ of climate during the later Pleistocene, which involved increased climatic variability at short timescales (in the range of hundreds to thousands of years, rather than tens of thousands of years in earlier periods) may have potentiated the evolution of culture by favoring it as a solution to the problem of managing a diverse and shifting environment that outpaces the rate of biological evolution. Natural selection can favor culture when culture allows rapid adaptation to local environmental heterogeneity, and when reliance on culture is more efficient than individual learning.

In addition to making the general case that culture is important in understanding human behavior, and that culture—as a whole—is an adaptation to variable environments, Richerson and Boyd explore the mechanics of gene-culture coevolution, laying out the ‘the forces of cultural evolution’ in their third chapter. The aim here is to develop a conceptual taxonomy for cultural evolution that can complement the existing framework for genetic evolution. In the genetic case, there are mutation, natural selection, and drift; in the cultural case there are ‘random forces’ (cultural mutation and cultural drift), ‘decision-making forces’, and ‘natural selection’.

The distinction between these latter two forces is a bit tricky. Under decision-making forces, Richerson and Boyd include any process in which the psychology of the adopters influences the dynamics of a cultural trait. Thus these forces include the transformation of cultural variants (beliefs, practices, etc.) through learning or invention, and the use of particular rules (conscious or unconscious) in selecting which cultural variants to adopt. Utilizing cultural learning rules, for example, one might adopt the most common beliefs about what sorts of food to eat, or imitate the successful individuals in one's community regarding what sorts of clothing to wear. (Richerson and Boyd argue that propensities to use these two cultural learning rules probably were favored by natural selection.)

Decision-making forces differ from the forces of natural selection because in the latter case transmission depends not “on what is going on in the brains of imitators,” but on how well cultural variants survive and reproduce in a way that is “independent of human desires, choices and preferences” (79). Thus natural selection does not occur through a cultural trait's effects on others’ decisions about whether to adopt the variant in question. Nor does natural selection occur when a trait proliferates due to unconscious effects on adopters that facilitate its spread. In the authors’ taxonomy, the memorability of a trait is not a feature that plays a role in natural selection, since memorability involves the psychology of the adopters.

Instead, natural selection of cultural variants results from “the effects of holding one cultural variant rather than others” (69). For example, natural selection occurs when a trait leads its possessor to behave in ways that make him unlikely to serve as a model for others. The authors cite drug addiction, which, if it lands its possessors in jail, removes the jailed individual from the population of cultural models and thus confers reduced fitness on the drug addiction trait. To prevent this from being an instance of a decision-making force, it must be the case that the drug addict's behavior and beliefs are made less visible to others in virtue of his incarceration and not the case that others choose not to imitate the jailed individual because he is in jail. (The latter might be an implementation of the cultural learning rule don't do what the jailbirds—or the otherwise socially unsuccessful—do.)

I found Richerson and Boyd's taxonomy and their application of the term ‘natural selection’ to the sort of process at issue in the jailed drug addict's case to be awkward and confusing. The authors eschew talk of cultural selection, but the distinction between natural selection in the ordinary sense (involving the survival and reproduction of biological individuals or genes) and cultural selection (involving the persistence and spread of cultural traits) is much more intuitive than the taxonomy the authors present. A classification that distinguishes natural from cultural selection permits a distinction between the biological and cultural fitness of various cultural variants that Richerson and Boyd's taxonomy cannot capture. As it stands, the authors don't have a clear way of conceptually delineating the fitness of cultural traits, and the rate at which a cultural trait spreads may not reflect very closely the effects of what they call natural selection because there are so many other distorting forces in operation. This generates a serious and confusing disanalogy between cultural and biological evolution. A different taxonomy could better preserve the analogy between genetic and cultural evolution and take advantage of the conceptual work that has gone into the development of biological evolutionary theory. Perhaps the taxonomy Richerson and Boyd develop is meant to be subversive, part of the program that enfolds culture within biology rather than distinguishing the two. But if so, more argument is needed to establish it.

Despite these complaints, the authors’ parsing of cultural evolutionary forces is in many ways a significant achievement, for it represents an admirable effort to make sense of the complex factors that influence cultural evolutionary dynamics. What's more, the book as a whole opens substantial new frontiers for the integration of the biological and social sciences in an effort to understand human behavior. The authors enter a sustained plea for the use of population-level models in this enterprise, and encourage the development of “quantitative ethnography” as a means of generating the relevant data on patterns of cultural variation and cultural change over time (251). The book itself is much less quantitative, and hence more digestible and accessible, than the authors’ earlier Culture and the Evolutionary Process. The argument for modeling would have been strengthened by walking through a concrete example within the text; however, the authors’ more technical papers, cited in the bibliography, provide a rich resource for the interested reader.

For philosophers of science and philosophers of biology, Not by Genes Alone opens vast territory for exploration. What is the best way to think about the relationship between cultural and biological evolution? How does cultural evolution challenge traditional evolutionary concepts and categories, in which vertical transmission (from biological parent to offspring) is the norm? What new evolutionary dynamics can horizontal (peer-to-peer) and oblique (e.g., from teacher to student) transmission generate? To what extent does the argument of this book—by emphasizing the causal power of culture to generate biological feedbacks—cut against reductionist accounts of human evolution? In what ways does biological evolution constrain culture, and vice versa? The authors are right to have great hopes for this book: it is one whose legacy is not yet written.