Philosophers of science should read Kenneth F. Schaffner’s new book twice. Schaffner wrote Behaving: What’s Genetic, What’s Not, and Why Should We Care? over the course of 2 decades. During that time, the science of behavioral genetics evolved significantly, and Schaffner’s philosophical thinking clearly evolved with it. The chapters of the book, however, are organized pedagogically, not chronologically. Read in their pedagogically ordered sequence, philosophers are ushered into the complicated world of behavioral genetics by way of fascinating and detailed case studies involving antisocial worms and debilitating mental health conditions; Schaffner then uses those case studies as a foundation to inform balanced assessments of precisely what genetic differences do and do not contribute to behavioral differences. When the chapters are read in their chronologically ordered sequence, philosophers are treated to a vision for how Schaffner’s own thoughts concerning genetic causation and explanation changed with the times.

The Pedagogical Reading: Schaffner organized Behaving with a pedagogical mind-set and for good reason. The science of behavioral genetics draws on concepts and methods from a range of disciplines, including quantitative-statistical genetics, molecular genetics, developmental psychology, psychiatry, and neuroscience. The philosophical issues inherent to the field derive from how those disciplines attempt to explain the world, and so Schaffner creates space early in the book to convey to readers how the science works. The first two chapters are a modern spin on the Socratic dialogue, this time involving a judge as the curious pupil and a behavioral geneticist as the educational guide. It is a great format to introduce the science of behavioral genetics; the judge asks the sorts of questions the reader probably has, and the behavioral geneticist answers in ways understandable to the nonspecialist. The next three chapters draw on research involving model organisms to assess questions of explanation, extrapolation, and reduction. Personality, depression, and schizophrenia are the focus of chapters 6 and 7, where issues of classification and replication are considered. The final, substantive chapter is all about the “why should we care?” question regarding behavioral genetics; it evaluates the extent to which behavioral genetics challenges free will. (Spoiler alert: It does not.)

Readers will find a common structure in almost all the chapters of Behaving. Schaffner opens by introducing a contested philosophical issue; he then spends the bulk of the chapter examining a case study in great detail; finally, he reflects back on the philosophical issue with the lessons from the case study in hand. Behaving, as a result, is full of rich case studies that reveal how the science of behavioral genetics actually works. Schaffner’s purpose is to get beyond the research results in order to evaluate how the results were generated with a close inspection of the research methods; the cases are part history of science and part philosophy of scientific practice, all with an eye to what we can reliably infer about genetic causation and explanation from the studies. Chapter 3, for example, includes an extended discussion of behavioral genetic research on C. elegans—the common roundworm that has become a model organism for behavioral genetics. Schaffner provides a brief history of how C. elegans became a model organism, basic facts about worm anatomy and physiology, and a thorough assessment of a famous study that tracked a difference in a behavioral trait (social vs. solitary feeding) to a genetic difference. Similarly, chapter 7 on schizophrenia provides a primer on how the mental disorder is diagnosed, a brief history of quantitative and molecular genetic research on the condition, and a detailed analysis of one model of schizophrenia that tracks the disorder down to neural cell signaling pathways.

When Schaffner’s chapters are taken in their entirety, the real virtue that stands out is his constant and careful judgment of precisely what the scientific research does and does not tell us about the role of genetic differences in generating behavioral differences. Behavioral genetics has been a topic of philosophical attention since at least the 1970s, and that literature has historically been extremely polarized. The impact of genetics on behavioral traits and trait differences was often treated as a zero-sum game in which genes were doing either everything or nothing at all. Philosophers on both sides explained away their critics’ disagreements with charges of sociopolitical bias. There is none of that in Behaving. Instead, Schaffner’s protocol is a statement of philosophical theses and then a consideration of the extent to which a detailed case study supports or refutes those theses. The chapter on C. elegans, as one example, starts with a series of theses from developmentally minded philosophers who criticize simple-minded and dichotomous ideas about gene action. Schaffner ultimately concludes that the C. elegans research supports a number of the developmentalist theses (like the idea that genes can only make a difference in a particular molecular, cellular, organismal, and environmental context), but it also challenges other theses (like the idea that genetic causation cannot be teased apart from environmental causation because of that contextual relationship). C. elegans is the resource again in chapter 5 where debates about reduction are scrutinized. In contrast to extreme ideas that behavior will either be completely reduced to molecular-genetic causes or alternatively exist as an emergent property completely unrelated to molecular-genetic causes, Schaffner makes the case for incremental and partial reductions. These partial reductions, Schaffner points out, may not reduce complex human behaviors to genetic processes, but they still shed light on how genetic differences percolate up through molecules, cells, and organisms, and those insights are often rewarded with Nobel prizes.

The Chronological Reading: Schaffner wrote Behaving over the course of 2 decades. It was published in 2016, but he worked on the earliest chapters in the 1990s. Human genetics underwent a radical evolution over that time span. The Human Genome Project took center stage in the late 1990s and early 2000s when human geneticists half hoped/half expected complex behavioral differences to be linked to single genetic differences (“candidate genes”). When it became clear that there were no single genes for complex traits like depression and schizophrenia, geneticists pursued two different tracks. One group tried combining the candidate genes with known environmental causes of human behavior, pursuing gene-environment interaction studies. Another group tried searching across the entire genome for very small genetic effects that collectively added up to a large genetic effect, pursuing genome-wide association studies. The gene-environment interaction route appeared to become the standard-bearer in the mid-2000s, but then replicability concerns about publication bias (common across the life and social sciences at that time) created consternation about that approach. The genome-wide association route then stepped up as the best game in town in the mid- to late 2000s. But it too proved frustrating. For any given behavioral trait today, genome-wide association studies have identified dozens or even hundreds of places in the human genome associated with differences in that trait, but those studies typically account for only a small fraction of the variation for the trait in question. What is more, because the total genetic effect is diffusely spread across many, many genomic regions, it is unclear whether these genome-wide association studies will translate their results into reliable causal explanations about the traits being investigated.

The chronological reading of Behaving reveals how Schaffner’s philosophical thought evolved with the science. Material in chapters 3 and 4 was initially published in the late 1990s (in this journal), and it conveys an optimism about how a complex trait in model organisms could be tracked down to a single genetic difference. Chapters 1 and 2 were written in the early 2000s for an edited volume on behavioral genetics, and a great deal of attention is paid there to the gene-environment interaction studies that were making headlines at that time. Chapter 5, written in the mid-2000s, reveals that, even in the simple organism C. elegans, the story told in chapter 3 turned out to be much more complicated when further research was undertaken. A portion of chapter 7 on schizophrenia was written for a 2013 Oxford Handbook, and there Schaffner’s focus shifts to the genome-wide association studies that were growing in influence. Finally, chapter 6 includes new material written most recently; not surprisingly, we find there Schaffner’s discussions of the replication crisis faced by candidate gene and gene-environment interaction studies as well as the disappointment associated with genome-wide association studies identifying so little variation. Schaffner sums up in chapter 6, “The bottom line on the past 15 years of research into and ‘discoveries’ of genetic effects on behavior, whether these are ‘normal’ personality traits or features of disorders such as anxiety or depression, needs to be a skeptical one” (162–63). Schaffner’s shift from an optimistic outlook to a skeptical one is not apparent on the pedagogical reading; it requires the chronological reading to track that evolution.

This is not to say that Behaving is just a collection of already-published papers. In fact, all of the material that was published in some form earlier has been extensively updated. The earliest chapters on C. elegans, for example, have been modernized with very recent citations on model organism research. Likewise, Schaffner adds a brief introduction to genome-wide association studies in chapter 2. The book is also full of references to “personal communication,” where Schaffner updates a discussion with a recent interview or e-mail exchange he has had with the scientist whose research he is discussing, so as to include the scientist’s latest thoughts on the topic.

Philosophers will be rewarded by reading Schaffner’s new book on the science of behavioral genetics twice. The pedagogical reading conveys balanced judgments in a space more accustomed to polarized accusations. The chronological reading conveys how those balanced judgments have evolved with a science that has changed dramatically over the last 20 years.