Beyond Versus: The Struggle to Understand the Interaction of Nature and Nurture

6 The Interaction of Nature and Nurture: An Integrated Concept

Fisher and Hogben, Jensen and Lewontin, Moffitt and Caspi, alongside their defenders and critics—for close to a century now, disputants at each turn of the interaction debates faced off over the following three questions: • the conceptual question—what is interaction? • the investigative question—why and how should interaction be investigated? • the evidential question—what is the empirical evidence for interaction? In the previous chapter, I intentionally avoided any mention of the interaction of nature and nurture because the issues there were about the nature/ nurture debate generally, not about the more specific interaction debates. The goal of that chapter was to construct an explanatory bridge between the variation-partitioning and mechanism-elucidation approaches in terms of the thing to be explained, the causal question about that thing, the thing that does the explaining, and the methodology used to provide that explanation . Population thinking about mechanisms offered that explanatory bridge, presenting a relationship wherein the answers and results from one approach could co-inform the questions and investigations of the other approach. With that general relationship in place, it is time to return to the conceptual, the investigative, and the evidential questions. I will proceed by treating each question in turn. The Conceptual Question: What Is Interaction? The conceptual, investigative, and evidential questions all arose time and again in the interaction debates. But there is an imbalance in terms of the weight of each of these questions, in that the investigative and the evidential questions presume an answer to the conceptual question. Questions 6 The Interaction of Nature and Nurture: An Integrated Concept 134 Chapter 6 about the way to investigate interaction or the empirical evidence for interaction presuppose having already answered what the investigation is investigating and what the evidence is evidence of. As a result, we must start with the conceptual question. Fisher had a very clear answer to this question. Interaction was a statistical measure of the breakdown in additivity between the main effects of nature and nurture. This is what I called the “biometric concept of interaction .” Fisher, when he first worried about the problem, paved the way for all future variation-partitioning answers to the conceptual question— interaction was a “deviation from summation formula” and the “nonlinear interaction of environment and heredity” (Fisher and Mackenzie 1923). For Fisher, interaction was conceptualized as an absence—an absence of summation, an absence of linearity, an absence of additivity. As we saw in chapters 3 and 4, Fisher’s answer to the conceptual question was carried forward by subsequent members of the variationpartitioning approach to nature and nurture as well as philosophical defenders of that approach; and, importantly, these subsequent members/ defenders were particularly critical of any discussions of interaction that conceptualized it differently. Jay Lush specifically considered (but then dismissed) the “nonadditive combination effects of heredity and environment ” (Lush 1937, 64), and Jensen bemoaned the “considerable confusion concerning the meaning of interaction” (Jensen 1969, 39). On the philosophy side, Neven Sesardic distinguished statistical interaction from commonsense interaction and similarly complained about confusions of the two (Sesardic 2005). And in the most recent episode of the interaction debates, critics of Moffitt and Caspi warned against “misconceptions about geneenvironment interactions”; the appropriate conception of interaction, these authors clarified, “always refers to a measure of statistical interaction” (Zammit, Owen, and Lewis 2010, 65). Hogben answered the conceptual question quite differently. Interaction was not simply the absence of perfect additivity between the hereditary and environmental causes of variation; interaction was the presence of a unique source of variation—what Hogben called the “third class of variability ,” which “arises from the combination of a particular hereditary constitution with a particular kind of environment” (Hogben 1932, 98). This is what I called the “developmental concept of interaction,” because, for Hogben, it was a uniquely developmental phenomenon. As we also saw in chapters 3 and 4, Hogben’s answer to the conceptual question was carried forward by subsequent proponents of the mechanismelucidation approach to nature and nurture as well as philosophical The Interaction of Nature and Nurture 135 defenders of it. Waddington defined interaction as “a difference of degree in environmental sensitivity to the development controlled by two genotypes ” (Waddington 1957, 94). And during the IQ controversy, Lewontin and Layzer both made much of interaction, emphasizing the “complicated developmental process in which genetic and environmental factors are inextricably mingled” (Layzer 1972, 275). Subsequently, a host of Lewontinians carried this concept of interaction...