Moments of Truth in Genetic Medicine

Chapter 5: Two Peas in a Pod: Twin Science and the Rise of Human Behavior Genetics

chapter 5 T W O P E A S I N A P O D Twin Science and the Rise of Human Behavior Genetics Twin studies require resolution of the question of zygosity—whether samesex twins are derived from a single fertilized egg or from two different fertilizations . The “twin method” compares these two types of twins, and knowing which twins fall into which category is crucial. Scientists in the immediate postwar period determined zygosity through blood typing, anthropometrics , fingerprint analysis, and comparison of obvious phenotypic qualities such as hair color, eye color, and facial features. But in 1961 a widely cited Swedish study compared the accuracy of these technical, metrological means of diagnosis with the accuracy resulting from asking the twins themselves a single question: “Were you and your twin as alike as two peas in a pod?” By this group’s calculation, of those twin pairs in which both twins answered yes, 98 percent were in fact monozygotic (identical). In other words, asking the twins themselves this particular question was as reliable in the determination of zygosity as were blood testing, phenotype analysis, and fingerprinting combined. Twins, researchers found, generally knew something about their own cellular origins (Cederlof et al. 1961). The paper by Cederlof and colleagues, and papers by several other groups replicating its results, are still cited as justifications for using questionnaires to determine zygosity in large-scale populations of twins. In fact, large-scale studies of twins would be impractical without this inexpensive strategy, the “questionnaire method of determining zygosity.” From my perspective this method fuses social and technical knowledge, or folk and scientific knowledge . An irretrievable gestational experience—having been derived from a single egg—is inferred on the basis of social consensus, and technical analysis testifies to the legitimacy of social perception: people know whether twins are monozygous or dizygous, and their knowledge has become a convenient surrogate for the results of the blood test, the cranial x-ray, examination of the birthing materials, or the DNA probe (Chen et al. 1999; Peeters et al. 1998; Goldsmith 1991).1 Twins are an important resource for research in human behavior genetics . Their knowledge of their own bodies has been embedded in the technical knowledge produced about behavior and genes. Folk or social knowledge has also been invoked to justify the behavior genetics research program. Behavior geneticists in the 1960s and 1970s commonly constructed their work as the mathematization of everyday experience. By demonstrating the genetic causes of human behavior, they suggested, they were validating what was already widely known by people in general: that talents, intelligence, and personality were inborn. In this chapter, I explore twins as physical and social resources for the explosion in human behavior genetics after about 1955. I emphasize the practices involved in the creation and use of vast population registries of twins. I look particularly at the National Academy of Sciences–National Research Council Veteran Twin Registry, developed after 1955 and eventually including almost sixteen thousand pairs of white male twins who had served in the armed forces in World War II. My chapter title, drawn from the “questionnaire method of determining zygosity” used in the Veteran Twin Registry and in many others, is suggestive of my central themes. The image of “two peas in a pod” resonates with Mendel’s peas and the critical role of peas in the history of genetics; with the agricultural origins of the eugenics movements around the world, which drew on the commonsense knowledge of farmers to make claims about human society; and with Darwin’s profound debt to folk knowledge of domestic breeding in the development of his theory of evolution by natural selection. I suspect that the “peas in a pod” image was simply a convenient and familiar way to express biological sameness , using a commonplace (Western) phrase that subjects would readily T W O P E A S I N A P O D 1 2 1 comprehend. But it is also a remarkably rich image, a cultural shorthand for both folk knowledge and the social meanings attached to things and people that are “the same.” By the late nineteenth century, heredity had long been understood to be involved in what might be called “instinctual” behaviors such as maternal love. The British eugenicist and statistician Francis Galton began promoting twins as resources for behavior genetics in 1875, a date that stands as the traditional starting...