Behavior Genetics and Twin Studies: Principles, Analytical Techniques, and Data Resources for Innovative Communication Research
Social scientists are often interested in identifying factors that lead to individual differences in traits and behaviors such as intelligence, obesity, criminal delinquency, political attitudes, and media use. Traditionally, individual differences in such traits and behaviors have been attributed to the influence of parents, culture, and other social-environmental forces of “nurture.” Yet, almost a century of research in the field of behavior genetics has shown that nurture plays only a small role in what makes people distinct psychologically and behavio- rally. Instead, variation in hundreds of complex traits and behaviors, from personality characteristics to tobacco use, is primarily rooted in genes, or “nature” (Plomin, 2018; Poldennan et ah, 2015).
The logical implication is that genes should also account for variability in the expression of communication traits and behaviors. However, few researchers have used behavior genetic approaches to investigate the biological origins of traits and behaviors of interest to the communication discipline. This chapter attempts to address this gap by providing readers with a brief introduction to behavior genetics and twin studies.
PRINCIPLES OF BEHAVIOR GENETICS AND TWIN STUDIES
Behavior genetics is a field of scientific research in which investigators pursue fundamental explanations for individual differences. The central goal of behavior genetics is to trace variation in the way organisms appear, think, feel, and act to root genetic and environmental causes.1 To do so, researchers rely on genetically informative research designs that allow them to isolate the effects of genes and the environment on traits and behaviors of interest. Using these designs, researchers have repeatedly shown that variation at the genetic level partly explains why many complex traits and behaviors—or, phenotypes—differ so extensively within populations of organisms.
In behavior genetics, the term phenotype is used as a catch-all that refers to any observable physical, physiological, cognitive, psychological, or behavioral characteristic of an organism. Hair color is a phenotype, as is a complex psychological trait such as extraversion. Critically, from a behavior genetic perspective, all phenotypic variation within populations of organisms can be traced back to a relative combination of genetic and environmental factors or an interaction between the two (see Figure 6.1). This perspective makes intuitive sense: all phenotypes can only originate in and be shaped by genetic and environmental forces, which in behavior genetics are treated as the only sources of phenotypic variation.
For example, consider Tay-Sachs disease. Phenotypic variation in the symptoms of a rare inherited illness such as Tay-Sachs is almost entirely explained by underlying genetic mechanisms. The reason why one person develops Tay-Sachs symptoms and another does not is entirely due to genetic variation between individuals, not variation in the social or physical environment. Conversely, a facial scar that results from an idiosyncratic childhood trauma is a phenotype that is almost entirely accounted for by random environmental variation, not genes. For personality traits, IQ, educational attainment, political ideology, voter turnout, and hundreds of other complex phenotypes, both genetic and environmental factors play a role (see Lockyer & Hatemi, 2018; Plomin, DeFries, Knopik, & Neiderhiser, 2016). The question is: To what degree do genetic and environmental factors explain variation in the expression of complex psychological and behavioral phenotypes? One behavior genetic research design, the twin study, is uniquely positioned to answer this question.
A twin study is a genetically informative research design used to determine the extent to which genetic and environmental factors contribute to individual differences in traits and behaviors. Twin studies represent only one behavior genetic design that relies on biological relationships among kin to disentangle genetic and environmental influences on phenotypes.2 However, twin studies are considered an important research design because they can capitalize on two unique properties associated only with twin research participants.
Figure 6.1 The Behavior Genetic Perspective
Source: Adapted from Blokland, Mosing, Verweij, & Medland (2013).
The first property relates to how twins develop in utero (Figure 6.2). Specifically, it is well established that identical or monozygotic (MZ) twins are conceived from a single zygote, or fertilized ovum, that divides into two separate yet indistinguishable embryos, ultimately leading to the production of two genetically identical same-sex individuals. Fraternal or dizygotic (DZ) twins, on the other hand, develop from separate zygotes that ultimately become same- or opposite-sex twins who share roughly half of their DNA in common just as frill siblings do.
Unlike full siblings, however, both fraternal and identical twins share highly similar social and physical environments. This is the second property. All twins share the same uterine conditions during their initial development. All twins are bom at roughly the same time, and all twins presumably experience a comparable upbringing by the same parents in the same culture.
Thus, twins act as a kind of “natural expeiiment” in which shared aspects of their environment is the factor that is “held constant'’ or “controlled” within pairs while shared DNA (roughly 50% for fraternal twins and 100% for identical twins) is the factor that varies between identical and non-identical pairs.3 Other first-degree relatives, as well as grandparents, cousins, and so on may share cextain degrees of genetic similarity, but none experience a similar upbringing in the way twins do. In fact, it is these two properties unique to twins—shared genetics and environments—that make twin studies a powerful and effective research design, and a design that is increasingly popular in the social sciences.
