Twin Studies in Behavioral Genetics
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The twin studies design has been extensively used in the behavioral genetics research.
The common assumptions on the twin studies research include the notion of the greater resemblance between monozygotic (identical) twins as opposed to fraternal (dizygotic) ones (Plomin & Daniels, 2011, p.566). As the twin studies research design assumes the presence of shared environment, it may be expected that the heredity would generally play a greater role in behavioral phenotype development.
For instance, in Kaplan’s (2012) study of adult intelligence heritability, the identical twins’ weighted average was .841, with almost 90% of their IQ phenotype’s variability explained by the common genetic heredity (Kaplan, 2012, p.1294). Similarly, Nishihara et al.’s (2011) measurements of relative magnitude of the factors influencing the association between social role function’s limitations and depressive symptoms in Japanese elderly male twins demonstrated that the genetic coefficient (-0.47) was more prominent than an environmental one (-0.31) (Nishihara et al., 2011, p.23). Thus, twin studies allow for the precise evaluation of the dominant role of genetic factors in the development of behavioral phenotypes.
On the other hand, twin studies may enable a more rigorous control of the “shared environmental background” (Johnson et al., 2009, p.218). The study of the interplay between genetic and shared environmental factors in the twins’ anti-sociality undertaken by Eaves, Prom, & Silberg (2010) may be a typical example of this approach, with the shared environmental factor of exposure to parental neglect accounting for 30% of the twins’ anti-sociality observations. In the same vein, McCaffery et al. (2008) managed to establish the relative correlations between genetic, shared and non-shared environmental factors for cigarette smoking and depressive syndromes among male and female adolescent twins, demonstrating the validity of this approach.
The non-shared environment factors may be successfully isolated with the help of the twin studies research design. For instance, differences in political beliefs and voting behavior among the identical (MZ) twins cannot be reasonably contributed to the genetic factors, as these behaviors are the result of complex cultural and ideological influences. However, after the shared environment factors’ influence (e.g. parental political attitudes or the socio-demographic group affinity) is controlled, the real impact of the non-shared environmental factors (e.g. personal political influences) may be objectively assessed (Johnson et al., 2009, p.219). This example shows how the twin studies design would enable the researcher to compare and contrast the genetic-environment behavioral influences and their interactions.
At the same time, twin studies have their own significant limitations. In particular, the equal environments assumption with respect to the environmental factors experienced by the identical / MZ and fraternal / DZ twins may not be properly validated by the current quantitative research, making the assessment of the relative importance of genetic and environmental factors in determining the DZ twins’ behavior rather incomplete (Rutter et al., 2001). Further, the problem of the twin studies findings’ extrapolations to the non-twin populations remains a pressing one, as it significantly limits the design’s universality (Nuffield Council on Bioethics, 2002, p.43).
However, in general, such limitations do not cancel the fact that twin studies remain one of the most popular research designs in behavioral genetics, and that their validity was proved by the numerous empirical studies’ results. Hence, their further development remains one of the most hopeful directions in behavioral genetics.