scholarly journals Twin studies with unmet assumptions are biased towards genetic heritability

2020 ◽  
Author(s):  
Edwin S. Dalmaijer
Keyword(s):  
Measurement Error ◽  
Cultural Transmission ◽  
Environmental Influences ◽  
Twin Studies ◽  
Property Taxes ◽  
Dizygotic Twins ◽  
Genetic Heritability ◽  
Equal Environments Assumption ◽  
Equal Environments ◽  
Shared Environments

SummaryFor a century [1,2], studies of monozygotic and dizygotic twins have yielded estimates of trait heritability. The clever logic behind them is that while both types of twins share environments, their genetic overlap is different. Hence, larger trait correlations between monozygotic compared to dizygotic twins indicate heritability (nature), whereas similar correlations indicate shared environmental influences (nurture), and low correlations indicate shaping through non-shared environments (external influences and measurement error). While many have written on the assumptions that both types of twins share equal environments [3–5], and that parental genetics and environment are independent [6,7]; fewer have put their data where their mouth is. Here, the impacts of unmet assumptions were investigated using a generative mixture model of twin phenotypes. The results indicated that violations of the equal environments assumption yielded large overestimations of heritability and underestimations of shared environmental influences. On the other hand, when parental genetics shaped twins’ shared environments, only minor non-linear biases against heritability emerged. Finally, realistic levels of measurement error uniformly depressed estimates for genetic and shared environmental factors. In sum, twin studies are particularly susceptible to overestimation of genetic and non-shared environmental influences. This bias could explain why some traits, such as attitudes towards property taxes [8], show suspiciously high heritability without a biologically plausible mechanism. Particularly in the context of traits with convincing mechanisms of cultural transmission [9–11] and complex gene-environment interactions [6], researchers should not allow biases in twin studies to overestimate heritability.

Has the “Equal Environments” Assumption Been Tested in Twin Studies?

Twin Research ◽  
2003 ◽  
Vol 6 (6) ◽  
pp. 486-489 ◽  
Author(s):  
Lindon Eaves ◽  
Debra Foley ◽  
Judy Silberg
Keyword(s):  
Genetic Factors ◽  
Twin Studies ◽  
Shared Environment ◽  
Environment Interaction ◽  
Genotype X Environment ◽  
Dizygotic Twins ◽  
Equal Environments Assumption ◽  
Twin Method ◽  
Equal Environments

AbstractArecurring criticism of the twin method for quantifying genetic and environmental components of human differences is the necessity of the so-called “equal environments assumption” (EEA) (i.e., that monozygotic and dizygotic twins experience equally correlated environments). It has been proposed to test the EEA by stratifying twin correlations by indices of the amount of shared environment. However, relevant environments may also be influenced by genetic differences. We present a model for the role of genetic factors in niche selection by twins that may account for variation in indices of the shared twin environment (e.g., contact between members of twin pairs). Simulations reveal that stratification of twin correlations by amount of contact can yield spurious evidence of large shared environmental effects in some strata and even give false indications of genotype x environment interaction. The stratification approach to testing the equal environments assumption may be misleading and the results of such tests may actually be consistent with a simpler theory of the role of genetic factors in niche selection.

Twin Studies of Political Behavior: Untenable Assumptions?

2008 ◽  
Vol 6 (4) ◽  
pp. 785-791 ◽  
Author(s):  
Jon Beckwith ◽  
Corey A. Morris
Keyword(s):  
Molecular Genetic ◽  
Genetic Research ◽  
Twin Studies ◽  
First Century ◽  
Behavioral Traits ◽  
Political Ideologies ◽  
Equal Environments Assumption ◽  
Twin Method ◽  
Genetic Techniques ◽  
Equal Environments

Using the “classical twin method,” political scientists John Alford, Carolyn Funk, and John Hibbing conclude that political ideologies are significantly influenced by genetics, an assertion that has garnered considerable media attention. Researchers have long used human twins in attempts to assess the degree of genetic influence on various behavioral traits. Today, this methodology has been largely replaced in favor of contemporary molecular genetic techniques, and thus heritability studies have seen a diminishing role in behavioral genetic research of the twenty-first century. One important reason the twin method has been superseded is that it depends upon several questionable assumptions, the most significant of which is known as the equal environments assumption. Alford, Funk, and Hibbing argue that this crucial assumption, and thus their conclusion, holds up under empirical scrutiny. They point to several studies in support of this assumption. Here, we review the evidence presented and conclude that these attempts to test the equal environments assumption are weak, suffering significant methodological and inherent design flaws. Furthermore, much of the empirical evidence provided by these studies actually argues that, contrary to the interpretation, trait-relevant equal environments assumptions have been violated. We conclude that the equal environments assumption remains untenable, and as such, twin studies are an insufficient method for drawing meaningful conclusions regarding complex human behavior.

Has the "Equal Environments" Assumption Been Tested in Twin Studies?

2003 ◽  
Vol 6 (6) ◽  
pp. 486-489 ◽  
Author(s):  
Lindon Eaves ◽  
Debra Foley ◽  
Judy Silberg
Keyword(s):  
Twin Studies ◽  
Equal Environments Assumption ◽  
Equal Environments

scholarly journals Twin Studies, Molecular Genetics, Politics, and Tolerance: A Response to Beckwith and Morris

2008 ◽  
Vol 6 (4) ◽  
pp. 793-797 ◽  
Author(s):  
John R. Alford ◽  
Carolyn L. Funk ◽  
John R. Hibbing
Keyword(s):  
Molecular Genetics ◽  
Human Behavior ◽  
Behavioral Genetics ◽  
Twin Studies ◽  
Quarter Century ◽  
Political Orientations ◽  
Equal Environments Assumption ◽  
And Control ◽  
Equal Environments

Beckwith and Morris raise concerns about the value of twin studies for understanding the role of genetics in complex human behavior, but virtually all of their concerns have been raised and rebutted before. When it comes to the equal environments assumption (EEA), the best approach is to test for and control possible violations of the EEA on heritability estimates rather than merely rejecting all empirical evidence because of the possibility of EEA violations. In many respects, since the study of the genetic basis of complex human behaviors now includes many methods in addition to twin studies, Beckwith and Morris's critique applies more to the behavioral genetics of a quarter century ago than to today's multifaceted behavioral genetics. Twin studies establish that there is a sizeable genetic component to political orientations, thereby giving cause to look further at the nature of that role by using other methodologies, including molecular genetics. We conclude by pointing out that the normative implications of the relevance of genes to human behavior are not nearly as worrisome as Beckwith and Morris seem to believe.

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