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.

The Genetics of Political Attitudes and Behavior: Claims and Refutations

2010 ◽  
Vol 12 (3) ◽  
pp. 200-217 ◽  
Author(s):  
Jay Joseph
Keyword(s):  
Genetic Factors ◽  
Political Attitudes ◽  
Molecular Level ◽  
Twin Studies ◽  
Theoretical Assumption ◽  
Study Results ◽  
Twin Method ◽  
And Behavior ◽  
Attitudes And Behavior ◽  
Equal Environments

Some political scientists have argued in recent years that twin research shows that genetic factors play an important role in shaping political attitudes, ideologies, and behavior. Moreover, some researchers claim to have identified genes for political traits at the molecular level. The author argues that the main theoretical assumption of the twin method, which holds that monozygotic and dizygotic twin pairs experience equal environments, is untenable. Therefore, the results of twin studies can be completely explained by nongenetic factors. The author also argues that recent gene discovery claims in political science are unlikely to be replicated. He concludes that because genetic interpretations of twin study results are confounded by environmental factors, political scientists have no reason to revise previous socialization theories of political traits.

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.

scholarly journals On The Influence of Genetic Factors on the Formation of Homosexuality by Data of Twin Studies

2019 ◽  
Keyword(s):  
Sexual Orientation ◽  
Genetic Factors ◽  
Dominant Role ◽  
Monozygotic Twins ◽  
Twin Studies ◽  
Correct Selection ◽  
Genetic Determination ◽  
Dizygotic Twins

Results of twin studies are presented; these demonstrate that in a number of cases genetic effects can play a role of mild predisposing factors for the development of homosexuality, but the main part in its formation is accounted for by psychological and social factors. The opinion that genetic factors play the only and dominant role in the genesis of homosexuality does not hold water due to the fact that if it were so then their concordance for homosexuality in monozygotic twins would be 100 %, but it is not observed in reality. The studies conducted with the correct selection of examinees revealed 20 % of the concordance for homosexuality in male monozygotic twins and 24 % in female ones (Bailey, J.M., et al. Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. J. Pers. Soc. Psychol. 78(3), 524‑536). The use of Holzinger’s formula for analyzing the obtained numerical findings demonstrated that in the above case the proportion between heritable and environmental factors for male persons was 0.2 (20 %) versus 0.8 (80 %), for female persons it being 0.15 (15 %) versus 0.85 (85 %). Earlier twin studies (Bailey, J.M., Pillard, R.C. (1991). A genetic study of male sexual orientation. Arch. Gen. Psychiatry. 48(12), 1089–1096) revealed that their concordance for homosexuality in siblings (biological brothers, who are not twins) was lower than in adopted brothers (9.2 % versus 11 %), it contradicting to the idea of genetic determination of same-sex attraction. Moreover, attention is also attracted by the fact that dizygotic male twins demonstrated a significantly higher concordance for homosexuality than siblings (22 % versus 9.2 %). But it is known that dizygotic twins, like siblings, have on an average only 50 % of common genes. If there were genetic determination, such differences would not exist; the revealed difference demonstrates environmental effects, since it is evident that family upbringing of dizygotic twins is much more similar. Also it is necessary to pay attention to the fact that the rate of homosexuality in adopted homosexual brothers (11 %) considerably exceeded recent estimations of the part of homosexuals in the general population and was actually equal to the value for siblings, once again convincingly demonstrating a significant role of the environment in the formation of sexual orientation. We should not also ignore the fact that upbringing of monozygotic twins is even more similar than that of dizygotic ones; this phenomenon can cause their larger concordance for homosexuality.

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.

scholarly journals Twin studies for the investigation of the relationships between genetic factors and brain abnormalities in bipolar disorder

2016 ◽  
Vol 25 (6) ◽  
pp. 515-520 ◽  
Author(s):  
L. Squarcina ◽  
C. Fagnani ◽  
M. Bellani ◽  
C. A. Altamura ◽  
P. Brambilla
Keyword(s):  
Bipolar Disorder ◽  
Family Environment ◽  
Brain Structure ◽  
Genetic Factors ◽  
Genetic Research ◽  
Twin Studies ◽  
Neural Mechanisms ◽  
Shared Environment ◽  
High Heritability ◽  
The Relationship

The pathogenesis of bipolar disorder (BD) is to date not entirely clear. Classical genetic research showed that there is a contribution of genetic factors in BD, with high heritability. Twin studies, thanks to the fact that confounding factors as genetic background or family environment are shared, allow etiological inferences. In this work, we selected twin studies, which focus on the relationship between BD, genetic factors and brain structure, evaluated with magnetic resonance imaging. All the studies found differences in brain structure between BD patients and their co-twins, and also in respect to healthy controls. Genetic effects are predominant in white matter, except corpus callosum, while gray matter resulted more influenced by environment, or by the disease itself. All studies found no interactions between BD and shared environment between twins. Twin studies have been demonstrated to be useful in exploring BD pathogenesis and could be extremely effective at discriminating the neural mechanisms underlying BD.

scholarly journals A Twin Study of Cigarette and Snus Initiation and Quantity of Use in Norwegian Adult Twins

2019 ◽  
Vol 22 (2) ◽  
pp. 108-113
Author(s):  
Kenneth S. Kendler ◽  
Steven H. Aggen ◽  
Nathan Gillespie ◽  
Nikolai Czajkowski ◽  
Eivind Ystrom ◽  
...  
Keyword(s):  
Public Health ◽  
Twin Study ◽  
Twin Studies ◽  
Self Report ◽  
Shared Environment ◽  
Cigarette Use ◽  
Environmental Correlations ◽  
Twin Models ◽  
Adult Twins

AbstractWhile snus has been the focus of increasing public health interest, twin studies have examined neither sources of individual variation for its use nor the sources of resemblance between snus and cigarette use. Twins from the Norwegian Institute of Public Health Panel were assessed by self-report questionnaire for the initiation of regular use and maximal quantity used for snus and cigarettes. Twin modeling was performed using OpenMx on data from 2767 twins including 856 complete pairs. Fitting univariate twin models produced similar results for cigarette initiation and quantity with estimates of additive genetic, shared environmental and unique environmental effects of approximately 77%, 0% and 23%, respectively. Estimates of snus initiation and quantity were, respectively, approximately 53%, 26% and 21%. Joint analyses suggested that the genetic, shared environmental and unique environmental correlations between cigarette and snus initiation and quantity were +.82, 0 and +.42, respectively. However, these results could not be statistically distinguished from a model which postulated that resemblance between cigarette initiation and quantity resulted from genetic and unique environmental correlations of +.47 and +.43. Compared with cigarette initiation and quantity of use in Norwegian twins, the role of genes was less prominent and shared environment more prominent for initiation and quantity of use of snus. Joint analyses of both tobacco phenotypes suggested, but did not confirm definitively, that genetic risk factors for cigarette and snus use were similar but not identical, while shared environmental factors existed that were specific to snus use.

Glucose and Insulin Metabolism in Twins: Influence of Zygosity and Birth Weight

Twin Research ◽  
2001 ◽  
Vol 4 (5) ◽  
pp. 350-355 ◽  
Author(s):  
Pernille Poulsen ◽  
Allan Vaag
Keyword(s):  
Type 2 Diabetes ◽  
Glucose Intolerance ◽  
Twin Studies ◽  
Metabolic Effects ◽  
Diabetes Research ◽  
Relative Role ◽  
Intrauterine Environment ◽  
Dizygotic Twins

AbstractSeveral epidemiological and metabolic studies have demonstrated an impact of the intrauterine environment on the development of disease in adult life, including Type 2 diabetes and glucose intolerance. Our finding of lower birth weights among monozygotic diabetic twins compared to their non-diabetic genetically identical co-twins confirms this association and, furthermore, eliminates the possibility that the association could be explained solely by common genes leading to both impaired intrauterine growth and increased risk of Type 2 diabetes. Due to an often shared placenta monozygotic twins may experience a more adverse intrauterine environment compared to dizygotic twins and may therefore be more prone to develop various metabolic abnormalities. Our findings of a higher glucose and insulin profile after oral glucose ingestion, and recently lower insulin-stimulated glucose uptake — indicating glucose intolerance and insulin resistance — among monozygotic compared to dizygotic twins may to some extent question the validity of classical twin studies in diabetes research where equal environmental influences in monozygotic and dizygotic twins is assumed. The potential role of an adverse intrauterine environment in causing Type 2 diabetes in humans, may to some degree alter our conception of the twin model in diabetes research including the interpretation of aetiological conclusions reached in previous classical twin studies of diabetes. However, our present knowledge is far too insufficient to discard the results from classical twin studies concerning the relative role of genes versus environment for the development of diabetes and its metabolic effects.

Genetic factors in anorexia nervosa

1999 ◽  
Vol 14 (4) ◽  
pp. 189-198 ◽  
Author(s):  
A. Kipman ◽  
P. Gorwood ◽  
M.C. Mouren-Siméoni ◽  
J. Adès
Keyword(s):  
Anorexia Nervosa ◽  
Genetic Factors ◽  
Monozygotic Twins ◽  
Twin Studies ◽  
Vulnerability Factors ◽  
Adoption Studies ◽  
Complex Disorder ◽  
Dizygotic Twins ◽  
The One ◽  
Familial Environment

SummaryAnorexia nervosa is a severe and complex disorder with incompletely known vulnerability factors. It is generally recognized that anorexia nervosa is a familial disorder, but the majority of twin studies have shown that the concordance rate for monozygotic twins is higher (on average 44%) than for dizygotic twins (on average 12.5%). This difference in concordance rates shows that genetic factors, more than common familial environment, may explain why the `anorexia nervosa' phenotype runs in families. In order to estimate the heritability in the broad sense of anorexia nervosa according to published familial and twin studies, we first assessed the intrapair correlation between monozygotic and dizygotic twins, and secondly calculated the deviation threshold of relatives of affected probands from the relative mean. In this review, we obtained an estimation of the heritability at 0.72 according to all published controlled familial studies (six references quoted in MEDLINE®), and 0.71 for all published twin studies (59 references quoted in MEDLINE®). This estimation is close to the ones previously proposed, between 0.5 and 0.8.Familial and twin studies may also help to define the boundaries of the phenotype, shedding light on the complex relationship between anorexia nervosa on the one hand, and bulimia nervosa, mood disorders, and alcoholism on the other. Demonstrating the importance of genetic factors in anorexia nervosa, and more specifically for anorexia of the restrictive type, requires not only prospective and adoption studies (which are still lacking), but also genetic polymorphisms analyses, which began very recently.

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