The equal environments assumption of classical twin studies may not hold

Richardson, Ken; Norgate, Sarah

doi:10.1348/000709904x24690

Cited by 76 publications

(58 citation statements)

References 41 publications

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“…There is evidence that MZ twins, even during childhood, are treated differently by their parents, teachers, and peers than DZ twins (28). However, even if the experience of the home environment during childhood was equitable among DZ and MZ twins, fraternal twins are likely to grow dissimilar more rapidly than identical twins as they mature, and as their distinct identities become more apparent to themselves and to others.…”

Section: Discussionmentioning

confidence: 99%

The Relative Importance of Genetics and Environment on Mammographic Density

Ursin

Lillie

Lee

et al. 2009

Cancer Epidemiology, Biomarkers &Amp; Prevention

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Background: Although several environmental factors predict mammographic density, estimates of its heritability have been quite high. We investigated whether part of the presumed heritability might be attributed to differential sharing of modifiable risk factors in monozygotic (MZ) and dizygotic (DZ) twins. Methods: We measured percent and absolute mammographic density using mammograms from 257 MZ and 296 DZ twin pairs. The correlation of intrapair mammographic density was compared according to zygosity across strata of modifiable risk factors. Portions of variance attributable to additive genetic factors, shared environment, and individual environment were calculated using a variance component methodology in the entire set, and within twin pairs stratified by environmental trait similarity. Results: Both percent density and absolute mammographic density were more highly correlated between

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Section: Discussionmentioning

confidence: 99%

The Relative Importance of Genetics and Environment on Mammographic Density

Ursin

Lillie

Lee

et al. 2009

Cancer Epidemiology, Biomarkers &Amp; Prevention

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“…The first one, which is the equal environments assumption, states that the environmental factors which contribute to risk for a disorder are equally correlated between MZ and DZ twin pairs. This assumption has at times received criticism [20,32], on the basis of the suggestion that many environmental factors are more often shared by MZ twins than DZ twins [23,25]. However, many of these supposed environmental factors are also influenced by genetic factors, which would make MZ twins more similar a priori.…”

Section: Discussionmentioning

confidence: 99%

Genetic and environmental influences on non-specific low back pain in children: a twin study

et al. 2008

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Aggregation of low back symptoms in families of children with low back pain (LBP) has been described. However, this may be due to genetic factors or common exposure to environmental factors. The aim of this study was to evaluate the relative contribution of genetic and environmental factors to childhood LBP by comparing the pairwise similarity of LBP in pairs of monozygotic (MZ) and dizygotic (DZ) twin children. Data was collected from 1995 to 1998 from a national sample of Finnish 11-year-old twins born between 1984 and 1987. This study sample constituted of 1,790 twin pairs: 610 MZ pairs, 598 same-sex DZ pairs, 582 opposite-sex DZ pairs. LBP pain was determined by using a validated pain questionnaire designed to assess musculoskeletal pains during the preceding 3 months. The outcome measure, LBP, was considered in three categories: none, once a month and at least once a week. Twin similarity in the report of LBP was quantified by correlations. Variance components for genetic and environmental factors were estimated by using biometric structural equation modelling techniques. The prevalence of LBP at least once a month was 15.7%, and at least once a week was 6.6%. The prevalence of frequent LBP in boys was significantly higher than that in girls (P = 0.04). In both genders, there were no differences in LBP reporting by zygosity (P [ 0.2). There were no statistically significant differences between polychoric correlations in male MZ and DZ pairs and between polychoric correlations in female MZ and DZ pairs, suggesting little genetic influence. Results obtained from the best-fitting genetic model suggests that, of the total variance in LBP, 41% (95% CI 34-48) could be attributed to shared environmental factors within families; and 59% (52-66) to unique (unshared) environmental factors. Our results suggest that genetic factors play, at most, a minor role in LBP in children; instead, symptoms seem to be related to a mixture of shared and unshared environmental factors. This study underscore the need for further highquality research, preferably prospective studies, to identify important modifiable risk factors in order to guide interventions that may prevent LBP in childhood.

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“…Another general supposition of twin modelling is known as the equal environment assumption Richardson & Norgate, 2005). We assume that MZ and DZ co-twins are exposed to common environmental factors to the same degree.…”

Section: Equal Environment Assumption (Eea)mentioning

confidence: 99%

The beauty, logic and limitations of twin studies

Røysamb¹,

Tambs²

2016

Nor J Epidemiol

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During the last fifty years more than 2700 twin studies have been published, examining the etiology of a high number of traits. Twin studies enable investigation of both genetic and environmental effects, and thereby also examination of causal factors involved in human traits and disorders. The beauty of twin studies resides in the potential of studying the unobserved by the logic of a design. The aim of this article is to outline central theoretical foundations and possible limitations, and to review selected key findings. We describe the inherent fundamentals of the classic and extended twin designs. The logic of the main analytic approaches is outlined, and the principles of univariate biometric modelling described. Next, we review different multivariate models, including the Cholesky, correlated factors, common factor, common pathway and phenotypic causality models. Additionally, the cotwin-control approach, representing a natural experimental design, and mimicking a counterfactual situation, is outlined. Central assumptions, threats and limitations of the twin design are discussed. In particular, we address the issue of missing heritability, non-random mating, the equal environment assumption and gene-environment correlations. Finally, we review some selected findings from the field of behavior genetics and twin studies.This is an open access article distributed under the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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The equal environments assumption of classical twin studies may not hold

Cited by 76 publications

References 41 publications

The Relative Importance of Genetics and Environment on Mammographic Density

The Relative Importance of Genetics and Environment on Mammographic Density

Genetic and environmental influences on non-specific low back pain in children: a twin study

The beauty, logic and limitations of twin studies

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