The Nature of the Shared Environment
While a standard part of twin modeling, the magnitude of shared environment (c2) is rarely examined by comparing estimates obtained using other methods. To clarify these effects on familial resemblance, we estimated c2 for 20 diverse phenotypes in: (i) monozygotic and dizygotic twins, (ii) all step-siblings, and (iii) reared together and apart half-siblings, ascertained from the Swedish general population. The mean c2 estimates (± 95% CIs) differed across methods and were higher from twins (0.18; 0.13–0.23) than from the step (0.12; 0.09–0.14) and half-sibs (0.09; 0.06–0.13). c2 estimates correlated moderately across these three methods (ICC = + 0.28). When step-siblings from blended (each sib biologically related to one parent) and adoption-like families (one sib offspring of both parents and one of neither), were examined separately, resemblance was much lower in the latter. We need to clarify the range of environmental processes now considered together under the term “shared environment.”
KeywordsShared environment Twins Step-siblings Half-siblings Sweden
This project was supported by Grant Nos. R01DA030005 and AA023534 from the National Institutes of Health, the Swedish Research Council (K2012-70X-15428-08-3), the Swedish Research Council for Health, Working Life and Welfare (In Swedish: Forte; Reg.nr: 2013-1836), the Swedish Research Council (2012-2378; 2014-10134) and FORTE (2014-0804) as well as ALF funding from Region Skåne awarded. The authors also wish to thank The Swedish Twin Registry at the Karolinska Institute, which provided the twin data for this study.
Compliance with ethical standards
Conflict of interest
Kenneth S. Kendler, Henrik Ohlsson, Paul Lichtenstein, Jan Sundquist and Kristina Sundquist declares that they have no conflict of interest.
Human and Animal Rights
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. We secured ethical approval for this study from the Regional Ethical Review Board of Lund University. There were no animals used in this study.
Informed consent was not obtained from individual participants included in the study as we were working with anonymized data.
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