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An Investigation of a Measure of Twins' Equal Environments

Published online by Cambridge University Press:  21 February 2012

Karen S. Mitchell*
Affiliation:
Department of Psychology, Virginia Commonwealth University, Richmond, Virginia, United States of America; Virginia Institute for Psychiatric & Behavioral Genetics, Virginia Commonwealth University, Richmond, Virginia, United States of America. s2ksmitc@vcu.edu
Suzanne E. Mazzeo
Affiliation:
Department of Psychology, Virginia Commonwealth University, Richmond, Virginia, United States of America; Department of Pediatrics, Virginia Commonwealth University, Richmond, Virginia, United States of America.
Cynthia M. Bulik
Affiliation:
Departments of Psychiatry and Nutrition, University of North Carolina at Chapel Hill, North Carolina, United States of America.
Steven H. Aggen
Affiliation:
Virginia Institute for Psychiatric & Behavioral Genetics, Virginia Commonwealth University, Richmond, Virginia, United States of America; Department of Psychiatry, Virginia Commonwealth University, Richmond, Virginia, United States of America.
Kenneth S. Kendler
Affiliation:
Virginia Institute for Psychiatric & Behavioral Genetics, Virginia Commonwealth University, Richmond, Virginia, United States of America; Department of Psychiatry, Virginia Commonwealth University, Richmond, Virginia, United States of America.
Michael C. Neale
Affiliation:
Virginia Institute for Psychiatric & Behavioral Genetics, Virginia Commonwealth University, Richmond, Virginia, United States of America; Department of Psychiatry, Virginia Commonwealth University, Richmond, Virginia, United States of America; Department of Human Genetics, Virginia Commonwealth University, Richmond, Virginia, United States of America.
*
*Address for correspondence: Karen S. Mitchell, Department of Psychology, Virginia Commonwealth University, PO Box 842018, Richmond, VA 23284-2018.

Abstract

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The equal environments assumption, which holds that trait-relevant environments are equally correlated among monozygotic (MZ) and dizygotic (DZ) twin pairs, is essential to twin designs. Violations of this assumption could lead to biased parameter estimates in twin models. A variety of methods and measures have been used to test this assumption. No studies to date have evaluated the measurement invariance of such items or examined the distribution of the underlying equal environments trait. The current study was an investigation of the psychometric properties of a self-report measure of twins' equal environments. Exploratory and confirmatory factor analysis results indicated that items loaded onto ‘child’ and ‘teen’ equal environments factors. Factor loadings and factor variances and their covariance were invariant for MZ and DZ twins; however, DZ twins had significantly lower factor means than MZ twins. Further, these items demonstrated adequate test–retest reliability. Lastly, the child and teen factors may be bimodally distributed, particularly for MZ twin pairs. Measurement invariance issues, as well as distributions of equal environments traits, should be considered when evaluating the equal environments assumption, in order to produce accurate parameter estimates in twin models.

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Copyright © Cambridge University Press 2007
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