The equal environments assumption of classical twin studies may not hold

Richardson, K and Norgate, SH ORCID: https://orcid.org/0000-0003-0716-2558 2005, 'The equal environments assumption of classical twin studies may not hold' , British Journal of Educational Psychology, 75 (3) , pp. 339-350.

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Abstract

The classical twin method – comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins – in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far-reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well-known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio-cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ–DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems.

Item Type:	Article
Themes:	Subjects / Themes > H Social Sciences > HQ The family. Marriage. Woman > HQ503 The family. Marriage. Home > HQ0767 Children. Child development Subjects / Themes > R Medicine > R Medicine (General) Health and Wellbeing Subjects outside of the University Themes
Schools:	Schools > School of Health and Society > Centre for Applied Research in Health, Welfare and Policy Schools > School of Health and Society > Centre for Health Sciences Research
Journal or Publication Title:	British Journal of Educational Psychology
Publisher:	British Psychological Society
Refereed:	Yes
ISSN:	00070998
Depositing User:	H Kenna
Date Deposited:	03 Aug 2007 10:33
Last Modified:	05 Mar 2019 15:27
URI:	http://usir.salford.ac.uk/id/eprint/93

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