Trait-specific testing of the equal environment assumption: The case of school grades and upper secondary school attendance

Abstract

Objective: This paper tests the equal environment assumption for school grades and upper secondary school attendance and describes the conditions under which violations are problematic. Background: A growing number of sociologists use twin-based research designs, particularly the Classical Twin Design (CTD), to differentiate between genetic and social causes of social inequalities. One key assumption of CTD is that environmental influences are shared by monozygotic and dizygotic twins to the same extent; called the equal environment assumption (EEA). This assumption is frequently contested and the target of concern, because violation can result in an overestimation of heritability and an underestimation of the role of the social environment. Method: Using data from the first wave of the German TwinLife study, the paper illustrates two approaches to test EEA for school grades and enrolment in upper secondary school (Gymnasium). The analysis is based on a sample of twins (N = 1,576) aged ten to twelve years. Results: The results show that the approaches are able to detect violations of EEA (though in different ways), depending on the environmental variables that might causally be involved in trait variance. Only in one case was a violation was observed; it had no effect on heritability estimates. Conclusion: While EEA holds for school grades, violations do not automatically invalidate CTD in case of upper secondary school attendance.