Monozygotic twins discordant for educational attainment: Definitions, the null hypothesis, and other challenges

Abstract

Discordant monozygotic twins offer a unique opportunity to examine environmental influences on human traits. We report on our attempts to exploit that opportunity within a database of Australian twins assessed longitudinally across four school grades in literacy and numeracy in nationally-administered “high-stakes” testing. The first challenge is to define discordancy within pairs; issues include degree of separation, longitudinal consistency, and consistency across assessment domains. Adopting a working definition of 1SD separation on one occasion, we show that longitudinal consistency is relatively weak, raising the issue of whether single instances of discordancy should be considered of interest or assigned to “error.” Insisting on consistency across domains runs the risk of missing environmental factors affecting just one domain, numeracy, for instance. We entertain the hypothesis that cross-domain consistency might signal broader biological factors such as illness in one twin but also show that testing that hypothesis is challenged by a possible causal path from literacy to numeracy. The second challenge is to identify the null hypothesis, and we suggest that it is that there are as many environmental factors in operation as there are pairs of discordant twins. A third challenge, perhaps particular to our database, is the validity of the high-stakes tests against school-based assessments. A fourth challenge is whether there are factors at work that throw doubt on the generalisability of our observations to students in general, such as twin competitiveness. Finally, the relatively modest number of consistently discordant pairs for highly heritable traits like literacy and numeracy (despite a database of several thousand families) challenges standard quantitative analyses and requires more reliance on interviews, with their subjective elements. We report the results of a series of parent interviews that shed light on these issues.