Although it is possible to test for differences in the association between MZ and DZ twin pairs by analyzing the groups separately, this results in a loss of statistical power. Another method involves testing for zygosity differences by including an interaction between zygosity and the exposure variable (in addition to the main effect of exposure) in the model. If the interaction term is not significant and DZ = MZ > 1.0, then there is no evidence of differential association by zygosity and it is possible to analyze MZ and DZ discordant pairs simultaneously. Alternatively, if DZ = MZ = 1, then the association is non-causal and attributable to shared familial influences.
