The discordant MZ-twin method: One step closer to the holy grail of causality

Twin studies are well known for their value in quantifying the contribution of genes to population variation in behaviors and personality traits. Twin studies also provide a unique opportunity to untangle the contribution of environmental experiences to emotional and behavioral development. This is particularly true when examining monozygotic (MZ) twins since they represent a pair of individuals naturally matched on both their genetic background and their shared environment, thus allowing the identification of environmental experiences unique to each twin which may impact developmental outcome. This article presents two analytical strategies based on the discordant MZ-twin method. It stresses the power of this method to establish plausible causal pathways between environmental factors and developmental outcomes, and provides examples from the socio-developmental literature to illustrate its application. It also describes the limitations of this method and its requirements for optimal utilization