The discordant twin design provides an elegant strategy for the control of potential genetic and environmental confounds. Similar to a matched pair analysis, in which cases are matched to controls for gender and age, the discordant twin design matches at-risk twins (e.g., those with early cannabis initiation) to control twins (e.g., cotwins without early cannabis use) thereby controlling for genetic and shared environmental influences (which increase twin similarity). In pairs of monozygotic/identical twins discordant for early cannabis use, any reduced educational attainment in the early-onset cannabis using twin can only be attributed to “causal” mechanisms or those environmental factors that are not shared by members of twin pairs, but not to shared genetic or environmental influences. The utility of this design has previously been demonstrated in multiple studies that have found increased likelihood of other illicit drug use in the twin who uses cannabis or alcohol at an early age when compared with their non-using or later initiating cotwin (Grant et al., 2006; Lynskey et al., 2003b).
