In cotwin-control analyses, twin pairs are matched for familial background and vary only in that one is at risk because of “exposure” (exposure is substance use/early use/dependence in these analyses) whereas the other twin is not. The test is whether the “exposed” twin is at increased risk of having a particular outcome (lower educational attainment in this case) compared to the “non-exposed” twin. These cotwin-control analyses permit the assessment of whether the associations observed between substance use behaviors and educational attainment in the general population are: a) attributable to an underlying familial liability (environmental or genetic), b) are causal (or due to other unmeasured environmental risk factors), or c) are influenced by both (a) and (b).
