All possible same-sex sibling pairs were drawn from the data (N=241 pairs from 146 families; details in eMethods S1). Linear mixed models (lmer function in R package lme425) were used, which nested individuals within sibling pairs and nested pairs within families. The mixed models included fixed effects for the primary covariates and WBV (when predicting regional volumes). The main effect of interest was membership in a sibling pair concordant or discordant for cannabis exposure. To test this, participants were divided into four groups – individuals from concordant unexposed pairs, individuals from concordant exposed pairs, exposed members of discordant pairs, and unexposed members of discordant pairs. This four-group factor was tested as a fixed effect using Helmert contrast coding (eTable 2). Contrast 1 compared exposed and unexposed siblings from discordant pairs to test a causal hypothesis, i.e. cannabis causes altered brain volumes (depicted as smaller volumes for exposed vs. unexposed individuals in Figure 1a). Siblings share 50% of their genes and much of their rearing environment. Therefore, within-pair volumetric differences would be preliminary evidence for causation, pending replication in MZ pairs.
