greater liability for cannabis use and altered brain volumes (because both siblings have used cannabis) than discordant pairs, i.e. graded liability. Contrast 3 compared volumes from concordant unexposed pairs to all other groups to test whether altered brain volumes and cannabis exposure were associated with a shared predisposition (Figure 1c). A significant effect here implies that both concordant exposed and discordant pairs are at the same genetic liability regardless of whether one or both siblings use cannabis.
