Fig. 1.

Hypothesized models of: (a) shared genetic risks across disorder and population trait variation, where the extreme end of a continuous distribution of a trait is associated with a continuous underlying genetic liability, and (b) shared genetic risks across different disorders, where squares labeled ‘P’ represent phenotypes, and squares labeled ‘G’ and ‘E’, represent genetic or environmental contributions, respectively, that can be shared or unique (indicated by the number of arrows pointing to phenotypes). All G factors are uncorrelated with one another and thus the entire genetic contribution to a phenotype can be modelled as the sum of the genetic factors contributing to it (e.g. for P1 this would be G1 + G2 + G3 + G5). The same is true for environmental factors (i.e. environmental contribution to P1 is E1 + E2 + E3 + E5). As an illustrative example, if P1 were ADHD, P2 were ASD, and P3 were MDD, then G1 represents any genetic variants that are shared between ADHD, ASD, and MDD; G2–G4 represents genetic variants shared between only two of these disorders (e.g. G2 would be genetic risk for ADHD and ASD but not MDD); and G5–G7 represent unique genetic risks (e.g. G5 is genetic risk that is unique to ADHD and not shared with either ASD or MDD). N.B. The shapes are not indicative of whether a variable is latent or measured.