Figure 1b is equivalent to a standard mediational model as commonly articulated in the psychological literature.6 Both these models generate the pattern of findings required by most of the criteria listed in Table 1. For example, both the liability-index and the mediational model would generate data that satisfy both criteria 4 (cosegregate with illness in families) and 5 (found at increased rates in unaffected family members) of Gottesman and Gould.2 Furthermore, both the liability-index and the mediational models would be useful in a developmental context. In both models, unaffected individuals with high scores on EP would be predicted to be at elevated risk for the development of PD.
