There are several ways by which this problem can be approached. One is to obtain good measures of λEP and λPD that could be incorporated into the analytic models. This might be carried out by obtaining test–retest reliability in a subset of the studied sample. An even more powerful approach would be to obtain longitudinal measures of EP and PD in the entire sample. Then, making the reasonable assumption that the genetic risks for them were temporally stable, the model depicted in Figure 2c could be applied. In this model, the estimates of β are now unconfounded from those of λEP and λPD. Such data would also now be much more likely to be able to discriminate the risk indicator from the mediating variable model for EP. A third possibility is to use data collected from pairs of relatives, which can also potentially discriminate risk indicator from mediational models.
