We present in Figure 6a a highly simplified version of such a model, which contains one set of genetic risk factors, one set of environmental risk factors, three EPs and a PD. In the model, EP1 reflects only the genetic risk factors and EP3 only the environmental risk factors. However, both sets of risk factors affect EP2. More complex models could easily be conceptualized. In particular, it seems likely that there exist joint models in which there are both shared risk factors and direct causal pathways between EPs and PDs. Cross-sectional data from relatives are generally not sufficient to identify all the parameters of such models, but there is the potential to do so with longitudinal studies of related individuals.
