Our treatment of the mediating variable EP model above assumed that the causal relationship between an EP and a PD all flowed in one direction, EP → PD. Is this a realistic assumption? Take the well-studied association between attentional difficulties and risk for schizophrenia. Although having attentional difficulties might contribute to risk for schizophrenia, the state of schizophrenic psychosis might itself also impair attention. Thus, in this instance, the association between EP and PD may be sometimes bidirectional (EP↔PD) rather than unidirectional (EP → PD). Furthermore, the ‘return’ path from EP to PD might vary as a function of clinical state, being stronger during times of acute symptoms and weaker in times of remission.
