Consider the difficulties of determining the molecular basis of schizophrenia. We have convincing evidence from twin, family and adoption studies that there is a substantial genetic contribution to the illness (Kendler & Diehl, 1993). Although diagnosis is not easy, the limitations of the diagnostic categories and the potential overlaps with other disorders are appreciated. Consequently we can categorize patients in ways that allow genetic analysis. Nevertheless, mapping genetic determinants by linkage or identifying genetic variants by association has proved extremely difficult, an observation that can be explained by assuming that schizophrenia has a complex genetic architecture, consisting of multiple genetic effects that individually alter susceptibility by a small degree and operate via interactions with each other and the environment (Riley & Kendler, 2006). The complex genetic architecture means that gene finding, using current methodologies, is extremely hard. Despite an enormous and rapidly growing molecular genetic literature, only a handful of candidate genes have been identified and there remains considerable controversy regarding the reliability of these findings (Riley & Kendler, 2006).
