Complex sentence: Family-linkage and twin studies have indicated that genetic factors often play an important role in the development of mental disorders in general, although the reliable identification of specific genetic susceptibility to particular disorders, through linkage or association studies, has proven difficult.
Split into simpler sentences: Family-linkage and twin studies have indicated that genetic factors often play an important role in the development of mental disorders. The reliable identification of specific genetic susceptibility to particular disorders, through linkage or association studies, has proven difficult.