This article introduces Shin Ihda's manifestation model of mood disorders. Ihda et al. reported eight cases of monozygotic twins with discrepancies in the onset and clinical presentation of mood disorders. The reports emphasize the prevalence of cyclothymic, melancholic-type, and manic-type personalities among the twins, as well as the differences in onset, clinical presentations, and course of the disorder, despite the twins being genetically nearly identical. Applying the twin study methodology, Ihda proposed the twins' developmental histories, personalities, and situations surrounding disorder onset as factors contributing to discrepancies in the clinical presentation and course of the disorder. Based on situation theory, he proposed a manifestation model of mood disorders, comprehensively explaining the development of a depression-prone personality through growth stages, situations that lead to depression or mania, and the disorder progression process based on inherent temperament. The model assumes five possible pathways: the revelation of underlying mood instability in defenseless cyclothymic personalities
the collapse of the illusionary feeling of omnipotence in manic-type individuals
the revelation of the need for objects in clinging individuals
the collapse of the illusionary feeling of integration with an authority figure through introjection in melancholic individuals
and neurosis. Ihda's insightful depiction of the specific interactions between genetics and environment has valuable implications for research and clinical practice.