iPSC technology has enabled us to accurately model the pathology of human diseases. Previous studies have identified the abnormal central nervous development, impaired synaptic functions and impaired neural circuit functions as the causes of psychiatric and neurodevelopmental disorders, such as schizophrenia and autism spectrum disorders, however, much remains unknown about the molecular and cellular etiology of these disorders. We have been focusing on disease-associated rare variants with high penetrance for analyzing the molecular and cellular etiology of psychiatric and neurodevelopmental disorders. We established iPSCs from patients with these disorders for which there is existing clinical information and differentiated these iPSCs into neurons. We identified impaired developmental and mature neural functions in the iPSC neurons from patients with rare variants with high penetrance, which provide important insights into the cellular basis of psychiatric and neurodevelopmental disorders. Patients'-derived iPSC neurons together with the patients' clinical information are versatile for understanding the molecular and cellular etiology of psychiatric and neurodevelopmental disorders and developing therapeutics for these disorders.