In June 1908, Stanley was moved to the McCormicks' Riven Rock estate in Montecito, California, where his schizophrenic older sister, Mary Virginia, had lived from 1898 to 1904 before being placed in a Huntsville, Alabama, sanitarium. While there, he was examined by the prominent German psychiatrist Emil Kraepelin and diagnosed with the catatonic form of dementia praecox. In 1909, Stanley was declared legally incompetent and his guardianship divided between Katharine and the McCormick family.
