Prevalence of Mental Illness, Cognitive Disability, and Their Overlap among the Homeless in Nagoya, Japan

BACKGROUND: While the prevalence of mental illness or cognitive disability is higher among homeless people than the general population in Western countries, few studies have investigated its prevalence in Japan or other Asian countries. The present study conducted a survey to comprehensively assess prevalence of mental illness, cognitive disability, and their overlap among homeless individuals living in Nagoya, Japan. METHODS: Participants were 114 homeless individuals. Mental illness was diagnosed based on semi-structured interviews conducted by psychiatrists. The Wechsler Adult Intelligence Scale-III (WAIS-III, simplified version) was used to diagnose intellectual/ cognitive disability. RESULTS: Among all participants, 42.1% (95% CI 33.4–51.3%) were diagnosed with a mental illness: 4.4% (95% CI 1.9–9.9%) with schizophrenia or other psychotic disorder, 17.5% (95% CI 11.6–25.6%) with a mood disorder, 2.6% (95% CI 0.9–7.5%) with an anxiety disorder, 14.0% (95% CI 8.8–21.6%) with a substance-related disorder, and 3.5% (95% CI 1.4–8.8%) with a personality disorder. Additionally, 34.2% (95% CI 26.1–43.3%) demonstrated cognitive disability: 20.2% (95% CI 13.8–28.5%) had mild and 14.0% (95% CI 8.8–21.6%) had moderate or severe disability. The percent overlap between mental illness and cognitive disability was 15.8% (95% CI 10.2–23.6%). Only 39.5% (95% CI 26.1–43.3%) of the participants were considered to have no psychological or cognitive dysfunction. Participants were divided into four groups based on the presence or absence of mental illness and/or cognitive disability. Only individuals with a cognitive disability reported a significant tendency toward not wanting to leave their homeless life. CONCLUSION: This is the first report showing that the prevalence of mental illness and/or cognitive disability among homeless individuals is much higher than in the general Japanese population. Appropriate support strategies should be devised and executed based on the specificities of an individual’s psychological and cognitive condition.