In a study in Western societies, homeless people have a higher prevalence of mental illness when compared to the general population. They also are more likely to be alcoholics and suffer from drug dependency. It is estimated that 20–25% of homeless people, compared with 6% of the non-homeless, have severe mental illness. Others estimate that up to one-third of the homeless suffer from mental illness. Studies have found that there is a correlation between homelessness and incarceration. Those with mental illness or substance abuse problems were found to be incarcerated at a higher frequency than the general population. Fischer and Breakey have identified the chronically mentally ill as one of the four main subtypes of homeless persons; the others being the street people, chronic alcoholics,