Reporting on research on homelessness, the Mental Illness Policy Organization notes that there are an estimated 744,000 homeless persons in America. Among this population, approximately 250,000 (almost one-third) suffer from mental illness such as schizophrenia and manic-depression.

Since the 1970s, the homeless population has steadily increased, especially those with serious mental illness. A University of Pennsylvania study of 5,000 homeless persons in New York City found that they cost the public $40,500 annually, on average. This amount is comprised of prison, shelter, emergency room, and psychiatric hospital costs.

Shifts in American public policy over the last few decades may account for the high number of mentally ill persons living on the street. Theories abound, but one factor in the increase is the deinstitutionalization of the mentally ill. Over the last few decades, there appears to have been a shift from treating many of the mentally in a hospital setting to abandoning them to the street, where they either remained or became imprisoned. To illuminate the point, consider that while research found 250,000 mentally ill persons living on the street, (a separate) 90,000 were hospitalized. It appears that the system for treating the mentally ill is itself ill.

As the United States Interagency Council on Homelessness points out, substance abuse can perpetuate homelessness as many shelters and public housing resources require sobriety as a condition of occupancy. This observation suggests that existing social policies may address homelessness, but are not tailored to address substance abuse, even though much of the research is clear that the majority of the homeless have substance abuse disorders (and many have co-occurring mental illness). The concern, of course, is that the level of care which substance-abusing and/or mentally ill homeless persons require involves high costs amidst national, state and local budget cuts. A single homeless person, especially one with co-occurring disorders, can present layered challenges to government social service agencies and non-profits, yet there is a general consensus that society should not abandon this already neglected population.

Does addiction cause homelessness, or does homelessness cause addiction? As the National Coalition for the Homeless points out, the answer is that addiction is both a cause and result of homelessness.

Mental illness often underlies addiction, which makes negative stereotypes about addiction all the more misplaced. While a century ago, alcoholism and drug abuse were perceived as a moral weakness, today there is reframing of perspectives on substance abuse. It is an illness, and although some may be more disposed to addiction than others, no one is immune from it.

According to research from the Substance Abuse and Mental Health Services Administration (SAMHSA), substance abuse is more common among the homeless than in the population in general. While statistical data on homelessness is rife with data collection complications, given that this population is hard to locate or may not be want to be surveyed, SAMHSA research findings estimate that:

-About 38 percent of the homeless abuse alcohol.
-About 26 percent abuse drugs other than alcohol.
-Alcohol abuse is more common among the older homeless population.
-Drug abuse is more common among the younger set

A survey conducted by the United States Conference of Mayors asked 25 cities to provide the top reasons for homelessness in their jurisdictions. Sixty-eight percent reported that substance abuse was the number one reason for homelessness among single adults. According to a separate research survey, two-thirds of the homeless who were interviewed reported that abuse of drugs and/or alcohol was a major cause of their homelessness.

According the United States Interagency Council on Homelessness, 70 percent of homeless US veterans suffer from a substance abuse disorder.