Historical Context

Multiple historical events have been linked with current conceptualizations of homelessness, including such disparate populations as those created by the 16th-century Elizabethan Poor Law, colonization of the North American continent, and itinerant workers in the late 19th century. For example, Elizabethan Poor Laws were the first attempt to provide service for landless and homeless poverty populations. In the 19th century, discussions of homelessness often focused on itinerant workers, or “hobos.” Historically, homelessness has not necessarily been identified as a “problem.” Wright points out that various descriptions of the homeless, some as recently as the 1950s and 1960s, have romanticized the lives of hobos and migrant workers. However, starting with changes in the population from the time of deinstitutionalization in the 1960s, there is a general consensus that homelessness has emerged as a serious and increasingly important social issue, and that this issue is closely interrelated with substance use and abuse and other psychiatric illness.

It is also important to consider the conduct of research on this population from a historical perspective. Although there are no doubt exceptions, early research on homelessness (for the sake of the current discussion, operationalized as published prior to 1970) was generally ethnographic or even anecdotal in method. Seminal works, such as those by Whyte, Gans, and Liebow, focused on the complex interactions among small groups of urban dwellers. Although more recent reexamination of these works demonstrates the significance of illicit substances in the lives of these “streetcorner” groups, questions of “how many” or prevalence of these disorders were not addressed in these studies.

The 1970s and 1980s witnessed an explosion of research on homelessness, with more than 500 published articles and books listed on the subject in those two decades. Unfortunately, most of this research was also methodologically flawed, presenting population descriptions incorporating a convenience sample, services-limited research consisting of program descriptions, or nonrandomized studies comparing different interventions. It was not until the late 1980s that leaders in the field called for research to move beyond demographic descriptions to conduct more complete and methodologically sophisticated research addressing complex epidemiologic issues.

In the last two decades, numerous methodologically sound cross-sectional studies have concluded that addiction and other psychiatric disorders are disproportionately prevalent in the homeless population. Unfortunately, because of sampling-related issues emanating from varied definitions applied to the problem of homelessness, changes in the population over time, and the lack of an acceptable national sample, our subsequent discussions of homelessness and associated comorbidities represent at best an incomplete snapshot of the problem. Thus answers to the specific questions of how many (e.g., What is the prevalence of psychiatric illness in the homeless population?) vary with these methodological differences, even among studies deemed methodologically adequate for most purposes. Given this situation, we present ranges of likely prevalence estimates rather than provide specific figures of undeterminable validity.

Operationalizing Homelessness

Historically, homeless samples in research studies have often been limited to service-using populations, especially individuals using services directed to homeless populations, such as overnight shelters. General consensus, however, is that this subset captures only a segment of the homeless population that may not be representative of the larger homeless population. Perhaps the most commonly accepted definition of homelessness is that of the 1987 Stewart B. McKinney Homeless Assistance Act, which defines a homeless person as:

(1) an individual who lacks a fixed, regular, and adequate nighttime residence and (2) an individual who has a primary nighttime residence that is (a) a supervised, publicly or privately operated shelter designed to provide temporary living accommodations, (b) an institution that provides a temporary residence for individuals intended to be institutionalized, or (c) a public or private place not designed for or ordinarily used as a regular sleeping accommodation for human beings.

Understanding differences among specific definitions of homelessness requires consideration of a number of factors. These factors include what personal circumstances are considered homeless (e.g., inclusion of individuals doubled up/marginally housed versus only counting individuals literally without housing), how long one must be homeless to be included (one night vs. a longer spell), and whether one self-identifies or is identified by some external criteria as homeless. Currently, operational definitions of homelessness have focused either on individuals who are literally homelessness or those marginally housed. Definitions of literal homelessness include not only those found in shelter settings, following the definition in the McKinney Act, but also individuals sleeping on the streets and in other locations not considered appropriate housing (e.g., subways, abandoned properties). Definitions of literal homelessness vary both in duration and in the types of nonhousing locations included. Inclusion of marginally housed individuals broadens the definition of homelessness to include individuals with precarious housing situations such as those living in single room occupancy buildings and staying with others without paying rent, and has been used to provide broader estimates of the lifetime prevalence of homelessness. Most recently, researchers have identified an additional dimension to measuring homelessness with given definitions that may affect estimates of population prevalence. Eyrich-Garg and colleagues have discussed differences between subjective (self-identified) and objective (identified by others) determinations of homelessness, and have demonstrated significant differences in risk patterns among samples of heavy-drinking women identified with different methods of determining homelessness.

As discussed in the preceding text, the many definitions of homelessness emerging from variations on an array of elements comprising this concept are destined to yield inconsistent sample characteristics and prevalence estimates. Because there is no unified definition of homelessness, there can be no single gold standard for determining the status of homelessness of individuals, and, therefore, it should be understood that in the remainder of this chapter, the relevant research to be reviewed necessarily consists of work derived from samples based on a variety of nonuniform definitions of homelessness from different perspectives. Although we are careful to identify both definitions of homelessness and the resulting types of samples included in specific studies (and to present critiques of current estimates in part based on this limitation), readers are encouraged to pay attention to these issues and remain cognizant of how these choices can subtly or even dramatically influence estimates of homelessness prevalence and observed characteristics of the population being studied.

Population Size Estimates

A number of methodological and conceptual issues must be considered in answering the question, how many homeless people are there? Similar to the complexities described around defining homelessness discussed in the previous section, issues requiring explication in interpreting estimations of population size include considerations of the sampling source and measurement methods (e.g., agency-based versus epidemiologic samples, neighborhood vs. urban area vs. national samples, point vs. recent or lifetime prevalence estimates vs. incidence).

Early prevalence estimates of homeless populations consisted of cross-sectional point prevalence estimates projected from samples counted at one or more overnight shelters. In one of the more thorough studies of this type, Burt and Cohen estimated that there were 194,000 adult users of homeless shelters and soup kitchens in cities of 100,000 or more in a given week in 1987. Although basing their estimate on national shelter numbers represented a methodological improvement on previous estimates, because their estimate excluded multiple other sources of homeless people (e.g., soup kitchens, unsheltered locations), it was generally considered a substantial underestimate. Other commonly discussed population estimates (e.g., census enumeration) attempted to determine the size of homeless populations on a given night using single enumeration methods. However, we agree with an assertion made by Burt and Cohen and endorsed by many others that these single-night estimations are also likely to miss substantial proportions of the literally homeless population, and thus represent significant undercounts. Populations underrepresented include the literal homeless (particularly those sleeping in hidden locations, such as in abandoned buildings) and those housed for single nights or for short spells. For these reasons, we will not further consider single-night estimates here.

Perhaps the best of the prevalence estimates emerge from the seminal work on homelessness of Burt et al. Using data from the 1996 National Survey of Homeless Assistance Providers and Clients (a survey of a variety of providers for two, 1-month periods) and extrapolating from previous estimations, they were able to arrive at reasonable estimates of how many service-using individuals were homeless on a given day or week, and estimating the total number of homeless individuals (both accessing and not accessing services) for the same periods. Readers wishing to understand more about how these estimates were reached are invited to explore the details of the various methods and estimates provided in this work.

In examining the various estimates, the best defensible figures of homeless service users who were homeless at the time of receipt of services were approximately between 440,000 and 840,000 in a given week and between 260,000 and 460,000 on a given night (including adults and children) in the National Survey of Homeless Assistance Providers and Clients. Using their methods for estimating the proportion of individuals not using services, Burt and colleagues argued that between 1.4 and 2.1 million adults were homeless in a given year. This number is not out of line with other estimates for approximately the same time period. More recently, using multiple enumeration strategies, the “Homelessness Counts” report gave a higher estimation of around 750,000 on a given night. It is important to note that in estimating lifetime prevalence, a telephone household survey found that 6.5% of adults had experienced a spell of literal homelessness at some time in their lives, and that 3% had been homeless within the past year, numbers far greater than any of the previous estimates.

An ongoing debate in the homelessness arena is the accuracy of these population estimates over time and their applicability to current population size and generalizability across locations. In terms of current population estimates, a relative consensus holds that the size of the homeless population increased in the 1980s and that the population size has remained stable or grown since. However, as the National Alliance to End Homelessness has pointed out, consistent enumerations are lacking beyond the flawed census attempts in 1990 and 2000, and, therefore, any discussions around changing size of the population are more speculative than factual. Thus the estimates presented here, while representing best available evidence, cannot be considered precise or even necessarily accurate. In terms of generalizability of findings across locations, Culhane and colleagues have used administrative records from homeless service providers to attempt to examine population size across multiple jurisdictions. Their results, although representing the state of the art, point out once again the difficulties in estimating population size, as they find rates ranging from 0.1% to 2.1% in different cities of the overall population on a yearly basis using approaches similar to those applied in administrative records data collection.

Although discussions of overall population size over time have been inconsistent at best and lacking at worst, some persuasive evidence points to recent changes in the composition of those who are homeless. North and colleagues, using three comparable representative samples each examined a decade apart within a single urban environment, noted significant increases in substance use and mood disorders among homeless cohorts over time. Their findings suggest that the homeless population may be changing, and that some of the differences found across studies are likely attributable to changing characteristics of the population rather than simply variation created by use of different sampling strategies and study of different environments. Furthermore, they argued that observed changes in the population over time may represent unintended consequences of changes in national policy.

Chronic Versus Short Term

Efforts to understand the composition of the homeless population require examination of linked issues of duration of homelessness and number of spells of homelessness that have long received considerable attention in the homelessness literature and have focused efforts to help this population toward specific subgroups with distinct characteristics. Currently, much of the federal policy is aimed at addressing the chronic homeless population. Classifications of homelessness generally break the population into some variation of three not-always-distinct groups: crisis/first-episode, episodic, and chronic. Estimates of proportions for the chronic subgroups vary from almost half falling into the chronic category to less than one-fourth and as low as 10%. Caton and colleagues examined predictors of remaining homelessness over 18 months in a cohort of newly homeless individuals, finding that shorter duration of homeless spells was associated with employment, no history of substance treatment or incarceration, and younger age. These observed differences indicate that these subgroups are distinct, with the additional implication that they may have differing treatment needs.

A Critique

Careful readers will note that much of this discussion of the homelessness population has included repeated cautions about the role of methodological issues in shaping the findings, including the definition of homelessness, sampling methods (e.g., service-using vs. non–service-using samples), and evolution of the population over time, to name only a few. Although numerous articles, books, and governmental reports have debated each of these issues separately and together, a broad conclusion from this literature is that it collectively yields only a vague understanding of the size and composition of the homeless population. We echo numerous other writers in noting the frustrations and complexities of integrating a large, methodologically flawed body of information that has been unable to describe this multifaceted population coherently or precisely. Much more could be written, including similar discussions of proportions of the population falling into various demographic subgroupings, but all would be marred by this same general critique. Given the focus of this chapter on substance use disorders and associated psychiatric and medical risk factors, we now move away from this general discussion of the homeless population to the central task of examining substance use disorders and comorbidities.

Association Between Substance Use Disorders and Other Risk Factors and Homelessness

Before launching a discussion of rates of risk factors in the homeless population, it is important to address the relationships of substance use disorders and other risk factors associated with homelessness. Generally, an implicit assumption in the popular literature holds that the disproportionate findings of these risk factors in the homeless population indicate that substance use and abuse/dependence and other mental illness cause these individuals to become homeless. However, evidence on onset of homelessness and psychiatric disorders has called into question this assumption.

Research on the causal nature of psychiatric disorders on homelessness has, in fact, concluded that the association between these factors is not simply unidirectional. O’Toole and colleagues found evidence for changes in alcohol and drug abuse patterns after first onset of homelessness, including escalating use for some individuals and diminished use among others. North and colleagues compared the relative timing of onsets of substance use disorders and other psychiatric disorders with first episode of homelessness and found that the proportion of homeless individuals with onset of their illnesses prior to the onset of their first episode of homelessness was similar to the proportion of a national community sample with onset of illness before an age comparable to that of the homeless sample’s age at first homelessness. Earlier assumptions of direct unidirectional causality from psychopathology, to homelessness have largely been abandoned by the experts who now argue that there are also multiple indirect effects related to having a psychiatric disorder that may not only increase individual risk for entering homelessness but also create barriers to exiting homelessness.

Prevalence Rates

When we use the term “substance use disorder,” we are referring to substance abuse or substance dependence as defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM; editions III, III-R, IV, IV-TR, or 5 depending on when the research was conducted). Meeting diagnostic criteria for either alcohol use disorder or for any specific drug use disorder (e.g., cocaine use disorder, cannabis use disorder, opioid use disorder) qualifies one for a diagnosis of substance use disorder.

There is a general scientific consensus that the prevalence of substance use disorder is disproportionately high in the homeless population. According to epidemiologic studies, the lifetime prevalence of substance use disorder is estimated to be in excess of two-thirds of homeless people. Women range between 31% and 63%, and men between 71% and 75%, Systematic shelter-selected samples (e.g., 30) yield similar lifetime prevalence rates. The current (12-month) prevalence of substance use disorder is estimated to be somewhere between 38% and 52%.

One study found that substance use disorders accounted for most of all lifetime psychiatric disorders among a representative sample of homeless people. A lifetime psychiatric diagnosis was detected in 88% of men and 69% of women, and a lifetime substance use disorder was identified in 84% of men and 58% of women.

Alcohol and Drug Use Disorder Comorbidity

The overlap between alcohol use disorders and drug use disorders is considerable in the homeless population. Approximately 61% of homeless women with lifetime alcohol use disorder also report a history of lifetime drug use disorder, and approximately 40% of men and women who report a history of lifetime drug use disorder also report a history of lifetime alcohol use disorder. A substantial proportion of the homeless population meets criteria for both alcohol and other drug use disorders. Approximately 36%–42% of homeless people meet lifetime criteria for both alcohol and other drug use disorders. Somewhere between 29% and 33% of women meet lifetime criteria for both alcohol and other drug use disorders, and approximately 38% of men meet lifetime criteria for both types of disorders. Approximately 18% of homeless people meet current (12-month) criteria for both alcohol and other drug use disorders.

Many homeless people with substance use disorders also have at least one other psychiatric disorder. Epidemiological studies have shown that of homeless people with lifetime alcohol use disorder, 32%–38% meet diagnostic criteria for a lifetime nonsubstance psychiatric disorder. Of men with a lifetime nonsubstance psychiatric disorder, 60% meet diagnostic criteria for lifetime alcohol use disorder and 24% meet diagnostic criteria for lifetime drug use disorder. Of women with a lifetime nonsubstance psychiatric disorder, 46% meet diagnostic criteria for lifetime alcohol use disorder and 20% meet criteria for lifetime drug use disorder.

Prevalence Rates

Alcohol use disorders are highly prevalent in the homeless population, ranging from approximately 53%–63% according to epidemiological studies of lifetime rates. Lifetime prevalence ranges from 17% to 40% in women and 56% to 68% in men. Most individuals who meet criteria for lifetime alcohol use disorder also meet criteria for current alcohol use disorder. Current-year prevalence rates for the disorder among homeless populations are estimated to be between 39% and 42%, approximately 32% in women and 41%–50% in men.

Studies comparing prevalence rates of lifetime and current (6-month) alcohol use disorder between homeless persons and housed groups have found significantly higher alcohol use disorder rates among the homeless. However, this finding may not generalize to particular homeless subgroups. For example, lifetime rates of alcohol use disorder do not appear to differ between first-time, nonmentally ill shelter-seekers and persons applying for government assistance. Shelter-using, mentally ill women have been found to have higher current (6-month) rates of alcohol use disorder than their housed, mentally ill female counterparts ; however, no significant difference was detected in the current (6-month) rates of alcohol use disorder between shelter-using, mentally ill men and housed, mentally ill men. The literature indicates that excessive alcohol use is a strong discriminator between homeless and housed families. For instance, single female parents who were homeless reported a history of using alcohol excessively 23 times more frequently than single female parents who were housed.

Overall Prevalence Rates

As with alcohol use disorder, drug use disorder is overrepresented in homeless populations. Lifetime prevalence of drug use disorder is estimated between 31% and 58%. According to North and colleagues, drug use disorders in the homeless population have increased significantly in prevalence over the past two decades. Women’s lifetime rates range between 23% and 51%, and men’s lifetime rates range between 40% and 61%. Current (12-month) prevalence of drug use disorder is estimated to be between 31% and 38%, approximately 32%–35% for women and 18%–38% for men. Current (12-month) prevalence among shelter-using mentally ill women appears to be in line with these estimates, but shelter-using mentally ill men appear to have a substantially higher prevalence of drug use disorder (77%).

Drugs of Choice

The literature is clear that the drug associated with the highest rates of abuse and dependence among the homeless is cocaine. Cocaine use disorder has grown substantially in the homeless population through the 1990s and 2000s. Lifetime cocaine use disorder prevalence rates range from 16% to 40% for women and from 37% to 46% for men. Current (12-month) cocaine use disorder rates are approximately 26%–29% for women and 24%–33% for men.

The drug with the second highest abuse/dependence rate is cannabis. During the mid-1980s, cannabis appeared to be the most prevalent drug of abuse (8% for women and 7% for men) in the homeless population, but that has changed. Lifetime cannabis use disorder prevalence estimates range between 7% and 28% for women and 30% and 37% for men. Current (12-month) prevalence of cannabis use disorder is approximately 8%–12% for women and 10%–16% for men.

Stimulant, opioid, and sedative use disorders have estimated lifetime prevalence rates between 3% and 10%. Current (12-month) prevalence is similar for stimulant, opioid, and sedative use disorders.

Some differences emerge for subgroups of the homeless population. First-time shelter-seeking men appear to have lifetime prevalence rates of cocaine (33%), cannabis (32%), and heroin (11%) use disorders that are similar to those of their housed counterparts. First-time shelter-seeking women, however, have elevated prevalence of cocaine (40%) and heroin (23%) but not cannabis (22%) use disorders, when compared with their housed counterparts.

Injection Drug Use

In this brief section, we present information on a particular type of drug ingestion—injection drug use. Note that we discuss use (as opposed to drug abuse/dependence) and that this use may or may not be part of a drug use disorder. Injection drug use is important to mention apart from diagnosis because sharing works (e.g., needles) is a risk factor for contracting and spreading HIV. Many cities view needle-sharing as such a problem that they fund needle-exchange programs in which people can exchange their used needles for sanitary ones free of charge.

Epidemiological studies have found that 22% of the homeless population has a lifetime history of injection drug use, with 10% of men and 5% women injecting within the past year. The relative proportions of injection drugs of choice were heroin (94%), cocaine (58%), stimulants (45%), other opiates (20%), sedatives (19%), and hallucinogens (7%). Sex and age appear to play a role in the likelihood of injection drug use. In one study of severely mentally ill homeless people, rates of injection drug use across study recruitment sites ranged from 16% to 26% for men and 6% to 8% for women. Younger persons in this subpopulation (e.g., under the age of 45) were more likely than older persons to have injected drugs at some point in their lives.

Looking Forward

Substance use disorders among the homeless have been studied for many years now. Research needs to continue to identify shifts in the abuse/dependence of popular substances. It is not yet known whether the recently documented trends of increasing oxycodone abuse in the general substance abuse treatment-seeking population and methamphetamine abuse observed in the general population are also reflected in the homeless population.

Mood Disorders

Major depression is the most prevalent nonsubstance disorder in the homeless population. The lifetime prevalence of major depression is estimated to be between 18% and 21%, with greater prevalence found among women (around 25%) compared with men (around 18%).

Major depression can be challenging to assess and difficult to interpret in homeless populations. North and colleagues studied the relationship between ambient weather and same-day assessments of major depression in homeless people using a structured diagnostic interview. They found that men were more likely to meet diagnostic criteria for major depression when the weather was cold and wet. Yet, this difference was not detected among women, who are often allowed to spend days in the shelters (probably because most of them have children) while men are thrust out into the day’s weather each day. North and colleagues concluded that the symptoms of major depression can be difficult to separate from the “miseries of homelessness,” including hardships of exposure to inclement weather, especially for men. It is possible that the methods used to measure major depression in many epidemiological studies do not distinguish between the major depression that people typically present with in psychiatric treatment settings and emotional distress and disillusionment among homeless people coping with the extraordinary hardships of being homeless (physical and mental discomforts, hunger, fatigue, social isolation, demoralization, lack of privacy, and the presence of danger). If this is the case, then the standard treatments for major depression (e.g., medication, talk therapy) may not be appropriate for distressed homeless people as with clinically depressed treatment populations. Treatments for major depression are likely to be ineffective for ameliorating the situational distress and misery of homelessness that may be difficult to differentiate from major depression.

Instead of using a systematic diagnostic instrument to provide diagnosis of major depression according to DSM criteria (e.g., the Diagnostic Interview Schedule, the Composite International Diagnostic Interview, or the Structured Clinical Interview for DSM), many studies use proxies such as symptom screens, measures of distress or depressed mood, or distinctions of “clinical caseness.” Studies based on such nondiagnostic measures have asserted that almost three-fourths (73%) of homeless samples can be defined as having clinically significant emotional distress or clinical caseness for some sort of depressive-like syndrome, and approximately one-third of epidemiological and first-time shelter-user samples (ranging from 33% to 37%) have been reported to have “extreme distress.” As we stated before, being homeless can make one extremely stressed and unhappy. Even diagnostic instruments may overrepresent distress as major depression in this population, but they are far less likely than nondiagnostic screening tools to fall into this error. We suspect that high rates, such as 73%, for emotional distress/clinical caseness are a result of inadvertent capturing of the agonies of homelessness and their obfuscation with diagnostic syndromes.

The next mood disorder we will discuss is bipolar disorder. Breakey and colleagues estimated the lifetime prevalence of bipolar disorder in homeless populations of the mid-80s to be around 7%–8% for both women and men. Another epidemiological study of homelessness estimated the lifetime prevalence of bipolar disorder to be around 11%. A study conducted in the early 2000s ⁶¹ estimated lifetime rates of bipolar disorder to be around 9%.

Anxiety Disorders

Anxiety disorders, including panic disorder, generalized anxiety disorder, and posttraumatic stress disorder, are prevalent in the homeless population as well. One epidemiological study estimated the lifetime prevalence of any anxiety disorder for shelter users to be around 39% and the 6-month prevalence to be around 22%. However, prevalence estimates of these disorders have been lower in more recent epidemiological studies. Similar to our discussion of the challenges of diagnosing major depression in the homeless population, it can be difficult to diagnose anxiety disorders correctly in this population. Real threats of violence, theft, lack of food, and a need to avoid the police in some cities (for fear of being arrested for vagrancy) understandably generate anxiety for many homeless people. This situational anxiety can be difficult to distinguish from symptoms of psychiatric disorders such as panic disorder and generalized anxiety disorder.

Posttraumatic stress disorder appears to be the most prevalent anxiety disorder among homeless people. Lifetime prevalence of posttraumatic stress disorder is estimated to be around 20% for all homeless people —34% for homeless women and 18% for homeless men. Current (12-month) prevalence is estimated to be slightly less (15%).

Panic disorder is typically the next most prevalent anxiety disorder in studies of homeless populations. The lifetime prevalence of panic disorder in the homeless population is estimated to be around 8%–9% —3% in women and 5% in men. Current (both 6-month and 12-month) prevalence of panic disorder appears to be slightly less overall (5%) but very similar when reported separately by sex (women: 3%; men: 4%).

Generalized anxiety disorder is the last anxiety disorder we will discuss. Lifetime rates of generalized anxiety disorders range from 7% to 14%. Current (both 6-month and 12-month) prevalence rates of the disorder appear to be slightly lower, ranging from 5% to 11%.

Psychotic Disorders

When asked to conjure up an image of a homeless person, most people imagine someone who is severely mentally ill. They think of someone who is psychotic (hearing voices and seeing images that do not exist) and who talks to or yells at imaginary others, such as someone with schizophrenia. Systematic research shows, however, that a very small percentage of the homeless population fits this description. Contrary to anecdotal evidence, psychotic disorders, most often represented by schizophrenia, are not nearly as prevalent as the news media’s sensationalistic presentation of them in the homeless population. Epidemiological studies have estimated the lifetime prevalence of schizophrenia to be between 4% and 17%. First-time homeless shelter–using men have reported a prevalence estimate within this range: 8%.

Personality Disorders

The most consistently measured personality disorder in the homeless literature is antisocial personality disorder. The prevalence of antisocial personality disorder appears to be remarkably, disproportionately high among homeless populations. Epidemiological data show that between 16% and 20% of the homeless population meet criteria for the lifetime disorder with approximately 10% of women and 25% of men qualifying for the diagnosis. Some researchers have argued that meeting criteria for antisocial personality disorder can be a functional and adaptive, survival pattern of behavior in the context of homelessness rather than a strictly pathological phenomenon. Therefore, they would argue that using the diagnosis in the homeless population is culturally insensitive. North et al. refuted this argument, contending that the onset of adult antisocial personality disorder almost always occurs prior to the onset of homelessness and correlates with childhood conduct symptoms, and, in their analysis, the rates of antisocial personality disorder did not decline significantly when they removed criterion symptoms related to homelessness from the algorithms. They concluded that although homelessness may exacerbate the manifestations of antisocial personality disorder, it is a valid diagnosis in this population.

Other personality disorders have received generally less attention in the scientific literature. A pioneering study in the 1980s by Bassuk and colleagues examined rates of DSM-III–diagnosed personality disorders in sheltered homeless families. In this study, an astounding 71% of homeless mothers in their sample met diagnostic criteria for at least one personality disorder. Diagnoses provided by psychiatrists yielded the following prevalence rates of various personality disorders: dependent (24%), atypical (10%), borderline (6%), narcissistic (4%), antisocial (4%), passive-aggressive (4%), mixed (4%), schizoid (3%), and histrionic (1%). The authors of the study were quick to point out that Axis II diagnoses are less reliable than Axis I diagnoses and that many external environmental factors (e.g., poverty, racism, and sexism) may play a role in determining observable features masquerading as personality traits in this population barraged by extraordinary stressors.

Overall Prevalence Rates

According to a major epidemiological study conducted in the early 2000s, 49% of homeless people have a lifetime history of at least one nonsubstance disorder, most of which is accounted for by major depression . This means that (1) half of the homeless population does not have any history of nonsubstance disorder and (2) of those with a lifetime history of nonsubstance disorder, very few have chronic and persistent severe mental illness (e.g., schizophrenia, bipolar disorder). We exclude major depression from chronic and persistent severe mental illness (see reference 83 for definitions).

The Roles of Sex and Race in Psychiatric Disorders Among the Homeless

A common theme among the psychiatric disorders in the homeless population is differences in findings by sex and race. Homeless Caucasian women have been found to have a greater prevalence of schizophrenia, major depression, and bipolar, panic, generalized anxiety, and posttraumatic stress disorders than homeless women of color. This indicates that the homeless women of color may have less major psychiatric illness than homeless Caucasian women. Although we cannot determine causality, these data lend support to ideas that racism, oppression, social inequities, and social injustices may play a proportionately greater role in the homelessness of women of color.

The prevalence of cocaine, opioid, and amphetamine use disorders is greater among homeless men than among homeless women, and more homeless women have alcohol and cannabis use disorders, major depression, and schizophrenia compared with homeless men.

Medical Illness

Medical illness is also disproportionately overrepresented in the homeless population. Life on the streets and in the shelter system can be dangerous, stressful, and hazardous to one’s health. It can be difficult to locate a free place to shower and wash one’s clothes to maintain proper hygiene; this makes it difficult to prevent as well as treat illness. Few homeless people have health insurance and can take preventive health care measures (perhaps, in part, because of competing immediate demands such as food and shelter). Many may wait for health conditions to become urgent or emergent before seeking medical attention and then use emergency rooms rather than regular outpatient services for treatment.

We cannot state that all medical problems in the homeless population are the direct result of homelessness. Housed low-income populations often have poor health as well. This poor health can be attributed to a variety of factors including poor diet, lack of preventive health care, and lack of exercise. Medical problems among those who are low-income and become homeless are generally problems that are well known to be associated with circumstances of extreme poverty and other associated social problems. It is, however, likely that being homeless exacerbates the health problems that are already endemic in these populations.

HIV/AIDS

HIV/AIDS has recently begun to receive increased attention in homeless populations. The prevalence of HIV/AIDS among the homeless and marginally housed populations has been estimated to be between 10% and 15% in San Francisco, 6% and 19% among the mentally ill homeless in New York City, and 16% among soup kitchen attendees in New York City. One study found that people with HIV/AIDS in Philadelphia were three times as likely to be homeless as people without the infection. Another study found that homeless injection drug users had a greater prevalence of HIV than housed injection drug users (19% vs. 11%).

Risk for engaging in risky sexual behaviors, which increase one’s chances of contracting HIV, is increased in association with intoxication with alcohol and other drugs in homeless populations as in other populations. Because many homeless people, particularly women, trade sex for food, clothing, drugs, or a place to stay, they are at heightened risk for contracting the virus. Homeless people, especially those who are most transient, may not have a reliable place to store their works; therefore, they are more likely than others to borrow injection equipment or visit a shooting gallery. This places them at even greater risk for contracting the virus.

Infectious Diseases

People who experience homelessness may be exposed to or carry infectious diseases. One study of people using soup kitchen services in New York City found high rates of hepatitis B virus exposure (21%), hepatitis B carrier (6%), hepatitis C seropositive (19%), and syphilis exposure (15%). The Centers for Disease Control and Prevention (CDC) reported the rate of tuberculosis among the homeless to be 6.5% in 1997.