Sociodemographics

Although there has been no change in social assistance in Berlin over the study period, there has been a significant decrease in affordable housing and insufficient construction of new social housing, despite a growing population. It has been reported that these factors are partly responsible for urban homelessness.^{Reference Parkinson, Batterham, Reynolds and Wood33} Furthermore, our data showed a significant increase (by 15%) in the proportion of homeless patients between 2008 and 2021. This result clearly exceeds the increase in homelessness in Germany's general population over that period, estimated to be 4.4% (excluding homeless refugees, who were only included since 2015 in the estimation model).^11,12 Although the number of inhabitants in the hospital's catchment area increased by 15.3% between 2008 and 2021, the in-patient capacity of the hospital increased by only 9.6%. An increase in homelessness is not yet considered in the planning of healthcare capacities, potentially resulting in a shift of people with mental disorders and need for in-patient treatment who are living in secure living conditions to out-patient settings or longer waiting times.

The homeless population in this study shows similar demographic characteristics to the homeless population of North Rhine–Westphalia, Germany.^{Reference Haussleiter, Lehmann, Ueberberg, Heinz, Zielasek and Gouzoulis-Mayfrank23} Here, too, patients experiencing homelessness were significantly younger and more often male.^{Reference Haussleiter, Lehmann, Ueberberg, Heinz, Zielasek and Gouzoulis-Mayfrank23} Other factors, such as involuntary admissions and shorter lengths of stay, are also consistent with this study.^{Reference Haussleiter, Lehmann, Ueberberg, Heinz, Zielasek and Gouzoulis-Mayfrank23} Young age is considered a risk factor for homelessness in psychiatric patients according to a Danish study.^{Reference Nordentoft, Knudsen and Schulsinger34} An explanation for the younger age of homeless patients is the considerably increased risk of death and association with high mortality, as shown by Nilsson et al.^{Reference Feodor Nilsson, Laursen, Hjorthøj and Nordentoft35} Another meta-analysis from high-income countries shows that homeless individuals are more likely to suffer from severe somatic illnesses, which lead to lower life expectancy.^{Reference Aldridge, Story, Hwang, Nordentoft, Luchenski and Hartwell36} Since our data do not provide information about first onset of the disorder or the last date of accommodation, we cannot say whether homelessness preceded the onset of mental illness or the mental illness caused the loss of housing.

Predictors of homelessness

In our regression model, being male, being born outside of Germany, the lack of out-patient treatment prior to admission, as well as the diagnoses of psychotic disorders, drug and alcohol dependence, reaction to severe stress and personality disorders, were identified as factors significantly associated with being homeless.

Being male was significantly associated with being homeless, which is in line with other study samples reporting high rates of males among homeless people.^{Reference Laporte, Vandentorren, Détrez, Douay, Le Strat and Le Méner37,Reference Bäuml, Brönner, Baur, Fischer, Pitschel-Walz and Jahn38} Males are more likely to be affected by substance misuse, psychotic disorders and involuntary treatment and are less likely to access social services and support systems, and as a result often remain untreated for longer periods.^{Reference Laporte, Vandentorren, Détrez, Douay, Le Strat and Le Méner37,39} The proportion of homeless females (21.4%) is consistent with national estimates for 2018 (25%),¹¹ but both estimates may be inaccurately low owing to hidden homelessness among females. Homeless females more often avoid social and medical support provided for homeless people, since they report a feeling of insecurity in, for example, male-dominated homeless shelters or due to previous traumatic experiences, which should be considered by the psychiatric care system.^{Reference Deleu, Schrooten and Hermans40,Reference Busch-Geertsema, Henke, Steffen, Reichenbach, Ruhstrat and Schöpke41} Overall, it could be considered that homeless people of different genders may experience homelessness differently and have different needs, which implies the need for gender-specific support services.

In our sample, the probability of being homeless was two times higher for people born outside of Germany than for people born in Germany. People who have had traumatic experiences in their lives were up to four times more likely to be homeless. There is a link between migration and stress-related disorders, with refugees suffering more often from post-traumatic stress disorder.^{Reference Bustamante, Cerqueira, Leclerc and Brietzke42} Migrants may have lower language skills, limited access to safe housing, healthcare and income support and ongoing family insecurities, which can enhance stress-related reactions and might contribute to the risk of homelessness.^{Reference Bustamante, Cerqueira, Leclerc and Brietzke42} Furthermore, during their first months in Germany, refugees are often housed in shelters falling under the definitions of homelessness and struggle to find regular housing after.

Among the six most common psychiatric diagnoses, reaction to severe stress and personality disorders showed a strong link in the regression model with being homeless. The link between reaction to severe stress and homelessness has been described, showing that homeless people often report some form of abuse^{Reference Torchalla, Strehlau, Li, Schuetz and Krausz43} and are at higher risk of experiencing traumatic events during homelessness.^{Reference Edalati, Krausz and Schütz44} A meta-analysis from 2021 has shown that the lifetime prevalence of adverse childhood experiences (ACEs) is substantially higher among people experiencing homelessness in the USA and Canada than in the general population and exposure to ACEs might be associated with prevalence of mental illness, substance misuse and victimisation.^{Reference Liu, Luong, Lachaud, Edalati, Reeves and Hwang45} In addition, another meta-analysis showed that personality disorders are common among homeless people in Western countries,^{Reference Fazel, Khosla, Doll and Geddes46} with an increased risk of dropping out of treatment.^{Reference Salavera, Tricás and Lucha47}

In our sample, the lack of out-patient treatment increased the probability of homelessness by up to five times. In a Canadian study based on longitudinal data on healthcare utilisation by homeless people and their age- and gender-matched low-income controls in a universal health insurance system, homeless people had substantially higher rates of emergency department and hospital use than the general population controls, especially driven by a subset of high users.^{Reference Hwang, Chambers, Chiu, Katic, Kiss and Redelmeier15} Out-patient care often comes with barriers for homeless people (inflexibility of appointment scheduling, lack of health insurance causing refusal of treatment by doctors, bureaucratic procedures). In our sample, only 29.1% of the homeless group were having any kind of medical out-patient treatment in the 6 months prior to admission, which is less than in a survey among homeless individuals from Hamburg (47% did not have any out-patient treatment in the 3 months prior).^{Reference Hajek, Van Rüth, Bertram, Schüler, Heinrich and Kretzler48}

According to a recent meta-analysis among Western countries, psychotic disorders are highly prevalent among homeless people, with pooled prevalence rates for schizophrenia spectrum disorders of 12.4% (95% CI 9.5–15.7%).^{Reference Gutwinski, Schreiter, Deutscher and Fazel8} In our sample, having a psychotic disorder was significantly associated with being homeless. In cities, the factor of urbanicity seems to be a risk factor for psychotic disorders by itself.^{Reference Gruebner, Rapp, Adli, Kluge, Galea and Heinz30} A recent US study underlined the importance of minimising the risk of homelessness by early treatment and detection of psychotic disorder, which was linked to youth-onset homelessness.^{Reference Iwundu, Chen, Edereka-Great, Businelle, Kendzor and Reitzel49}

Drug and alcohol dependence were significantly associated with homelessness in our sample. A US study showed alcohol and drug dependence to be associated with first-time homelessness, underlining the importance of early detection, destigmatisation and treatment of substance dependence in preventing homelessness.^{Reference Thompson, Wall, Greenstein, Grant and Hasin50} Lack of effective aftercare for patients with substance use disorders coupled with being discharged back into homelessness can lead to a high risk of relapse.^{Reference Stein, Dixon and Nyamathi51}

The significantly high proportion of people with a drug use disorder living in sociotherapeutic facilities is surprising, since abstinence is usually required and is often a barrier for many homeless people with substance use. On the other hand, the high rates of substance use among homeless people in our sample indicates the need for sociotherapeutic facilities with lower thresholds regarding substance consumption.

Barriers to treatment and length of stay

Some institutional processes, such as waiting lists and telephone enrolment in drug and alcohol treatment programmes, establish barriers for people without housing to receive psychiatric treatment. The shorter length of stay for homeless people compared with privately housed people in our sample supports findings from a study comparing length of stay for psychiatric in-patients among five European countries.^{Reference Dimitri, Giacco, Bauer, Bird, Greenberg and Lasalvia18} In Germany, homelessness predicted a shorter length of stay, in contrast to a longer length of stay in most other countries.^{Reference Iwundu, Chen, Edereka-Great, Businelle, Kendzor and Reitzel49} These results could reflect an established system of support for homeless people in Berlin outside of the hospital setting (e.g. charitable medical support, substance use support programmes), but might also reflect difficulties of supporting homeless people during psychiatric in-patient treatment efficiently, resulting in treatment drop-out. In combination with the finding of significantly higher rates of compulsory admissions among homeless patients in our sample, compulsory treatment might lead patients to keep their stay as short as possible and get discharged as soon as the legal basis for a compulsory admission has run out, increasing mistrust in institutions.

Limitations

There are several limitations to our study that must be outlined. Routine data can be prone to errors. The generalisability of our results to other areas, especially rural areas, is limited since our study region represents an urban district. Another limitation to generalisability is the exception of German studies showing a shorter length of stay for homeless patients compared with studies in other European and North American countries. Still, the catchment area of this study is characterised by a high proportion of European migrants and relatively low socioeconomic status and is thus comparable to other European urban areas. Furthermore, the study sample was limited to an in-patient psychiatric treatment setting and did not include day clinics, the emergency room and out-patient centres of the hospital. Future research should expand the homelessness sample, by including homeless people from different in- and out-patient healthcare services. We did not include data for housing status after treatment, insurance status or form of discharge, such as ‘against medical advice’, owing to absence of standardised documentation. Since Germany has a mandatory universal healthcare system, numbers of uninsured people are low. Nevertheless, approximately 61 000 people (less than 1%) in Germany remain without healthcare insurance.Reference Zehnder⁵²

Finally, owing to the cross-sectional design we cannot draw causal relationships between homelessness and associated factors, such as gender, place of birth or type of mental disorder. Future research should include longitudinal designs when examining protective and risk factors of homelessness among people with mental disorders.