Well-controlled epidemiological studies show elevated rates of psychotic disorders in densely populated areas [1,2]. Fifty-five per cent of today's world population lives in urban areas and further urbanization is expected in the next decades . The topic of urbanicity and mental health is therefore vital and timely [4▪,5,6]. Previous research associated psychosis risk with urban birth [7,8] upbringing [9,10], and current city living [11,12]. Explanations were sought in characteristics of the urban environment that reflect social stressors and socio-environmental adversity , which are increased by low social cohesion and high deprivation (e.g. low income, employment and education), inequality and social fragmentation [13–19]. In addition, lack of green space in itself or as proxy for urban stress, environmental pollution and toxin exposure has been suggested to increase risk . Urban factors may do most harm in individuals with a genetic liability for psychosis. Yet, it has also been proposed that particularly those at risk might be drawn toward city living . Knowledge about urban risk factors is crucial to enable urban designs that mitigate risk. Here we review the recent literature on the link between urbanicity and psychosis.
MATERIAL AND METHODS
Search strategy and study selection
We conducted a systematic literature search in the online databases PsycINFO, Embase and PubMed, covering May 2017 until November 2018. Search terms were: ‘urbanicity’ or ‘urban population’ or ‘green space’ or ‘nature’ or ‘urban area’ or ‘urbanisation/urbanization’ or ‘population density’ and ‘psychosis’ or ‘psychoses’ or ‘psychotic disorder’ or ‘schizophrenia’ or ‘schizophrenia spectrum disorder’ or ‘delusion’ or ‘hallucination’ or ‘paranoia’.
Reference lists of key studies were inspected for additional articles. Studies were included if they: were written in English; peer-reviewed; included patients with a self-reported or clinically assessed diagnosis of a (non-affective or affective) psychotic disorder; and assessed urbanicity effects (i.e. associations with population density). Studies were excluded if they were reviews, doctoral theses/dissertations or book chapters or reported only subclinical psychotic experiences/subclinical symptoms in the general population. We identified 647 unique articles. Titles and abstracts were screened by I.L.J. Eighty-five articles were eligible and the full text was assessed by I.L.J. and was discussed by I.L.J. and A.K.F., resulting in 17 included studies (see Fig. 1).
Urbanicity and psychosis risk
Five studies on worldwide urbanicity–psychosis associations yielded mixed findings. Data from a large Danish, male-only sample associated urban birth and moving toward higher urbanicity before age 10 with elevated risk for schizophrenia spectrum disorders in adulthood . The association with urban birth was most robust and remained significant after adjustment for IQ, social class, parental education, familial psychiatric history and second-generation migrant status. Schofield et al. confirmed the association between urbanicity exposure at age 15 and the risk for non-affective psychosis in native Danes, but not in migrants from Africa, Asia, Europe or the Middle East. A European multicentre study by Jongsma et al.[24▪▪] showed no significant overall association between urbanicity and the incidence of psychosis. However, analyses by country revealed significant associations between urbanicity and higher psychosis risk in the United Kingdom and the Netherlands, but not in France, Italy and Spain [24▪▪]. Data from the United Kingdom also indicated that individuals with first episode psychosis (FEP) are more likely to reside in urban areas compared to the general population . Urban living was also found to be associated with elevated schizophrenia risk in China  and WHO Survey data from several low and middle-income countries (LMIC) revealed a higher risk for psychotic disorders (self-report) with urbanicity in Estonia. However, a lower psychosis risk with higher urbanicity was present in Mali, Senegal and the Philippines. The overall association was nonsignificant [27▪▪].
Urbanicity, symptoms and hospitalization
Across different European sites, urban living was associated with higher general, negative, disorganized, and manic symptoms in FEP. Interestingly, urbanicity was associated with higher positive and negative symptoms in the United Kingdom and lower positive and negative symptoms in Spain. No such associations were present in the Netherlands, France or Italy . On the Italian island of Sardinia, lower hospitalization rates for schizophrenia and other psychiatric disorders have been found for individuals from towns with more than 10,000 residents than for individuals from smaller municipalities (7.1 vs. 9.3/1,000) .
Explaining the urbanicity–psychosis association
Twelve studies examined possible explanatory mechanisms of the urbanicity–psychosis link. Two studies investigated coping with urbanicity and the associations between urbanicity and the brain.
Urbanicity, social and economic mechanisms
Several social and economic mechanisms might account for the urbanicity–psychosis link. Four studies showed that migrants/ethnic minorities have a higher psychosis incidence than native populations [23,24▪▪,25,30]. Although migrant or ethnic minority status has been suggested as underlying factor of urbanicity effects, Schofield et al.'s  findings implicate a more complicated relationship. Only after ethnic density (i.e. concentration of a particular group in an area) was included in the analysis, psychosis risk was elevated for European and the Middle Eastern, but not for Asian or African migrants. Thus in some groups, urbanicity–psychosis associations appear when ethnic density is low . Kirkbride et al. showed that regardless of urbanicity, Black-Caribbean, African and Pakistani, but not Arabic or Bangladeshi minorities have a higher psychosis risk. Another investigation  used the same data to disentangle effects of urbanicity, racial/ethnic diversity, – density and – fragmentation, economic deprivation and social isolation on psychosis risk. Economic deprivation, social isolation and urbanicity emerged as independent risk factors when ethnicity was accounted for. Some patterns of urban risk differed for affective and non-affective psychosis, with increased risk in rural areas for some groups (see Table 1) [25,30]. In the European multicentre study [24▪▪], psychosis risk was not significantly associated with urbanicity when ethnic minority status, owner occupancy of housing, household status (single person) and unemployment were accounted for; with similar patterns for affective and non-affective psychosis.
Urbanicity, resources and treatment
Urbanicity can be a proxy for risk-increasing factors, but can also signify risk-reducing factors, such as healthcare or resource access. Individuals who were prenatally exposed to the Chinese famine in 1959–1962 showed significantly elevated schizophrenia rates in rural, but not urban areas. The urban population received food from state grain stores, showing how urbanicity can affect resource access . Another Chinese study showed that patients in rural compared to urban areas were less likely to take antipsychotics (35.4 vs. 17.5%) . Lower education, living alone and number of admissions were predictors of not taking antipsychotics in urban areas, whereas female sex, low BMI, higher positive symptoms, lower anxiety and insight emerged as predictors in rural areas. These patterns could reflect urban variation in status associated with sex, care and family structures. Research from India on rural–urban differences in time to treatment (indicated by duration of untreated psychosis) showed that despite greater proximity to psychiatric care, better education and income of families, urban patients did not access care earlier than rural patients .
Urbanicity and nature
Green space might reduce psychosis risk through stress reduction and reduced exposure to pollution. A Danish register study showed increased psychosis risk in those who during childhood (at age 10) lived in the least green compared to those who lived in the greenest areas. Risk decreased with accumulated green space exposure. The effects of green space were attenuated, but still significant when urbanicity and socio-economic factors were taken into account [34▪▪]. Evidence from the Netherlands showed that patients with psychotic disorders tend to reside in less green neighbourhoods than the general population. Yet, greener living was unrelated to the duration of psychiatric admissions .
Coping with urbanicity
Swiss research showed that FEP patients prefer rural to urban environments and that they perceive density of people and buildings; levels of sensory stimulation; mobility obstacles and difficult role management in public situations as stressors. Coping strategies included planning and regulating mobility to avoid crowded areas; creating isolation from the environment through technology (e.g. headphones), close social contact (e.g. friends) or internal dialogue and creating atmospheres of comfort (e.g. in nature) [36▪▪].
Urbanicity and genetic risk
One study examined whether the genetic risk for psychosis, indicated by polygenic risk scores, accounts for city living and associations between schizophrenia and urbanicity at birth and during upbringing (i.e. at age 15). Urbanicity was associated with increased schizophrenia risk. Adjustment for polygenic risk scores did not change the association between schizophrenia and urban birth and slightly attenuated the association with urban upbringing [37▪].
Urbanicity and the brain
A Dutch investigation on total grey matter volume (GMV) and urban upbringing in individuals with non-affective psychosis, healthy siblings and control participants [38▪], associated urban upbringing with reduced GMV in male patients only.
This review shows that urban environments do not increase the risk for psychosis by default, but that their effects are conditional on multiple factors. For instance, city living can offer benefits, like access to healthcare or resources, especially in less developed countries [31,32]. However, it is also characterized by lack of green space and high-intensity social encounters that might be particularly stressful for individuals with an increased liability for psychosis [36▪▪,39].
The North European findings support previously established urbanicity–psychosis associations [22,25,40,41], with most robust effects for urban birth and childhood urbanicity . Danish population register data show that the urbanicity–psychosis link cannot be explained by the genetic risk for psychosis, which could lead to a tendency for urban living [37▪]. Interestingly, South European data did not support the link between urbanicity and psychosis (in Italy urbanicity emerged as protective factor) and a North–South divide also appeared for associations between urbanicity and psychotic symptoms [28,29]. Thus, even in relatively homogenous western, high-income countries (e.g. comparable drug use, religion, education and healthcare standards), urbanicity effects are diverse and it is unclear why international differences occur. Possible explanatory factors include differences in social cohesion, control and isolation in rural areas; differential pressures of modern urban life or geographic variations in diet, climate or exposure to disease agents. Clear-cut urbanicity effects could not be identified in LMIC either [27▪▪]. It is possible that in some LMIC positive urbanicity effects, such as the availability of resources counteract negative urbanicity effects. Alternatively, some negative effects of (northern) high-income societies, such as migration/racial discrimination and social economic disparities might not play a large role in LMIC. Overall, this review reveals heterogeneity and complex variation. In Estonia, China, Denmark, the Netherlands and the United Kingdom urbanicity was associated with a higher psychosis risk. In Mali, Senegal, the Philippines, Spain, Italy and France urbanicity was associated with a lower psychosis risk or unrelated to risk. There is no obvious common denominator unifying the two lists of culturally and geographically diverse countries. In the search for explanations, it is important to consider that definitions of urbanicity varied between studies. Pooled analysis covers interesting and potentially important international patterns of variation and future research will need to analyze data by country to identify shared risk factors and protective factors.
Numerous social and economic mechanisms may account for urbanicity effects . Among others, the reviewed studies investigated migrant/ethnic minority status, racial/ethnic diversity, – density and – fragmentation, socio-economic status and social isolation. Migrant/minority status has been viewed as particularly important, because migrants often leave poor living conditions with limited opportunities to seek improvement in urban areas . This exposes them to multiple factors that can increase psychosis risk . For instance, residential mobility itself has been associated with an increased risk, suggesting harmful effects of social instability [25,45]. The current findings support an elevated psychosis risk for some (e.g. African, Caribbean, Middle Eastern/Pakistani, European non-Scandinavian and Asian), but not all migrant groups (e.g. Arabic, non-British White) [23,24▪▪,25,30]. Migrant status did not explain urbanicity effects in the United Kingdom and Denmark [23,25,30], but did so across multiple other European countries [24▪▪]. Interestingly, ethnic density, for example living in areas with others from the same cultural background, appears to mitigate psychosis risk in European and Middle Eastern groups . Protective effects could be social support networks and reduced discrimination . It remains unclear why this effect has been found only for specific migrant groups. Future research needs to consider time (e.g. childhood vs. adulthood) and reasons for migration (e.g. war refugees, economic migration within or between culturally (dis)similar countries), as well as the different conditions people encounter in their new homes (e.g. attitudes towards migrants) as possible explanations. Also with regard to urbanicity and other socio-economic factors, the current evidence is mixed. In the United Kingdom economic deprivation, social isolation and urbanicity (but not racial/ethnic diversity, – density or – fragmentation) emerged as independent predictors of psychosis risk when ethnicity was accounted for . However, across Europe [24▪▪] no significant associations between urbanicity and psychosis risk were found when minority status and socio-economic indicators (e.g. owner occupancy of housing, single-person household and unemployment) were considered.
Exposure to green space during childhood reduced psychosis risk, independent of urbanicity effects [34▪▪]. Effects of nature might be particularly salient during childhood, where a calmer and less polluted environment with opportunities for outdoor activities may enhance resilience and/or protect the developing brain. Such effects are supported by research that linked urban upbringing to reductions in GMV in males with psychosis [38▪]. Urban upbringing may increase psychosis risk through stress sensitization, mediated by gene–environment interactions with dopamine genes  and stress during development has previously been associated with reduced GMV . Still, it remains unclear why this effect would be specific to males. Further research will be necessary to show how the city affects the developing brain . Research that investigated current green space exposure showed that individuals with a psychotic disorder tend to live in less green neighbourhoods. Although this was unrelated to clinical outcomes , recent evidence suggests positive effects of nature on well-being  and that FEP patients use nature to cope with urban stressors [36▪▪]. Thus, exposure to nature could still impact positively on patients’ well-being if directly integrated in treatment during admission.
Diverse social, economic and environmental factors combine to explain urban risk with regard to incidence, prevalence and the course of psychotic disorders in ways we are only starting to understand. This review highlights the need for global, multidisciplinary investigations that incorporate epidemiology, neuroscience, experimental psychology, sociology and urban planning, an approach which was recently presented as ‘neurourbanism’ . This research needs to pinpoint toxic (e.g. pollution, disease exposure, unemployment, social fragmentation or migration), as well as protective urban effects (e.g. resources, political freedom and low social control) and their interactions. Sensitive periods of maximum risk (e.g. urban birth, upbringing and current urbanicity) should be systematically investigated, while considering effects of cumulative urban exposure. The newly generated knowledge will be paramount for urban design that mitigates risk and the understanding of the cause of psychosis.
Financial support and sponsorship
A.K.F. was supported by a NARSAD Young Investigator Grant from the Brain & Behaviour research Foundation (24138). L.K. was supported by a VICI grant from the Netherlands Organization for Scientific Research (NWO) [452-07-007,453-11-005]; and an ERC Consolidator grant (648082 SCANS).
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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