The urban environment and mental health

Living in or growing up in an urban environment has been associated with the elevated prevalence of mental disorders (Peen, Schoevers, Beekman, & Dekker, Reference Peen, Schoevers, Beekman and Dekker2010). Most research in this realm has focussed on disorders in the schizophrenia spectrum. However, particularly in the last decade, investigations of other mental health conditions have emerged. Compelling evidence supports the urbanicity and non-affective psychosis link [OR 1.72 (Krabbendam & Van Os, Reference Krabbendam and Van Os2005); meta-analytic IRRs of 1.68 (Vassos et al., Reference Vassos, Agerbo, Mors and Pedersen2016) and 2.25 (Castillejos, Martín-Pérez, & Moreno-Küstner, Reference Castillejos, Martín-Pérez and Moreno-Küstner2018)]. Findings are most robust for urban residence at birth and during childhood, suggesting an important impact of urban factors on the developing organism (March et al., Reference March, Hatch, Morgan, Kirkbride, Bresnahan, Fearon and Susser2008; Paksarian et al., Reference Paksarian, Trabjerg, Merikangas, Mors, Børglum, Hougaard and Agerbo2018; Toulopoulou, Picchioni, Mortensen, & Petersen, Reference Toulopoulou, Picchioni, Mortensen and Petersen2017). Urban birth and provincial city upbringing have also been associated with bipolar disorder [IRR 1.18 (Marcelis, Navarro-Mateu, Murray, Selten, & Van Os, Reference Marcelis, Navarro-Mateu, Murray, Selten and Van Os1998), 1.21 and 1.23 (Pedersen & Mortensen, Reference Pedersen and Mortensen2006)]; and urban birth and current urbanicity have been associated with the prevalence and incidence of unipolar mood disorders [e.g. IRR 1.16 (Vassos et al., Reference Vassos, Agerbo, Mors and Pedersen2016); pooled OR of 21 studies 1.39 (Peen et al., Reference Peen, Schoevers, Beekman and Dekker2010), for a review with mixed results see Rautio et al. (Reference Rautio, Filatova, Lehtiniemi and Miettunen2018)]. Associations with current urbanicity have also been found for anxiety disorders [pooled OR of 12 studies 1.13 (Peen et al., Reference Peen, Schoevers, Beekman and Dekker2010)]. Urban residence at birth and during childhood has been associated with autism spectrum disorders [IRRs birth: 2.28; childhood: 2.85 (Lauritsen et al., Reference Lauritsen, Astrup, Pedersen, Obel, Schendel, Schieve and Parner2014), birth: 1.41 (Vassos et al., Reference Vassos, Agerbo, Mors and Pedersen2016)]. The evidence for substance use is mixed, depending on geographic region and substance [IRRs 1.76–2.47 (Peen et al., Reference Peen, Schoevers, Beekman and Dekker2010; Vassos et al., Reference Vassos, Agerbo, Mors and Pedersen2016), pooled OR of 13 studies 1.31 (Peen et al., Reference Peen, Schoevers, Beekman and Dekker2010)]. Links between urbanicity and eating disorders were mostly, but not always inconclusive [e.g. (Mitchison & Hay, Reference Mitchison and Hay2014), Mulders-Jones, Mitchison, Girosi, and Hay (Reference Mulders-Jones, Mitchison, Girosi and Hay2017), Penkalla and Kohler (Reference Penkalla and Kohler2014), Vassos et al. (Reference Vassos, Agerbo, Mors and Pedersen2016)].

Some epidemiological studies suggest that the risk of urbanicity is increased in those with a genetic liability for mental health conditions, as in the case of psychosis (Krabbendam & Van Os, Reference Krabbendam and Van Os2005; van Os, Kenis, & Rutten, Reference van Os, Kenis and Rutten2010).

At the same time, those at higher risk might be drawn towards living in urban or deprived areas [i.e. selective migration (Colodro-Conde et al., Reference Colodro-Conde, Couvy-Duchesne, Whitfield, Streit, Gordon, Kemper and De Zeeuw2018; Sariaslan et al., Reference Sariaslan, Fazel, D'Onofrio, Langstrom, Larsson, Bergen and Lichtenstein2016)], but such mechanisms can only explain a small part of the urbanicity effect (Paksarian et al., Reference Paksarian, Trabjerg, Merikangas, Mors, Børglum, Hougaard and Agerbo2018). Given that hereditary risk for mental health disorders involves multiple common genetic variants of small effects, future studies could investigate how urbanicity interacts with polygenic risk scores. Using this approach for other environmental exposures, recent results from the EU-GEI study suggested that both early-life adversity and cannabis use interacted with molecular genetic risk state in the development of psychosis (Guloksuz et al., Reference Guloksuz, Pries, Delespaul, Kenis, Luykx, Lin and van Os2019).

Importantly, emerging evidence shows the geographic variation in the relation between urbanicity and psychosis. For instance, EU-GEI showed no significant overall association between current urbanicity and psychosis, with opposite effects in the UK and the Netherlands (IRRs = 1.17 and 1.89) and France, Spain and Italy (1.01, 1.01, 0.72) (Jongsma et al., Reference Jongsma, Gayer-Anderson, Lasalvia, Quattrone, Mulè, Szöke and Cristofalo2018). A study of low- and middle-income countries showed higher psychosis rates with urbanicity in Estonia, but opposite patterns in Mali, Senegal and the Philippines (DeVylder et al., Reference DeVylder, Kelleher, Lalane, Oh, Link and Koyanagi2018). For affective psychosis, some reported a lower incidence with urbanicity (Kelly et al., Reference Kelly, O'Callaghan, Waddington, Feeney, Browne, Scully and Larkin2010; Kirkbride et al., Reference Kirkbride, Hameed, Ioannidis, Ankireddypalli, Crane, Nasir and Jones2017) and living in urban environments or higher unit density neighbourhoods has been associated with a lower incidence of depression and anxiety in Peru and the USA (Loret de Mola et al., Reference Loret de Mola, Stanojevic, Ruiz, Gilman, Smeeth and Miranda2012; Miles, Coutts, & Mohamadi, Reference Miles, Coutts and Mohamadi2012). This variation in risk for different disorders, time of exposure (e.g. developmental v. current) and geographic region stresses the need for systematic research that assesses periods of risk in conjunction with multiple individual (e.g. polygenic risk) and contextual factors (e.g. specific risk attributes of urban environments). The key epidemiological findings are summarized in Box 1.

Risk attributes of the urban environment

Urban risk does not only lie in population density, but arises from an accumulation of social and environmental stressors (Galea, Reference Galea2011; Rapp et al., Reference Rapp, Kluge, Penka, Vardar, Aichberger, Mundt and Heinz2015; see Box 2).

Social and economic factors, such as deprivation/poverty and social fragmentation, lack of social capital, cohesion and trust may explain urban risk [(Castillejos et al., Reference Castillejos, Martín-Pérez and Moreno-Küstner2018; Drukker, Krabbendam, Driessen, & van Os, Reference Drukker, Krabbendam, Driessen and van Os2006; Galea et al., Reference Galea, Ahern, Nandi, Tracy, Beard and Vlahov2007; O'donoghue et al., Reference O'donoghue, Lyne, Renwick, Lane, Madigan, Staines and Clarke2016; Zammit et al., Reference Zammit, Lewis, Rasbash, Dalman, Gustafsson and Allebeck2010), for reviews see De Silva, McKenzie, Harpham, & Huttly (Reference De Silva, McKenzie, Harpham and Huttly2005); Ehsan & De Silva (Reference Ehsan and De Silva2015); McKenzie (Reference McKenzie2008)]. Yet, some contradictory evidence shows higher social cohesion and incidence of distress and common mental health disorders in rural areas (Loret de Mola et al., Reference Loret de Mola, Stanojevic, Ruiz, Gilman, Smeeth and Miranda2012). Overall, similar rates of poverty and social exclusion have been reported in urban and rural European areas, yet there were regional variations with higher social exclusion and inequality particularly in western European cities (Eurostat, Reference Koceva, Brandmüller, Lupu, Önnerfors, Corselli-Nordblad and Coyette2016). This is interesting, as perceived social inequality has been suggested to increase the risk for mental distress, especially when the situation is unchangeable [Social Defeat Hypothesis (Selten, van der Ven, Rutten, & Cantor-Graae, Reference Selten, van der Ven, Rutten and Cantor-Graae2013)]. Social defeat could explain elevated risk in low socio-economic status (SES)/higher inequality neighbourhoods, which might be more typical, yet not specific to cities (Blumenthal & Kagen, Reference Blumenthal and Kagen2002). Further, densely populated urban areas are characterized by a high frequency of social encounters. These may contribute to negative urban effects for individuals with a liability for mental health conditions through increased stress sensitivity (Myin-Germeys, Delespaul, & Van Os, Reference Myin-Germeys, Delespaul and Van Os2005) and/or, as in case of psychosis and depression, deficits in social cognition (Green, Horan, & Lee, Reference Green, Horan and Lee2015; Weightman, Air, & Baune, Reference Weightman, Air and Baune2014).

A factor that has frequently been investigated in the context of urban risk is migration, which is thought to increase the risk through social instability (Bhugra, Reference Bhugra2004; Cantor-Graae & Pedersen, Reference Cantor-Graae and Pedersen2013; McKenzie, Reference McKenzie2008; Price, Dalman, Zammit, & Kirkbride, Reference Price, Dalman, Zammit and Kirkbride2018). However, migrant status is not a risk per se and its effects vary between countries and ethnic groups (Jongsma et al., Reference Jongsma, Gayer-Anderson, Lasalvia, Quattrone, Mulè, Szöke and Cristofalo2018; Kirkbride et al., Reference Kirkbride, Hameed, Ioannidis, Ankireddypalli, Crane, Nasir and Jones2017; Schofield et al., Reference Schofield, Thygesen, Das-Munshi, Becares, Cantor-Graae, Pedersen and Agerbo2017). Social support structures, shared social history, positive identification with one's own ethnic group (Anglin, Lui, Espinosa, Tikhonov, & Ellman, Reference Anglin, Lui, Espinosa, Tikhonov and Ellman2018; Veling, Hoek, Wiersma, & Mackenbach, Reference Veling, Hoek, Wiersma and Mackenbach2010) and low discrimination appear to render individuals more resilient (Schofield et al., Reference Schofield, Thygesen, Das-Munshi, Becares, Cantor-Graae, Pedersen and Agerbo2017; Veling et al., Reference Veling, Susser, Van Os, Mackenbach, Selten and Hoek2008) showing that effects of individual characteristics (e.g. migrant status) depend on context. Finally, it is important to consider that the perception of the social stress associated with the urban environment is also influenced by the mental health and wellbeing of the individual (Corcoran et al., Reference Corcoran, Mansfield, Giokas, Hawkins, Bamford and Marshall2017). For example, a large population study in adolescents showed that those who perceived higher levels of threat in their neighbourhood were more likely to have psychotic experiences (Newbury et al., Reference Newbury, Arseneault, Caspi, Moffitt, Odgers and Fisher2018). This effect remained after accounting for levels of crime, individual disorder, neighbourhood- and family-level SES, suggesting that the subjective perception of the urban environment is an important target for research and possibly intervention.

An important physical factor through which cities could impact mental health may be lack of green space (Bratman, Hamilton, & Daily, Reference Bratman, Hamilton and Daily2012; Fong, Hart, & James, Reference Fong, Hart and James2018; Gascon et al., Reference Gascon, Triguero-Mas, Martinez, Dadvand, Forns, Plasencia and Nieuwenhuijsen2015; Lee & Maheswaran, Reference Lee and Maheswaran2010; van den Berg et al., Reference van den Berg, Wendel-Vos, van Poppel, Kemper, van Mechelen and Maas2015). Low compared to high green space presence was associated with a 1.52-fold increased schizophrenia risk in a Danish case registry study (Engemann et al., Reference Engemann, Pedersen, Arge, Tsirogiannis, Mortensen and Svenning2018), with most profound effects for low green space presence during early childhood. Moreover, individuals with psychosis have been found to reside in less green areas than the general population (Boers, Hagoort, Scheepers, & Helbich, Reference Boers, Hagoort, Scheepers and Helbich2018). Lack of green space has also been associated with the prevalence of anxiety and autism spectrum disorders (de Vries et al., Reference de Vries, ten Have, van Dorsselaer, van Wezep, Hermans and de Graaf2016; Wu & Jackson, Reference Wu and Jackson2017), but findings are mixed for mood and substance use disorders (Banay Rachel et al., Reference Banay Rachel, James, Hart Jaime, Kubzansky Laura, Spiegelman, Okereke Olivia and & Laden2019; de Vries et al., Reference de Vries, ten Have, van Dorsselaer, van Wezep, Hermans and de Graaf2016).

As a flipside to reduced green space, the features of urban environments (e.g. noise, light, social encounters) may lead to cognitive overload for attention, memory or cognitive control (Bratman et al., Reference Bratman, Hamilton and Daily2012) and salience processing in general (Winton-Brown, Fusar-Poli, Ungless, & Howes, Reference Winton-Brown, Fusar-Poli, Ungless and Howes2014). Some evidence linked urbanicity to reduced cognitive development in children (Gouin et al., Reference Gouin, Flamant, Gascoin, Rouger, Florin, Guimard and Hanf2015). Schizophrenia risk has been associated with lower cognitive functioning and urbanicity, possibly indicating reduced coping ability with the eventfulness of cities (Weiser et al., Reference Weiser, Van Os, Reichenberg, Rabinowitz, Nahon, Kravitz and Noy2007). Noise pollution or noise-induced stress has also been associated with measures of annoyance, displaced aggression, reduced wellbeing and cognitive functioning (Dzhambov & Dimitrova, Reference Dzhambov and Dimitrova2014; Goines & Hagler, Reference Goines and Hagler2007; Ohrstrom, Reference Ohrstrom2004; Wright, Peters, Ettinger, Kuipers, & Kumari, Reference Wright, Peters, Ettinger, Kuipers and Kumari2014). In line with the biophilia hypothesis (Kellert & Wilson, Reference Kellert and Wilson1993), these findings may pinpoint impaired relaxation (e.g. SRT) or cognitive restoration (e.g. ART), which are associated with nature sights and sounds or reduced cognitive load (Berto, Reference Berto2014; Bratman et al., Reference Bratman, Hamilton and Daily2012). Often exposure to noise occurs simultaneous to exposure pollutants from road or air traffic, which might be an independent mechanism that transduces urban risk [e.g. exposure to ultrafine particles, heavy metals as lead and cadmium, or nitrogen oxide (Buoli et al., Reference Buoli, Grassi, Caldiroli, Carnevali, Mucci, Iodice and Bollati2018; Newbury et al., Reference Newbury, Arseneault, Beevers, Kitwiroon, Roberts, Pariante and Fisher2019), which affect particularly individuals with lower SES in urban areas (Cesaroni et al., Reference Cesaroni, Badaloni, Romano, Donato, Perucci and Forastiere2010; Laurent, Bard, Filleul, & Segala, Reference Laurent, Bard, Filleul and Segala2007)]. Though, a recent review suggested that pollution only accounts for a small part of the risk for mental health problems, at least in psychosis (Attademo, Bernardini, Garinella, & Compton, Reference Attademo, Bernardini, Garinella and Compton2017).

Other, possibly related, salutogenic effects of natural environments could include better immune function, lower blood pressure and/or health behaviours, such as physical and social activity, which directly affect the individuals and unborn offspring (Ebisu, Holford, & Bell, Reference Ebisu, Holford and Bell2016; Fong et al., Reference Fong, Hart and James2018; James, Banay, Hart, & Laden, Reference James, Banay, Hart and Laden2015; Kuo, Reference Kuo2015; Maas, van Dillen, Verheij, & Groenewegen, Reference Maas, van Dillen, Verheij and Groenewegen2009; Rook, Reference Rook2013; Rook, Lowry, & Raison, Reference Rook, Lowry and Raison2013; Twohig-Bennett & Jones, Reference Twohig-Bennett and Jones2018). Interestingly, visiting green spaces appears to affect the mental health of city dwellers positively (Alcock, White, Wheeler, Fleming, & Depledge, Reference Alcock, White, Wheeler, Fleming and Depledge2014; Bratman, Hamilton, Hahn, Daily, & Gross, Reference Bratman, Hamilton, Hahn, Daily and Gross2015). Lack of green space during upbringing and adult life could be due to various SES-related factors, but low use of green space could lie in reduced motivation and withdrawal from activity that accompanies mental health conditions (Van den Berg et al., Reference Van Den Berg, Van Poppel, Van Kamp, Andrusaityte, Balseviciene, Cirach and Smith2016), highlighting the potential importance of activating interventions.

In sum, studies on urban upbringing and current urban living show that a variety of individual and context-related factors of the urban environment increase the risk for mental ill-health. A common denominator of urbanicity risk appears to lie in the presence of stressors and the lack of opportunity for stress relief associated with specific social and physical characteristics of the urban environment (Gong, Palmer, Gallacher, Marsden, & Fone, Reference Gong, Palmer, Gallacher, Marsden and Fone2016). Importantly, the urban environmental risk factors may cluster and have additive, if not synergistic effects (Kuepper, van Os, Lieb, Wittchen, & Henquet, Reference Kuepper, van Os, Lieb, Wittchen and Henquet2011; Morgan et al., Reference Morgan, Reininghaus, Reichenberg, Frissa, Hotopf and Hatch2014), which require further systematic investigation of the interplay of social, environmental and person characteristics.

The urban environment and the brain

Neuroscientific studies may contribute to our understanding of the neurobiological processes mediating the effect of urban living on mental health (Meyer-Lindenberg & Tost, Reference Meyer-Lindenberg and Tost2012). For instance, neuroimaging can investigate hypothesized emotional and cognitive brain mechanisms or brain areas and connections that are susceptible to urban effects. The social stress hypothesis is supported by an initial functional neuroimaging (fMRI) study that probed the blood-oxygen-level-dependent (BOLD) brain to stress as a function of urban upbringing and current city living. In healthy individuals, current city living was associated with higher activation of the amygdala during social stress. Urban upbringing moreover was associated with an increased activity of the perigenual anterior cingulate cortex (pACC), which connects to frontal and limbic brain areas and has been implicated in emotional processing, contingency learning and cognitive control (Stevens, Hurley, & Taber, Reference Stevens, Hurley and Taber2011; Palomero-Gallagher et al., Reference Palomero-Gallagher, Hoffstaedter, Mohlberg, Eickhoff, Amunts and Zilles2018). In another study, current city living was associated with higher activation of the amygdala, medial orbital cortex and pACC during a task measuring reward activation and modulation (Krämer, Diekhof, & Gruber, Reference Krämer, Diekhof and Gruber2017). The results indicate a fronto-limbic hypersensitivity during stress and reward processing. The finding is in line with the hypotheses derived from experimental and epidemiological studies, which suggest that exposure to urban (social) stress leads to neural sensitization and sensitization of physiological stress systems as the hypothalamus–pituitary–adrenal axis (Selten & Cantor-Graae, Reference Selten and Cantor-Graae2005; Selten et al., Reference Selten, van der Ven, Rutten and Cantor-Graae2013; Steinheuser, Ackermann, Schonfeld, & Schwabe, Reference Steinheuser, Ackermann, Schonfeld and Schwabe2014). In addition, urban upbringing has been associated with less efficient prefrontal processing during a working memory task, suggesting the involvement of cognitive control processes (Reed et al., Reference Reed, D'Ambrosio, Marenco, Ursini, Zheutlin, Blasi and Callicott2018). Although the evidence is still limited, structural neuroimaging associated urban upbringing with reduced dorso-lateral prefrontal cortex (dlPFC) and (in men only) pACC volumes (Akdeniz et al., Reference Akdeniz, Schäfer, Streit, Haller, Wüst, Kirsch and Meyer-Lindenberg2017; Haddad et al., Reference Haddad, Schäfer, Streit, Lederbogen, Grimm, Wüst and Meyer-Lindenberg2014), reduced cortical thickness in frontal and temporal–parietal cortices and increased volume of the precuneus (Besteher, Gaser, Spalthoff, & Nenadić, Reference Besteher, Gaser, Spalthoff and Nenadić2017; Lammeyer, Dietsche, Dannlowski, Kircher, & Krug, Reference Lammeyer, Dietsche, Dannlowski, Kircher and Krug2019), as well as white matter changes in the left superior longitudinal fasciculus (Lammeyer et al., Reference Lammeyer, Dietsche, Dannlowski, Kircher and Krug2019). Few studies have linked urban living to brain phenotypes in disorder. In males with a psychotic disorder, urban upbringing was associated with reduced grey matter volume (Frissen, van Os, Peeters, Gronenschild, & Marcelis, Reference Frissen, van Os, Peeters, Gronenschild and Marcelis2018), but not with cortical thickness (Frissen et al., Reference Frissen, van Os, Lieverse, Habets, Gronenschild and Marcelis2017), or functional connectivity (Peeters et al., Reference Peeters, van de Ven, Gronenschild, Patel, Habets, Goebel and Marcelis2015). Importantly, it is largely unclear what accounts for these neural alterations. Initial studies, which are discussed in the following paragraph, have focussed on social stressors and green/blue space as potential explanatory factors of structural and functional brain changes.

An important new direction is the study of gene–urbanicity interactions for brain phenotypes. In a subgroup of the sample studied in Lederbogen et al. (Reference Lederbogen, Kirsch, Haddad, Streit, Tost, Schuch and Deuschle2011), a functional variant of the neuropeptide S receptor 1 interacted with urban upbringing on the amygdala stress response during the social stress task (Streit et al., Reference Streit, Haddad, Paul, Frank, Schäfer, Nikitopoulos and Witt2014). Urban upbringing also interacted with dopamine genes in altering prefrontal function during a working memory task, a finding that was replicated in two independent samples (Reed et al., Reference Reed, D'Ambrosio, Marenco, Ursini, Zheutlin, Blasi and Callicott2018). Future studies of gene–urbanicity interaction could capitalize on the identification of polygenic risk scores for the disorder as an index of molecular genetic risk, rather than focusing on single nucleotide polymorphisms.

In sum, initial evidence links urbanicity to changes in neural activation and structure. While tentatively supporting existing theories, the findings need to be interpreted with great caution given their correlational nature. It will be important for future research to draw upon insights from the animal literature and to directly investigate the impact of different urban attributes on the brain in experimental and experience-based studies. Initial studies that attempted to directly unravel the effects of the city will be discussed in the following paragraph.

Risk attributes of the urban environment and the brain

Many studies have investigated neural correlates of social phenomena that are relevant to the hypothesized mechanisms of urbanicity, although not explicitly framed in this context. For example, low childhood SES has been associated with a range of brain structural and functional changes (Farah, Reference Farah2017; McDermott et al., Reference McDermott, Seidlitz, Nadig, Liu, Clasen, Blumenthal and Raznahan2019), with the strongest effects in the most disadvantaged children (Noble et al., Reference Noble, Houston, Brito, Bartsch, Kan, Kuperman and Libiger2015). A quantitative meta-analysis of social environmental stressors based on 54 studies and 3044 participants concluded that the experience of social environmental stress was associated with an altered BOLD response across several brain regions. Increased BOLD of the right amygdala was a robust finding across multiple studies (Mothersill & Donohoe, Reference Mothersill and Donohoe2016). This effect was similar for the (n = 34) studies including adults and those including children/adolescents (n = 21). The meta-analysis included different types of social stressors, some of which are not relevant to urban social risk (e.g. childhood trauma), but are nevertheless informative on the key role for the amygdala in the neural effects of the social environment. Finally, recent evidence linked psychosocial stressors (childhood adversity, migration and urban living) in healthy volunteers to reduced volume of the amygdala (Weissman et al., Reference Weissman, Lambert, Rodman, Peverill, Sheridan and McLaughlin2019) and increased connectivity between striatal and cortical regions, involved in salience and reward processing (McCutcheon, Bloomfield, Dahoun, Mehta, & Howes, Reference McCutcheon, Bloomfield, Dahoun, Mehta and Howes2019). In addition, evidence from the animal literature shows that repeated social defeat is associated with sensitized neurons and microglia over several weeks (Weber et al., Reference Weber, McKim, Niraula, Witcher, Yin, Sobol and Godbout2019). Such changes at cell level might drive higher-level changes in brain structure, and could contribute to various mental health conditions that have been associated with stress sensitization and reward processing deficits (Gerin, Hanson, Viding, & McCrory, Reference Gerin, Hanson, Viding and McCrory2019; Weissman et al., Reference Weissman, Lambert, Rodman, Peverill, Sheridan and McLaughlin2019; Whitton, Treadway, & Pizzagalli, Reference Whitton, Treadway and Pizzagalli2015).

Future studies could apply more detailed methods to assess the experience of social stress in urban and rural neighbourhoods, as is already done in epidemiological studies [e.g. Binbay et al. (Reference Binbay, Drukker, Alptekin, Elbi, Tanık, Özkınay and van Os2012)], allowing for a comparison of groups that differ on well-defined social dimensions of residential environments. Social cognitive neuroscience has developed validated paradigms that could be used in conjunction with this approach to study how individuals with different urban social experiences respond to social stressors. For example, the experience of social exclusion, social inferiority or transgression of personal space can be experimentally induced and studies using these paradigms have reliably associated negative social experiences with activation in brain areas that are involved in the processing of negative emotions and cognitive control (Kennedy, Gläscher, Tyszka, & Adolphs, Reference Kennedy, Gläscher, Tyszka and Adolphs2009; Kishida, Yang, Quartz, Quartz, & Montague, Reference Kishida, Yang, Quartz, Quartz and Montague2012; Zink et al., Reference Zink, Tong, Chen, Bassett, Stein and Meyer-Lindenberg2008).

Recent studies have started to investigate the associations between urban physical factors and the brain. Kühn et al. (Reference Kühn, Düzel, Eibich, Krekel, Wüstemann, Kolbe and Lindenberger2017) used data on green and blue space from the European Urban Atlas in conjunction with structural brain imaging, and reported that older adults who lived close to forests had increased amygdala integrity, based on three different neuroimaging sequences of grey and white matter density (voxel-based morphometry, mean diffusivity from diffusion tensor imaging and magnetization-transfer ratio). The study also investigated associations with urban green, water and wasteland and additional regions of interest, the pACC and dlPFC, for which volume reductions have previously been linked to urban upbringing (Akdeniz et al., Reference Akdeniz, Schäfer, Streit, Haller, Wüst, Kirsch and Meyer-Lindenberg2017; Haddad et al., Reference Haddad, Schäfer, Streit, Lederbogen, Grimm, Wüst and Meyer-Lindenberg2014). However, none of the other associations was significant, suggesting that neuro-regenerative effects of nature are specific to (non-urban) green space and primarily working on brain regions that are implicated in emotional (threat) processing, rather than those related to cognitive control. It is possible that non-urban green space is a proxy for personal space (i.e. low social exposure), which is likely to act positively on the brains' threat system. Although, others showed that lifelong access to residential greenness (using satellite-based normalized difference vegetation index) was positively associated with grey and white matter volume in prefrontal and premotor areas and the cerebellum in school-aged children (Dadvand et al., Reference Dadvand, Pujol, Macià, Martínez-Vilavella, Blanco-Hinojo, Mortamais and Sunyer2018), possibly relating to neural plasticity and regeneration. Interestingly, the regions that were associated with greenness were also positively associated with working memory and inversely associated with inattentiveness, suggesting that impaired structural integrity could underlie the previously discussed cognitive effects of urban environments [e.g. (Reed et al., Reference Reed, D'Ambrosio, Marenco, Ursini, Zheutlin, Blasi and Callicott2018)]. Others found that in children between 8 and 12 air pollution exposure was mainly associated with reduced functional connectivity in the default mode network activity and stimulus-driven mental operations (Pujol et al., Reference Pujol, Martínez-Vilavella, Macià, Fenoll, Alvarez-Pedrerol, Rivas and Querol2016) rather than structure or membrane metabolites, which might point toward stress-related mechanisms. Neurotoxicity-related structural changes, as supported by animal studies (Fonken et al., Reference Fonken, Xu, Weil, Chen, Sun, Rajagopalan and Nelson2011; Levesque, Surace, McDonald, & Block, Reference Levesque, Surace, McDonald and Block2011), could appear further down the line of development.

The initial neuroimaging studies suggest stress sensitization through environmental stressors, neurotoxicity and neuro re- and degeneration as possible neurobiological pathways that mediate urbanicity effects on cognitive functioning and mental health. Future studies need to systematically investigate multiple mechanisms that could underlie the urban effect on the brain (e.g. exposure to toxic or noise pollution, social stressors). Longitudinal research will be necessary to unravel developmental effects. To improve our insight into which specific urban features are involved, experimental and experience-based studies that investigate immediate responses to specific physical and social characteristics of urban environments will be indispensable.

Experimental and experience-based studies: testing causality of urban factors

Experimental studies use randomized designs to investigate the psychological or psychophysiological effects of nature/urban-related experimental stimuli or short-term experience of nature or urban environments. Drawing on ART and SRT, these studies have included cognitive outcomes as well as outcomes related to mental health and psychophysiology. Studies differ in type (e.g. images v. actual presence) and duration (e.g. minutes to hours) of the nature v. urban experience, but overall systematic reviews confirmed the positive effects of nature on mental wellbeing and cognition, although some studies yielded inconclusive findings (Bowler, Buyung-Ali, Knight, & Pullin, Reference Bowler, Buyung-Ali, Knight and Pullin2010; Bratman et al., Reference Bratman, Hamilton and Daily2012; Ohly et al., Reference Ohly, White, Wheeler, Bethel, Ukoumunne, Nikolaou and Garside2016). The effects on psychophysiological indicators, such as blood pressure and heart rate variability, are less well-studied (Bowler et al., Reference Bowler, Buyung-Ali, Knight and Pullin2010). However, there are individual studies that suggest that a brief experience of nature changes physiological health markers (Li et al., Reference Li, Otsuka, Kobayashi, Wakayama, Inagaki, Katsumata and Kagawa2011; Park, Tsunetsugu, Kasetani, Kagawa, & Miyazaki, Reference Park, Tsunetsugu, Kasetani, Kagawa and Miyazaki2010), an effect that may extend to simulated environments (i.e. viewing nature v. urban scenes) (Brown, Barton, & Gladwell, Reference Brown, Barton and Gladwell2013). Moreover, positive effects on mood and cognition of a short walk in nature v. urban environments have been observed in individuals with major depression (Berman et al., Reference Berman, Kross, Krpan, Askren, Burson, Deldin and Jonides2012). The experimental approach has also been applied to study the effects of specific urban environments. For example, passing through a deprived urban environment increased anxiety and reduced trust in patients with persecutory delusions (Ellett, Freeman, & Garety, Reference Ellett, Freeman and Garety2008), as well as in healthy individuals (Nettle, Pepper, Jobling, & Schroeder, Reference Nettle, Pepper, Jobling and Schroeder2014). In patients with persecutory delusions, going out in a busy shopping area had similar effects (Freeman et al., Reference Freeman, Emsley, Dunn, Fowler, Bebbington, Kuipers and Garety2014).

Recently, studies also started to investigate the direct neural effects of short-term urban v. nature experience. Healthy participants underwent resting-state fMRI before and after a 90 min walk in a nature as opposed to an urban environment (Bratman et al., Reference Bratman, Hamilton, Hahn, Daily and Gross2015). The walk in nature reduced self-reported rumination and activation in the sub-genual prefrontal cortex (sgPFC), whereas the urban walk did not. The sgPFC is linked to self-focused behavioural withdrawal and rumination, supporting a restorative effect of nature, possibly by distracting participants from negative feelings. It is important to note that the exposure conditions should be carefully matched for characteristics that are not directly related to nature or urban environments, such as pleasantness or level of threat. That is, unpleasant nature scenes may well have marked negative effects (Pretty, Peacock, Sellens, & Griffin, Reference Pretty, Peacock, Sellens and Griffin2005), just as beautiful urban scenes may have positive effects (Seresinhe, Preis, MacKerron, & Moat, Reference Seresinhe, Preis, MacKerron and Moat2019). A solution to this problem would be simulated exposures that are matched for such characteristics. Virtual reality has successfully been applied to study the effect of social environmental stress in psychosis, and may combine experimental control with good ecological validity (Veling et al., Reference Veling, Susser, Van Os, Mackenbach, Selten and Hoek2008), for a systematic review see Valmaggia, Day, and Rus-Calafell (Reference Valmaggia, Day and Rus-Calafell2016).

In sum, there is convincing evidence for positive effects of the short-term experience of nature on wellbeing and cognition, which may occur through largely unconscious processes affecting attention and autonomous nervous system activity. The experimental approach may be instrumental in identifying specific associations between elements of natural or urban environments and outcomes that are relevant to mental health. At the same time, not all relevant elements of natural or urban environments lend itself to experimental designs with human participants (e.g. pollution) and it is unclear how short-term effects relate to the effects of years of exposure. Within these constraints, future research could set out to systematically delineate the optimal duration and type of the nature experience (e.g. Barton & Pretty, Reference Barton and Pretty2010; Bratman et al., Reference Bratman, Hamilton and Daily2012). Furthermore, it would be relevant for experimental studies to systematically incorporate individual differences in sensitivity to the natural or urban environment, for example, related to baseline mental health, as epidemiological research points to person–environment interactions. It has also been postulated that explicit cognitions (e.g. connectedness to nature) may mediate the beneficial effects of nature experiences on attention and mental health (Mayer, Frantz, Bruehlman-Senecal, & Dolliver, Reference Mayer, Frantz, Bruehlman-Senecal and Dolliver2008). Zooming in on the conscious experience of the natural or urban environment to understand the impact of the environment on the psyche is at the focus of the experience-based approach that is discussed next.

Experience-based studies start from the notion that the lived experience and sense-making of subjects are crucial for analysing the effects of urban or natural milieus on mental health. Therefore, the role of natural and urban environments in mental health needs to be understood as the result of people's actions and experience in context (Cresswell, Reference Cresswell2014). Such in situ experiences can be captured by studies in which individuals provide quantitative and/or qualitative information, while they walk through specific neighbourhoods or natural landscapes. This may provide a much more fine-grained insight into how the effects of urban stressors identified in epidemiological studies depend on specific contexts (Söderström et al., Reference Söderström, Empson, Codeluppi, Söderström, Baumann and Conus2016).

Corcoran et al. (Reference Corcoran, Mansfield, de Bezenac, Anderson, Overbury and Marshall2018) collected data on walkers' in situ judgements of threat and trust in two urban neighbourhoods, which differed in terms of deprivation. Perceptions of trust and threat were influenced by the perception of neighbourhood affluence, but also by the mental health and wellbeing of the walkers. Experience-based studies may further elucidate specific features of the urban environment that elicit stress in individuals with psychopathology. For example, a recent study used video recordings of patients' urban walks in conjunction with video-elicitation in patients with first-episode psychosis and showed that situations of stress are related to demographic density, sensory environments (in places like shopping malls), obstacles to fluid pedestrian mobility and unchosen social interactions, whereas creating sensory ‘bubbles’, programming mobility and creating places of comfort were tactics used to handle urban stress (Söderström et al., Reference Söderström, Empson, Codeluppi, Söderström, Baumann and Conus2016; Söderström, Söderström, Codeluppi, Empson, & Conus, Reference Söderström, Söderström, Codeluppi, Empson and Conus2017). Interestingly, density, a known stressor, served as a protective context for one individual with psychosis, because being in an anonymous crowd (in contrast to being with close friends and relatives) triggered feelings of belonging without being too exposed. Other studies based on ethnographic data have shown that ‘niches’ (Bister, Klausner, & Niewöhner, Reference Bister, Klausner, Niewöhner, Blok and Farías2016) or ‘atmospheres’ (Duff, Reference Duff2016; Söderström et al., Reference Söderström, Empson, Codeluppi, Söderström, Baumann and Conus2016; Söderström et al., Reference Söderström, Söderström, Codeluppi, Empson and Conus2017) of recovery are important for urban mental health, despite urban changes which very often reduce such possibilities for economically precarious people. These findings converge with those of recent studies in psychology on the role of space in mental health (McGrath & Reavey, Reference McGrath and Reavey2018).

Recently, the study of the experience of the city has been complemented by the use of Ecological Momentary Assessment (EMA; Shiffman, Stone, & Hufford, Reference Shiffman, Stone and Hufford2008), which involves the repeated sampling of current experiences in real-time and real-world contexts. EMA has high ecological validity because assessments are made in the natural flow of real life and in different situations, which makes it possible to understand the variability in mental states in relation to the environment, without explicit reflection on these relationships from the side of the individual. Applying EMA, Bakolis et al. (Reference Bakolis, Hammoud, Smythe, Gibbons, Davidson, Tognin and Mechelli2018) observed that specific natural features of the built environment (i.e. seeing trees, hearing birds sing) were associated with higher levels of mental wellbeing, which lasted for several hours and which was more prominent in individuals with higher trait impulsivity (Bakolis et al., Reference Bakolis, Hammoud, Smythe, Gibbons, Davidson, Tognin and Mechelli2018). EMA can be enriched with geographically explicit information (i.e. GEMA; Kirchner & Shiffman, Reference Kirchner and Shiffman2016) through combination with global positioning systems and geographic information systems. By linking subjective experiences with objective measures of mobility and place, GEMA can reveal continuous and dynamic interactions between people and place. For example, focusing on the relation between location and stress in adolescents, a recent GEMA study suggested that being around urban green space was associated with lower stress (Mennis, Mason, & Ambrus, Reference Mennis, Mason and Ambrus2018). This may point to the restorative effects of nature experience, or to the tendency to seek out urban green spaces at times of lower stress, or explicitly for purposes of stress reduction. Recent advances in portable neuroimaging greatly enrich the possibilities to experimentally investigate the neural correlates of nature and urban encounters, e.g. with mobile EEG devices while participants walk in urban v. nature environments and talk about their experiences, an approach which has recently successfully been piloted in older individuals (Tilley, Neale, Patuano, & Cinderby, Reference Tilley, Neale, Patuano and Cinderby2017).

In sum, experience-based studies provide a richness of quantitative and qualitative detail about the interactions between contextual characteristics and individual reflexive experiences. Combined with measures that tap autonomous nervous system activity, or even brain electrical activity, this may contribute to a much more fine-grained understanding of the relation between presence in urban or nature environments and mental health. These studies are suited to analyse how combined features of the milieu – constituting ‘the urban’ – affect people's experience and less suited to investigate the effects of isolated risk factors.