interested community, to develop a land manage-ment and planning tool for the protection andfuture sustainable use of the region’s wetlands.Another example is that of one of the remaining

perennial springs (La Carbonera) surrounding thecity of Querétaro, located in a semi-arid region ofcentral Mexico. Because the area’s wetlands havebeen studied for several years, it was possible torescue the spring, together with strong local stake-holder support and action. This is clearly a suc-cessful case in which locals worked closely withacademics to achieve a common goal. This peri-urban spring has been managed, and the area iscurrently used to conduct recreational and educa-tional activities while also contributing to the re-storation of several critical ecological processes.

Toward livable cities

The discipline of urban ecology has made greatstrides over the past three decades. To furtherour knowledge of urbanization’s effects on people,biodiversity, and ecosystem processes, urban ecol-ogists must shift from studying patterns to un-tangling the emerging mechanistic processesbehind the reported patterns (15). However, itis also crucial to create the tools and proceduresfor transforming scientific knowledge into action.The need for these advances is pressing, as thereis a growing discontent among urban dwellersworldwide, related to the erosion of their qualityof life. Many urban dwellers are now calling forthe creation of green, sustainable cities that arealso healthy and resilient (16). Incorporating urbanecology principles into the design, construction,and management of cities will require the co-operation, alliance, and synergy of all stake-holders, thus reforming the way we conceiveand prepare land to fulfill the needs of modernhuman agglomerations.

REFERENCES

1. United Nations Department of Economic and Social Affairs,Population Division, World Urbanization Prospects: The 2014Revision, Highlights (United Nations, 2014).

2. I. Douglas, P. James, Urban Ecology: An Introduction(Routledge, 2015).

3. M. J. McDonnell, in Urban Ecology: Patterns, Processes, andApplications, J. Niemelä et al., Eds. (Oxford Univ. Press, 2011),pp. 5–13.

4. M. von der Lippe, I. Säumel, I. Kowarik, Die Erde 136, 123–143(2005).

5. S. T. Pickett et al., J. Environ. Manage. 92, 331–362 (2011).6. D. Hope et al., Proc. Natl. Acad. Sci. U.S.A. 100, 8788–8792 (2003).7. A. J. Felson, M. A. Bradford, T. M. Terway, Front. Ecol. Environ

11, 362–367 (2013).8. D. Sol, C. González-Lagos, D. Moreira, J. Maspons, O. Lapiedra,

Ecol. Lett. 17, 942–950 (2014).9. M. J. McDonnell, A. K. Hahs, Annu. Rev. Ecol. Evol. Syst. 46,

261–280 (2015).10. H. Slabbekoorn, M. Peet, Nature 424, 267 (2003).11. P. O. Cheptou, O. Carrue, S. Rouifed, A. Cantarel, Proc. Natl.

Acad. Sci. U.S.A. 105, 3796–3799 (2008).12. M. F. J. Aronson et al., Proc. Biol. Sci. 281, 20133330 (2014).13. A. K. Hahs et al., Ecol. Lett. 12, 1165–1173 (2009).14. I. MacGregor-Fors, R. Ortega-Álvarez, Eds., Ecología urbana:

Experiencias en América Latina (2013); http://www1.inecol.edu.mx/libro_ecologia_urbana.

15. M. J. McDonnell, A. K. Hahs, Urban Ecosyst. 16, 397–409 (2013).16. United Nations Human Settlements Programme (UN-Habitat),

State of the World’s Cities 2008/2009: Harmonious Cities(Earthscan, 2008).

10.1126/science.aaf3630

PERSPECTIVE

Living in cities, naturallyTerry Hartig1 and Peter H. Kahn Jr.2

Natural features, settings, and processes in urban areas can help to reduce stressassociated with urban life. In this and other ways, public health benefits from, street trees,green roofs, community gardens, parks and open spaces, and extensive connectivepathways for walking and biking. Such urban design provisions can also yield ecologicalbenefits, not only directly but also through the role they play in shaping attitudes toward theenvironment and environmental protection. Knowledge of the psychological benefits ofnature experience supports efforts to better integrate nature into the architecture,infrastructure, and public spaces of urban areas.

Crowding, noise, andotherstressful urban condi-tions increase the risk ofmental disorders such asanxiety and depression

(1). However, urban areas alsohave environmental assets thatsupportmental health. For ex-ample, parks, green spaces, streettrees, and community gardenscan facilitate physical activity,social contacts, and stress re-duction (1, 2).How canpsychol-ogical benefits from encounterswith natural features and pro-cesses offset the psychologicalcosts of other urban living con-ditions? Answers to this ques-tionwill help improve the qualityof life of today’s growing ur-ban populations (2, 3) (Fig. 1).In his classic work on urban

psychology, Stanley Milgramopened on a positive note, ar-guing that “cities have greatappeal because of their vari-ety, eventfulness, possibility ofchoice, and the stimulation ofan intense atmosphere thatmany individuals find a desir-able background to their lives”(4). He also saw that citiesoffer “unparalleled possibil-ities” for face-to-face contactand communication (4). Yet,when considering how psy-chology can contribute to un-derstanding the experienceof living in a city, he turnedto the negative; he highlightedoverload as a psychological concept useful forlinking objective urban circumstances such ashigh population density to observable behaviorssuch as incivility. Such contrasting perspectives

continue to inform psychological research onurban life.

Urban-rural differences in mental disorders

Numerous studies have suggested that urban liv-ing conditions underminemental health,whereasconditions in rural areas support health. Peenet al. (5) performed a meta-analysis of 21 studies,done after 1985, that investigated the prevalenceof mood disorders among non-institutionalized

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1Institute for Housing and Urban Research and Departmentof Psychology, Uppsala University, Uppsala, Sweden.2Department of Psychology and School of Environmentaland Forest Sciences, University of Washington,Seattle WA, USA.

Fig. 1. Together with ecological benefits such as climate changemitigation and the protection of biological diversity, the renaturing ofcities opens opportunities for people to engage with features andprocesses of the natural world; for example, when tending plants in acommunity garden.

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adults in relatively affluent countries. They foundthat the odds of mood disorder were 28% higherin the urban areas than in the rural areas; how-ever, the results were highly heterogeneous, per-haps because “rural” and “urban” were defineddifferently in different studies. They recognized theproblem with integrating results when urbansettings in some studies resemble rural settingsin others.Judd et al. (6) have argued that binary urban

and rural categories are insufficient. Numerousenvironmental factors can affect the stress thatpeople experience in urban and rural settingsand, thus, the prevalence of disorders such asanxiety and depression. These include, for exam-ple, residential density, housing quality, air qual-ity, transportation options, access to health andwelfare services, and access to parks and greenspaces. All of these factors might play a role in

whether a given individual develops a disorder,but they may do so in different ways in differentcombinations. Together with individual vulner-abilities and broader contextual characteristics,research must consider the independent and com-bined effects of these factors in order to elucidatehow specific urban conditions may undermine orsupport mental health.

Findings of urban-rural differences in mentaldisorders might be taken as warnings about theconsequences of urbanization, but there is a riskof confusing cause and effect. The social, political,and economic forces that drive urbanization in aparticular society may also generate mental illnessthrough other mechanisms. For example, in somecountries, many people have left rural areas forurban ones under threat of violence; this appearsto have boosted the prevalence ofmajor depressivedisorder (7). Poor conditions in the spontaneoussettlements they create may exacerbate the effectsof victimization, disruption of social relations, andloss of traditional occupations, but urbanizationitself is not the initial cause of disorder.

Psychological benefits of nature experience

Research on the experience of nature also sug-gests that urban living conditions can under-

mine mental health, whereasrelatively natural conditionscan support it. The term “na-ture” means different thingsin different contexts. Recog-nizing personal and culturalaspects of the experience ofnature, psychological researchconsiders how different peo-ple encounter nature in diversecontexts, from viewing indoorplants in urban offices to walk-ing in wilderness areas.A significant portion of this

research concerns the restor-ative effects ofnature experience,such as regaining the abilityto concentrate and reducingblood pressure after intensivemental work (3, 8). Theoriesabout restoration describe howencounters with nature involvepsychological distance fromeveryday demands and inter-ested engagementwith environ-mental features and processes.These components of restor-ative experience have counter-parts in many analyses of whypeople engage in outdoor rec-reation, done in the UnitedStates and elsewhere since theearly 1960s (8). Complement-ingwhat people have long saidthey seek through outdoor rec-reation and substantiatingtheoretical claims, laboratoryand field experiments have re-peatedly shown that spendingtime in natural environments

or viewing scenes of nature canquickly help peopleto lift their mood, improve their ability to directattention, and reduce physiological arousal to agreater degree than do urban streets and othercomparison conditions (2, 8).As in studies of urban-rural differences inmen-

tal disorders, comparisons of single examples ofurban and natural environments have been crit-

icized for neglecting relevant forms of variation(8). Experimental evidence regarding plausiblemechanisms has nonetheless encouraged largeobservational studies on urban green space andhealth. Such studies consider plausible cumu-lative effects of green space and greenery nearan urban home. Most such studies focus on quan-tity, but some have also used quality indicators(9). Most of the observational studies have used across-sectional design, but recent longitudinalstudies have enabled stronger inferences regard-ing beneficial effects of access or proximity to na-ture. For example, using 5 consecutive years ofdata for each of 1064 participants in the BritishHousehold Panel Survey, Alcock et al. (10) showedthat those who relocated from a less green (58%local area coverage) to a more green (74%) urbanarea showed improved mental health over thenext 3 years. In contrast, those who moved frommore (74%) to less (59%) green urban areas showeda decline in mental health before the move, fol-lowed by a return to the pre-move baseline.Much research on nature experience assumes

that too few generations have passed for naturalselection to shift the adaptedness of affective andcognitive functioning from the conditions of hom-inid evolution (8), and that humans are thereforepoorly adapted to living in urban environments,broadly defined. This natural-urban antithesisneglects the fact that urban environments includesettings that are supportive of human function-ing; noisy, polluted, car-filled streets linedby anon-ymous towers are not representative of all urbanpossibilities. This false antithesis also neglects rea-sons why people gathered in cities millenniaago, and the consequences of that move for theinterplay of natural selection and socioculturaldevelopment. Here too, the environmental catego-ries and the urbanization process need closerexamination.

The design of cities andhuman-nature relations

Psychological research has substantiated long-standing reasoning about parks and green spacesas health resources for urbanpopulations (8, 11, 12).This reasoning has guided the creation of parksin many cities, such as Central Park in New Yorkand Mount Royal Park in Montreal. Extendingsuch precedents, researchers, design professio-nals, citizen groups, and others are workingtogether to create sustainable urban fabrics inour increasingly urbanized world. These efforts—under banners such as green urbanism, greeninfrastructure, biophilic design, and renaturing—seek a better synthesis of natural processes andecosystem functions with architecture and urbaninfrastructure through acts of creation, preserva-tion, and ecological restoration. Such efforts areneeded for psychological as well as ecologicalpurposes. The evidence mentioned above andmore like it warn against assuming that peoplecan simply adapt to increasing urban density andits concomitants without negative consequencesfor health and well-being.By providing opportunities for people to ex-

perience nature in cities and to experience cities

SCIENCE sciencemag.org 20 MAY 2016 • VOL 352 ISSUE 6288 939

Fig. 2. People living in cities can encounter nature in the context ofmany different activities. Their experiences can enhance their devel-opment, health, and well-being, and they can shape their attitudestoward the environment and environmental protection.

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as natural, such efforts can shape attitudes towardthe environment (Fig. 2). People in increasinglylarge and dense urban areas may have few or nocontacts with the natural world in everyday life.Environmental generational amnesia refers tothe psychological process whereby each genera-tion constructs a conception of what is environ-mentally normal based on the natural worldencountered in childhood (13). A problem arisesinsofar as the amount of environmental degra-dation increases across generations, but eachgeneration tends to take that degraded conditionas the nondegraded condition: the normal ex-perience. This helps to explain inaction on en-vironmental problems; people do not feel theurgency or magnitude of problems because theexperiential baseline has shifted. Providing op-portunities for people to experiencemore robust,healthy, and evenwilder forms of nature in citiesoffers an important solution to this collective lossof memory and can counter the shifting baseline(14). Such opportunities include, for example,large green spaces and parks, rivers restored tosome former free-flowing condition, expansiveviews over water and land, and extensive con-nective pathways for walking and biking.Thus, cities designedwell, with nature inmind

and at hand, can be understood as natural, sup-portive of both ecosystem integrity and publichealth. Further psychological studies can describehow specific improvements in available oppor-tunities for nature experience come to affectmental health and environmental attitudes (15).How will they change if car-clogged spaces giveway to natural places where children can playwildly and others reflect quietly?

REFERENCES

1. American Psychological Association Task Force on UrbanPsychology, Toward an Urban Psychology: Research, Action, andPolicy (American Psychological Association, Washington, DC, 2005).

2. T. Hartig, R. Mitchell, S. de Vries, H. Frumkin, Annu. Rev. PublicHealth 35, 207–228 (2014).

3. European Commission Directorate-General for Research andInnovation, Towards an EU Research and Innovation PolicyAgenda for Nature-Based Solutions and Re-Naturing Cities: FinalReport of the Horizon 2020 Expert Group on 'Nature-BasedSolutions and Re-Naturing Cities' (European Commission,Brussels, 2015).

4. S. Milgram, Science 167, 1461–1468 (1970).5. J. Peen, R. A. Schoevers, A. T. Beekman, J. Dekker, Acta

Psychiatr. Scand. 121, 84–93 (2010).6. F. K. Judd et al., Aust. N.Z. J. Psychiatry 36, 104–113

(2002).7. A. J. Ferrari et al., Psychol. Med. 43, 471–481 (2013).8. T. Hartig et al., in Forests, Trees and Human Health, K. Nilsson

et al., Eds. (Springer, Dordrecht, 2011), pp. 127–168.9. S. M. E. van Dillen, S. de Vries, P. P. Groenewegen,

P. Spreeuwenberg, J. Epidemiol. Community Health 66, e8(2012).

10. I. Alcock, M. P. White, B. W. Wheeler, L. E. Fleming,M. H. Depledge, Environ. Sci. Technol. 48, 1247–1255 (2014).

11. T. S. Eisenman, J. Plann. Hist. 12, 287–311 (2013).12. D. C. Rouse, I. F. Bunster-Ossa, Green Infrastructure: A

Landscape Approach (Planning Advisory Service Report 571,American Planning Association, Chicago, 2013).

13. P. H. Kahn Jr., Technological Nature: Adaptation and the Futureof Human Life (MIT Press, Cambridge, MA, 2011).

14. P. H. Kahn Jr., P. H. Hasbach, Eds., The Rediscovery of the Wild(MIT Press, Cambridge, MA, 2013).

15. F. G. Kaiser, T. Hartig, A. Brugger, C. Duvier, Environ. Behav. 45,369–398 (2013).

10.1126/science.aaf3759

PERSPECTIVE

Meta-principles for developing smart,sustainable, and healthy citiesAnu Ramaswami,1* Armistead G. Russell,2 Patricia J. Culligan,3

Karnamadakala Rahul Sharma,1 Emani Kumar4

Policy directives in several nations are focusing on the development of smart cities, linkinginnovations in the data sciences with the goal of advancing human well-being andsustainability on a highly urbanized planet. To achieve this goal, smart initiatives mustmove beyond city-level data to a higher-order understanding of cities as transboundary,multisectoral, multiscalar, social-ecological-infrastructural systems with diverse actors,priorities, and solutions. We identify five key dimensions of cities and present eightprinciples to focus attention on the systems-level decisions that society faces to transitiontoward a smart, sustainable, and healthy urban future.

By the year 2050, the number of peopleliving in cities is expected to increase byabout 2.5 billion (1). It is estimated thatover 60% of the urban areas that will existby 2050 have yet to be built, indicating

that there will be massive new infrastructurerequirements, particularly in Asia and Africa(2). Simultaneously, existing cities worldwideare aging and much in need of infrastructurereplacement.Infrastructures—defined broadly as the systems

that provide water, energy, food, shelter, trans-portation and communication, waste management,and public spaces (3)—are essential to supporthuman well-being and economic development.However, aggregated globally, these seven infra-structure sectors currently place a large burdenon the environment and have a considerable im-pact on human health (Fig. 1). Urban demandsdominate these effects; for example, ~70% ofglobal greenhouse gas (GHG) emissions are at-tributable to cities (1). Because physical infra-structures have life spans of 30 to 50 years, thelarge imminent global requirement for new urbaninfrastructure presents a historic opportunity forchange. The question is, how can urban infra-structure transformations in the 21st century ad-vance the environmental sustainability and humanwell-being of our cities by taking advantage ofthe enormous potential offered by data scienceand technology?Although information and communication

technologies are important for developing smart,sustainable, healthy cities (4), we argue that alarger understanding of urban infrastructuresystems is necessary to move from data to infor-mation to knowledge and, ultimately, to actionfor urban sustainability and human well-being.

With infrastructure as the focus, we identifyfive key dimensions of cities and present eightprinciples to help guide urban transforma-tions toward sustainability and health, draw-ing on examples from the United States, China,and India.

Key dimensions

Economic opportunity is a key driver for urban-ization, and infrastructure is a prime enabler.Multi-city data sets are emerging that describescaling relationships among urban populationgrowth, gross domestic product (GDP), house-hold incomes, and infrastructure-related param-eters such as financial investments, energy andwater use, and land and road expansions (5).Cities with different economic structures (e.g.,highly industrial, highly commercial, or mixedeconomy) are known to exhibit different socio-spatial patterns of development (i.e., urban form)that affect infrastructure design. Yet basic city-leveldata on urban GDP, sectoral employment, andhousehold incomes are sparse in many developingnations and in smaller cities and towns, wheremuch urban growth is projected to occur.Urban form or morphology describes the evolv-

ing interaction between physical space and hu-man activity in cities. Numerous data sets, fromcensus data to aerial and satellite photographsand remote sensing information, are being in-tegrated to enable planners to characterize urbanform. Urban complexity science is advancingnew measures (4) that focus not only on pop-ulation density, connectivity, proximity to jobsand services, and diversity and intensity of urbanactivities but also on understanding self-similarityacross scales (from blocks to neighborhoods tocities) and patterns of social segregation (e.g.,of migrant and informal populations in a city).Urban form represents the foundation uponwhich infrastructure develops, shaping energyand material use; access to and contiguity ofwater bodies, green space, and other criticalecosystems; and urban equity and well-being.Infrastructure design and socio-spatial dis-

parities within cities are emerging as criticaldeterminants of human health and well-being.

940 20 MAY 2016 • VOL 352 ISSUE 6288 sciencemag.org SCIENCE

1Hubert H. Humphrey School of Public Affairs, University ofMinnesota, Minneapolis, MN 55455, USA. 2School of Civil andEnvironmental Engineering, Georgia Institute of Technology,Atlanta, GA 30332-0355, USA. 3Department of CivilEngineering and Engineering Mechanics, Columbia University,New York, NY 10027, USA. 4ICLEI–Local Governments forSustainability, South Asia Secretariat, New Delhi 110020,India.*Corresponding author. Email: anu@umn.edu

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(6288), 938-940. [doi: 10.1126/science.aaf3759]352Science Terry Hartig and Peter H. Kahn Jr. (May 19, 2016) Living in cities, naturally

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