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Mental Health & Function

By: Kathleen L. Wolf, Ph.D., and Katrina Flora
December 26, 2010
Cite: Wolf, K.L., & K. Flora. 2010. Mental Health and Function - A Literature Review. In: Green Cities: Good Health ( College of the Environment, University of Washington.

Encounters with nearby nature help alleviate mental fatigue by relaxing and restoring the mind. Within built environments parks and green spaces are settings for cognitive respite, as they encourage social interaction and de-stressing through exercise or conversation, and provide calming settings. Having quality landscaping and vegetation in and around the places where people work and study is a good investment. Both visual access and being within green space helps to restore the mind’s ability to focus. This can improve job and school performance, and help alleviate mental stress and illness.

Fast Facts

The Brain and the Environment

The brain, complex and vulnerable, is the only organ that undergoes substantial maturation after birth. This process is shaped in part by response to stimuli in our surroundings (including both negative and positive conditions), and continues throughout our lives.1 Substantial research shows that natural scenes evoke positive emotions, facilitate cognitive functioning, and promote recovery from mental fatigue for people who are in good mental health. The experience of nature can also provide respite for those who experience short-term and chronic mental illness.2

Mental Fatigue Recovery

Nature: An Urban Respite

The constant stimuli of city life can be mentally exhausting, and life in the city can actually dull our thinking.3 In navigating the outdoor environment, one must continually monitor traffic and pedestrian flow while constantly focusing on where one is going and the means to get there. Constant response to even such low-level stimuli cannot be maintained indefinitely. A few minutes in a crowded city setting can cause the brain to suffer memory loss and reduced self-control. Even brief glimpses of natural elements improve brain performance by providing a cognitive break from the complex demands of urban life.4

Attention Fatigue and Recovery

Our immediate environment can prompt both negative and positive subconscious effects. A glance at an object that even remotely resembles a snake, for instance, may initiate an instantaneous fear response. Similarly, the presence of plants subconsciously and beneficially impact how the brain responds even when we do not focus attention on such surroundings.

In today’s lifestyles and work, we must focus our attention on critical information or tasks. Maintaining that focus by screening out distractions overloads our capacity for conscious attention.5 Yet, exposure to settings that are visually interesting (having “high fascination”) have been found to aid directed attention recovery.6 Comparing memory retention in people viewing low versus high fascination scenes in built and natural environments, respectively, people viewing natural environments performed significantly better (see Figure 1).6 So, in the case of offices and schools, where one must focus on tasks, the addition of natural features could significantly improve attention and content retention rates.

Figure 1: memory scores, comparing fascination and nature/built conditions

Cognitive Function


Office workers may spend entire days indoors, and many decorate their workspaces with plants or pictures of natural settings to compensate for lack of a window view. In one study, people in windowless workspaces introduced twice as many nature elements to their work area as those who had window views of natural areas.7

Office workers report that plants make for more attractive, pleasant, and healthy work environments,1,8 but what impact do plants and nature views have on work performance? Studies show improved employee morale, decreased absenteeism, and increased worker efficiency result from such workplace enhancements.9 Having plants within view of workstations decreases both illness incidence,10 and the amount of self-reported sick leave.11 One study found that workers with workstation views that included green elements were more satisfied at work and had more patience, less frustration, increased enthusiasm for work, and fewer health problems.11 Not having nature views or indoor plants are associated with higher levels of tension and anxiety in office workers.12

College Settings

Learning, like tasks at work, requires focused, direct attention and high-level cognitive functioning. When plants were added to a college computer lab, the study participants were more productive (with 12% quicker reaction times on tested computer tasks) and showed less stress—though there was no difference in number of errors made on the test. Additionally, participants reported feeling more attentive and better able to concentrate in the presence of plants.9 In other studies, participants performed better on creative tasks in rooms having foliage plants, versus those without, and the authors conclude that nature may provide inspiration and a source of stimulation for creativity.13 College students with more natural views from their dorm windows scored higher on tests of capacity to direct attention (CDA) and rated themselves as able to function more effectively.14 In another study of college students, those who participated in a nature walk performed higher on a subsequent CDA test than those who went on an urban walk or relaxed in a comfortable room with magazines and light music prior to the test.15


In recent times, children have less opportunity to be outdoors, in terms of both time and space.16 Some schools provide nature experiences as part of a class, recess, or special activity, as they recognize the potentially significant affects on learning and mental health.

Educational theory suggests that contact with nature facilitates children’s development of cognitive, emotional, and spiritual connections to social and biophysical environments around them.17 Ecological theory also suggests that contact with nature is important for children’s mental, emotional, and social health because imagination and creativity, cognitive and intellectual development, and social relationships are encouraged in outdoor activity, all of which improve the child’s mental health and function.18

Nature can provide both background and objects for play and learning.19 Among older children, exposure to nature encourages exploration and building activities, which can improve problem-solving abilities, ability to respond to changing contexts, as well as participation in group decision-making. Younger children often use outdoor settings having plants, stones, and sticks as props for imaginative play, which is key to social and cognitive development.20 One study of children’s play found that a cluster of shrubs was the most popular place to play on an elementary schoolyard because it could be transformed into many imaginary places: a house, spaceship, etc.21

Mental Illness and Nature Response

Attention Deficit Disorder (ADD)

Over 2 million children in the U.S. have been diagnosed with Attention Deficit Disorder (ADD), a condition that has detrimental effects on social, cognitive, and psychological growth.22 Studies show that childhood ADD symptoms can be reduced through activities in green settings and that “green time” may be an important supplement to established drug-based and behavioral treatments.23 In one study, the greenness of a child’s home did not significantly affect ADD symptom severity, but greenness of play setting was related to a reduction of symptom severity (see Table 1).22 Children who played in windowless indoor settings had significantly more severe symptoms than those who played in grassy, outdoor spaces with or without trees.22

In another study, children diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) performed better on an objective concentration test after exposure to a relatively natural urban setting as compared to a less natural urban setting.24 Children with ADD can benefit from spending more time in green settings on a daily basis, and during attention demanding activities (like school and homework).22 Providing nature experiences in the school day and class environment is important for all children, and particularly so for those with ADD.

Table 1: activities judged as best and worst for ADD symptoms by parents22

Alzheimers and Dementia

Nature experiences provide mental health benefits for the elderly as well, including Alzheimer’s patients. Alzheimers is a type of dementia that causes memory impairment, intellectual decline, temporal and spatial disorientation, impaired ability to communicate and make logical decisions, and decreased tolerance to high and moderate levels of stimulation. Certain environments can provide prosthetic support for dementia patients to compensate for their reduced cognitive capabilities.25 For example, spaces that have dead-ends or are crowded can increase frustration and anxiety in Alzheimer’s-diagnosed residents.25 Supportive outdoor spaces include these design features: looped pathways; tree groves or sites to act as landmarks for orientation; non-toxic plants; even, well-lit paths with handrails; seating areas with the suggestion of privacy; and use of low-key fragrances and colors to soothe, rather than negatively stimulate, the patient.25

Studies have found that nature experiences can be of particular benefit to dementia patients. Exposure to gardens can improve quality of life and function of dementia patients by reducing negative behaviors up to 19% (see Table 2).25 Those patients who have access to gardens that are designed to positively stimulate the senses and promote positive memories and emotions are less likely to express negative reactions and fits of anger. After gardening activities, dementia and stroke patients exhibited improved mobility and dexterity, increased confidence, and improved social skills.26,27 Better sleep patterns, improved hormone balance, and decreased agitation and aggressive behavior have all been observed in dementia patients in association with contact with nature and the outdoors.28

Table 2: nature and behavior improvements in demential patients

Cognition and Illness

Clinical reports have noted the loss of concentration and distractibility in patients experiencing serious illness.29 Studies have tested the correlation between stress and cognitive function under various conditions in women diagnosed with breast cancer. The impairment of CDA has been observed to set in before the start of a cancer treatment, suggesting that attentional fatigue has an early onset and is a result of the diagnosis itself.30 This is likely due to the mentally demanding and stressful nature of diagnostic tests and treatment planning. Participation in activities and/or interacting with natural environments was shown to ameliorate and help stave off mental fatigue both before and after breast cancer treatment or surgery.29

Stress Relief

In addition to physiological symptoms, stress can lead to depression, schizophrenia, anxiety, exhaustion, and fatigue syndromes.31 Stress can occur at any time in life; however, such responses are especially prominent at later age due to physical, psychological, and social changes—for example, in response to chronic disease, disability, death of loved ones, or financial hardship.32 Stress can also negatively affect people’s perceptions of their well-being, including a poor perception of their own mental health.32 Physical activity has been linked to improvements in mental health and stress;33 many studies connect urban park use to decreased stress levels and improved moods. In one study, the longer participants stayed in a park, the less stress they exhibited.34 More than 100 studies have shown that relaxation and stress reduction are significant benefits associated with spending time in green areas.35


Depression also occurs at any age and can be helped through improved social connections (to decrease the feeling of isolation) and exercise, both of which are promoted by having nearby green outdoor spaces. In one study, 71% of people found a reduction in depression after going on an outdoor walk versus a 45% reduction by those who went on an indoor walk.36 Another study investigated major depression disorder (MDD) and found that an exercise program can be just as effective as antidepressants in reducing depression among patients.37 The value of green spaces in encouraging exercise is relevant to treating depression symptoms.

Can Technological Nature Be Effective

Couldn’t we simply substitute aspects of the natural world with technological depictions of nature? Can technology provide an adequate substitute in places where the natural world is some distance away?

When comparing subjects’ reactions in windowless offices with and without plasma TV “windows” showing natural scenes, participants preferred the offices with plasma-display windows and noted increased psychological well-being and cognitive functioning as a result of this connection to the natural world.38 In another study comparing viewing formats, outdoor views through glass windows were more restorative than blank walls, but plasma windows were no more restorative than blank walls to the subjects’ sense of well-being. Subjects’ heart rates were lower in offices with the glass windows than in those with plasma windows and blank walls.39

It seems that artificially represented nature is not an effective substitute for directly perceived nature as it does not provide equivalent benefits and positive experiences. Such technological representations could be useful to some degree in situations where it is difficult to incorporate “real” nature, as in space shuttles, submarines, or other extreme environments where there is an unavoidable disconnect from the natural world.

Nature in the Community

Green Space for Physical Activity

Play and exercise are an important part of children’s and adults’ development and brain function. As children, play can help develop cognitive thinking and reasoning abilities.17 Later in life, exercise likewise helps increase and maintain the brain’s cognitive capacity.40 Researchers have found that exercise boosts the growth of new nerve cells and improves learning and memory in adult mice: newly formed nerve cells were concentrated in the hippocampus, the part of the brain that is key in memory formation, spatial learning, and conscious recall of facts, episodes, and unique events.41 Urban green spaces encourage exercise and are a more restorative environment than indoor settings, with a greater positive effect on mental health.42 Additionally, urban green spaces offer a free, accessible, public environment in which to exercise and play to those who cannot afford a private gym membership.

Social Connections

A neighborhood than incorporates easily accessible green spaces into its design may also improve social cohesion and interaction. As a result, the mental health of individuals may also remain positive due to a decreased chance of depression and feelings of isolation and increased self-esteem. Effective social support networks have been found to restore feelings of personal control and self-esteem by buffering the effects of stress and poor health.43,44

Green spaces, such as community gardens or even the shade of a large tree, encourage social contact by serving as informal meeting places and sites for group and shared activities.45 Green spaces can serve as a sort of ecotherapy, as marginalized people can find empowerment, respite from stresses, and personal involvement in environmental stewardship.46 Green spaces in close proximity to homes encourage exercise, which can improve mental health.47 As described earlier, studies indicate that having views of nearby nature and living within green spaces can improve worker productivity,48 reduce stress,49 improve school performance,50 and lessen the symptoms of ADD.22 Useable and safely accessible gardens or green spaces not only foster a sense of community, but also provide psychological benefits among its members.16

Specifically concerning the elderly, social interaction is important as less loneliness is correlated with lower mortality rates, depression, and cognitive impairment.51,52 Additionally, in a study of elderly populations that prefer natural over built environments, there is a positive correlation between familiarity of the environment and restorativeness.53 To promote this, the elderly require easily accessible spaces due to their more limited mobility, so having parks and green spaces in close proximity to their neighborhoods or care centers is especially important.

Landscape Design and Planning for Mental Health and Function

Park Design

Parks are often scattered about cities, and many cities have too few parks. Based on decades of research findings, parks should be managed as systems, not just for the usual purposes of beauty and recreation, but also to help citizens function at their best. The National Parks and Recreation Association recommends that there be park space within 2 miles of every residence (with ¼- to ½-mile distances optimal for walkability) and that a city’s park system provide 5 to 8 acres of park space for every 1,000 residents.

Planting design within a park is also important. The “savannah hypothesis” argues that people prefer open landscapes with scattered trees, similar to African landscapes in which humans evolved.54 However, this theory has been recently challenged by evidence showing that the psychological benefits of green space are positively correlated with the diversity of its plant life.55 People who spent time in a park with greater plant species richness scored higher on various measures of psychological well-being than those subjects in less biodiverse parks.56

Building/Infrastructure Design

Planters, gardens, green roofs, and other features can be incorporated into building design to address mental health and cognitive function.20 For example, the soft rhythmic movements of a trees or grass in a light breeze or the light and shade created by cumulus clouds, called Heraclitean motion, are movement patterns that are associated with safety and tranquility, aiding the development of a calm, stable mental state; lighting or space design that mimics Heraclitean motion could be incorporated into building design to create calm, peaceful areas that aid patients’ recovery or improve workers’ or students’ productivity. Bright daylight supports circadian rhythms, enhances mood, promotes neurological health, and affects alertness; increasing the use of natural light and reducing dependence on electric lighting can also significantly improve mental health and function.

Design can also encourage learning and exploration by creating spaces that are not immediately interpreted but allow discovery through sensory exploration.57 Effective architectural design is not easy to achieve: built objects and spaces that are too complex at first glance can become daunting, overwhelming, and too difficult to understand, while those that are easily scanned do not encourage interaction.20 If the built environment simulates the layered complexity of ecosystems, a person’s sensory systems will be engaged to explore and learn about the built object or space, which encourages cognitive function through a high level of visual fascination and mystery.57

Project support was provided by the national Urban and Community Forestry program of the USDA Forest Service, State and Private Forestry. Summary prepared by Kathleen Wolf, Ph.D. and Katrina Flora, December 26, 2010.


1. Grinde, B., and G.G. Patil. 2009. Biophilia: Does Visual Contact with Nature Impact on Health and Well-Being? International Journal of Environmental Research and Public Health 6, 9: 2332-343.

2. Han, K.T. 2010. An Exploration of Relationships Among the Responses to Natural Scenes: Scenic Beauty, Preference, and Restoration. Environment and Behavior 42, 2: 243.

3. Lehrer, J. January 2, 2009. How the city hurts your brain  - And what you can do about it. Boston Globe.

4. Berman, M.G., J. Jonides, and S. Kaplan. 2008. The Cognitive Benefits of Interacting with Nature. Psychological Science 19, 12: 1207-212.

5. Kaplan, S. 1995. The Restorative Benefits of Nature: Toward An Integrative Framework. Journal of Environmental Psychology 15, 3: 169-182.

6. Berto, R., M.R. Baroni, A. Zainaghi, and S. Bettella. 2010. An Exploratory Study of the Effect of High and Low Fascination Environments on Attentional Fatigue. Journal of Environmental Psychology 30, 4: 494-500.

7. Heerwagen, J.H., and G.H. Orians. 1986. Adaptations to Windowlessness. Environment and Behavior 18, 5: 623.

8. Bringslimark, T., T. Hartig, and G.G. Patil. 2007. Psychological Benefits of Indoor Plants in Workplaces: Putting Experimental Results Into Context. Hortscience 42, 3: 581-87.

9. Lohr, V.I., C.H. Pearson-Mims, and G.K. Goodwin. 1996. Interior Plants May Improve Worker Productivity and Reduce Stress in a Windowless Environment. Journal of Environmental Horticulture 14, 97-100.

10. Fjeld, T., B. Veiersted, L. Sandvik, G. Riise, and F. Levy. 1998. The Effect of Indoor Foliage Plants on Health and Discomfort Symptoms Among Office Workers. Indoor and Built Environment 7, 4: 204.

11. Kaplan, R. 1993. The Role of Nature in the Context of the Workplace. Landscape and Urban Planning 26, 1-4: 193-201.

12. Chang, C.Y., and P.K. Chen. 2005. Human Response to Window Views and Indoor Plants in the Workplace. Hortscience 40, 5: 1354-59.

13. Shibata, S., and N. Suzuki. 2002. Effects of the Foliage Plant on Task Performance and Mood. Journal of Environmental Psychology 22, 3: 265-272.

14. Tennessen, C.M., and B. Cimprich. 1995. Views to Nature: Effects on Attention. Journal of Environmental Psychology 15, 1: 77-85.

15. Hartig, T., M. Mang, and G.W. Evans. 1991. Restorative Effects of Natural Environment Experiences. Environment and Behavior 23, 1: 3-26.

16. Maller, C.J., C. Henderson-Wilson, and M. Townsend. 2009. Rediscovering Nature in Everyday Settings: Or How to Create Healthy Environments and Healthy People. Ecohealth 6, 4: 553-56.

17. Isenberg, J.P., and N. Quisenberry. 2002. Play: Essential for All Children; A Position Paper. Association for Childhood Education International, 9 pp.

18. Heerwagen, J.H., and G.H. Orians. 2002. The ecological world of children. In: Kahn, P.H.J., and S.R. Kellert (eds.), Children and Nature: Psychological, Sociocultural, and Evolutionary Investigations. MIT Press, Cambridge MA, pp. 29-64.

19. Kahn Jr., P.H., and S.R. Kellert. 2002. Children and Nature: Psychological, Sociocultural, and Evolutionary Investigations. MIT Press, Cambridge MA.

20. Heerwagen, J. 2009. Biophilia, health, and well-being. In: Campbell, L., and A. Wiesen (eds.), Restorative Commons: Creating Health and Well-Being Through Urban Landscapes. Gen. Tech. Rep. NRS-P-39. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station.

21. Kirkby, M. 1989. Nature As Refuge in Children’s Environments. Children’s Environments Quarterly 6, 1: 7-12.

22. Taylor, A. F., F.E. Kuo, and W.C. Sullivan. 2001. Coping with ADD: The Surprising Connection to Green Play Settings. Environment and Behavior 33, 1: 54-77.

23. Taylor, A.F., and F.E. Kuo. 2009. Children with Attention Deficits Concentrate Better After Walk in the Park. Journal of Attention Disorders 12, 5: 402-09.

24. Kuo, F.E., and A.F. Taylor. 2004. A Potential Natural Treatment for Attention-Deficit/Hyperactivity Disorder: Evidence From a National Study. American Journal of Public Health 94, 9: 1580.

25. Mooney, P., and P.L. Nicell. 1992. The Importance of Exterior Environment for Alzheimer Residents: Effective Care and Risk Management. Healthcare Management Forum 5, 2: 23-29.

26. Rappe, E. 2005. The Influence of a Green Environment and Horticultural Activities on the Subjective Well-Being of the Elderly Living in Long-Term Care. University of Helsinki, Department of Applied Biology.

27. Ulrich, R.S. 2002. Health Benefits of Gardens in Hospitals. In: Plants for People, Proceedings of the International Exhibition Floriade.

28. Chalfont, G.E., and S. Rodiek. 2005. Building Edge: An Ecological Approach to Research and Design of Environments for People with Dementia. Alzheimer's Care Today 6, 4: 341.

29. Cimprich, B., and D.L. Ronis. 2003. An Environmental Intervention to Restore Attention in Women with Newly Diagnosed Breast Cancer. Cancer Nursing 26, 4: 284.

30. Cimprich, B., H. So, D.L. Ronis, and C. Trask. 2005. Pre-Treatment Factors Related to Cognitive Functioning in Women Newly Diagnosed with Breast Cancer. Psycho-Oncology 14, 1: 70-78.

31. Grahn, P., and U.K. Stigsdotter. 2010. The Relation Between Perceived Sensory Dimensions of Urban Green Space and Stress Restoration. Landscape and Urban Planning 94, 3-4: 264-275.

32. Orsega-Smith, E., A.J. Mowen, L.L. Payne, and G. Godbey. 2004. The Interaction of Stress and Park Use on Psycho-Physiological Health in Older Adults. Journal of Leisure Research 36, 2: 232-257.

33. U.S. Department of Health and Human Services. 1999. Physical Activity and Health: A Report of the Surgeon General. Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Atlanta GA, 278 pp.

34. Hull, R.B., and S.E. Michael. 1995. Nature-Based Recreation, Mood Change, and Stress Restoration. Leisure Sciences 17, 1: 1-14.

35. Davis, J. 2004. Psychological Benefits of Nature Experiences: An Outline of Research and Theory. Naropa University.

36. Anon. 2007. Ecotherapy: The Green Agenda for Mental Health. Mind: For better mental health, London, pp., 36 pp.

37. Blumenthal, J.A., M.A. Babyak, K.A. Moore, W.E. Craighead, S. Herman, P. Khatri, R. Waugh, M.A. Napolitano, L.M. Forman, M. Appelbaum, D.P. Uurali, and K.R. Krishnan. 1999. Effects of Exercise Training on Older Patients with Major Depression. Archives of Internal Medicine 159, 19: 2349-356.

38. Kahn Jr., P.H., R.L. Severson, and J.H. Ruckert. 2009. The Human Relation with Nature and Technological Nature. Current Directions in Psychological Science 18, 1: 37.

39. Kahn Jr, P.H., B. Friedman, B. Gill, J. Hagman, R.L. Severson, N.G. Freier, E.N. Feldman, S. Carrère, and A. Stolyar. 2008. A Plasma Display Window? - The Shifting Baseline Problem in a Technologically Mediated Natural World. Journal of Environmental Psychology 28, 2: 192-99.

40. Colcombe, S., and A.F. Kramer. 2003. Fitness Effects on the Cognitive Function of Older Adults: A Meta-Analytic Study. Psychological Science 14, 2: 125-130.

41. van Praag, H., B.R. Christie, T.J. Sejnowski, and F.H. Gage. 1999. Running Enhances Neurogenesis, Learning, and Long-Term Potentiation in Mice. Proceedings of the National Academy of Sciences of the U.S.A. 96, 23: 13427-431.

42. Pretty, J., R. Hine, and J. Peacock. 2006. Green Exercise: The Benefits of Activities in Green Places. Biologist 53, 3: 143-48.

43. Krause, N., and B.A. Shaw. 2000. Giving Social Support to Others, Socioeconomic Status, and Changes in Self-Esteem in Late Life. The Journals of Gerontology. Series B, Psychological Sciences and Social Sciences 55, 6: S323.

44. Krause, N. 1987. Life Stress, Social Support, and Self-Esteem in An Elderly Population. Psychology and Aging 2, 4: 349-356.

45. Health Council of the Netherlands. 2004. Nature and Health: The Influence of Nature on Social, Psychological and Physical Well-Being. Health Council of the Netherlands and RMNO, The Hague.

46. Burls, A. 2007. People and Green Spaces: Promoting Public Health and Mental Well-Being Through Ecotherapy. Journal of Public Mental Health 6, 3: 24-39.

47. Bell, S., V. Hamilton, et al. 2008. Greenspace and Quality of Life: A Critical Literature Review. Greenspace Scotland, Edinburgh, 75 pp.

48. Kaplan, R. 1993. Urban Forestry and the Workplace.  In: P.H. Gobster (ed.), Managing Urban and High-Use Recreation Settings. International Symposium on Society and Natural Resources. USDA Forest Service, St Paul MN, pp. 41-45.

49. Ulrich, R.S., R.F. Simons, B.D. Losito, E. Fiorito, M.A. Miles, and M. Zelson. 1991. Stress Recovery During Exposure to Natural and Urban Environments. Journal of Environmental Psychology 11, 3: 201-230.

50. Wells, N.M. 2000. At Home with Nature: Effects of "Greenness" on Children's Cognitive Functioning. Environment and Behavior 32, 6: 775-795.

51. Lubben, J.E. 1988. Assessing Social Networks Among Elderly Populations. Family & Community Health 11, 3: 42.

52. Almedom, A.M. 2005. Social Capital and Mental Health: An Interdisciplinary Review of Primary Evidence. Social Science & Medicine 61, 5: 943-964.

53. Berto, R. 2007. Assessing the Restorative Value of the Environment: A Study on the Elderly in Comparison with Young Adults and Adolescents. International Journal of Psychology 42, 5: 331-341.

54. Heerwagen, J.H., and G.H. Orians. 1993. Humans, habitats, and aesthetics. In: Kellert, S.R., and E.O. Wilson (eds.), The Biophilia Hypothesis. Island Press/Shearwater Books, Washington D.C., pp. 138-172.

55. Williams, K.J.H., and J. Cary. 2002. Landscape Preferences, Ecological Quality, and Biodiversity Protection. Environment and Behavior 34, 2: 257-274.

56. Fuller, R.A., K.N. Irvine, P. Devine-Wright, P.H. Warren, and K.J. Gaston. 2007. Psychological Benefits of Greenspace Increase with Biodiversity. Biology Letters 3, 4: 390-94.

57. Kaplan, R., and S. Kaplan. 1989. The Experience of Nature: A Psychological Perspective.  Cambridge University Press, New York.