Several of the reviewed studies have focused on psychological well-being effects arising from exercising in a natural environment. In a study investigating the effects of green exercise initiatives in the UK, Pretty et al. [77] found that participants’ mood and self-esteem, as measured by self-report surveys, were significantly improved after exercise. The authors interpreted this as evidence that exercise in natural environments can improve psychological well-being, although a control treatment with exercise in a non-natural setting was not included, and so it is not clear whether the natural environment itself was contributing to the effect, or whether exercise alone is sufficient. However, in a separate but topically related study, Pretty et al. [32] compared the emotional responses of participants exposed to a sequence of natural and urban landscape images whilst running on a treadmill in a laboratory. Mood and self-esteem both improved with exposure to natural scenes, suggesting that exercise may deliver greater benefits when it occurs in the presence of nature.

Despite the results of these studies, several papers report little or no relationship between the setting in which exercise takes place and psychological well-being benefit. A study in Japan monitored emotional change after exercise in a laboratory and in the green space around a university campus [78]. Positive emotions were found to be significantly higher after exercise, regardless of the running environment [78]. In a related study comparing the effects of exercise in a park and an urban setting in Sweden, Bodin and Hartig [12] found that anxiety, depression and anger, measured using self-report questionnaires, were significantly reduced after running, but there was no significant difference between the running environments. Consequently, we cannot yet conclude definitively that a natural setting increases the psychological well-being benefits of exercise.

Intentional interactions with nature such as gardening [64,70] and watching wildlife [41] also promote psychological well-being. Similarly, interacting with nature may have specific psychological well-being benefits for children. In a study on children’s mental and social health, Maller [62] found that direct contact with nature, as facilitated by curriculum-based nature activities in schools, had a positive impact on self-esteem and mental-well-being. These studies rely primarily on data derived from the perceptions of parents and teachers; experimental research designs that that utilize objective assessments of the psychological well-being effects of interacting with nature on children in a series of controlled environments may be useful for further investigating these effects. Despite the limitations of the reviewed studies, it is clear that interacting with nature may deliver several positive psychological well-being benefits to children.

In addition, there is emerging evidence that childhood interactions with nature may influence attitudes towards nature in later life [79]. While this is not necessarily a benefit per se, there is much interest from a sustainability perspective in how attitudes and behaviours that are positive toward nature develop [56,80,81]. A nationwide survey of US adults currently living in urban areas found that growing up in natural environments, or participating in activities such as gardening, visiting parks and taking environmental classes during childhood had a strong influence on positive environmental attitudes in adult life [78]. This was an important study as it highlighted the potential for long-term effects of experiences with nature during childhood. Chawla [82] reviewed several studies in this area, noting that they are limited by survey response bias, sampling bias and the lack of adequate control groups. Long-term longitudinal studies of children living in both urban and natural settings are necessary to determine the importance of these interactions for triggering environmentally responsible behaviour in later life. These studies are of particular importance given that a majority of the human population is now living within urban areas [4,83], where opportunities for people to interact and bond with nature are greatly diminished. Children raised in nature-deficient areas may be deprived of these early interactions with nature and may therefore be less likely to appreciate and support conservation initiatives in adulthood [84]. There are several validated tools available for measuring nature appreciation, including the Connectedness to Nature Scale [85], the Nature Relatedness Scale [86] and the New Ecological Paradigm [87], and, as well as being used to track the development of nature appreciation, they can be used to test whether nature-oriented participants are more likely to derive benefits from interactions with nature than those with less prior affinity to nature.

It has been hypothesised that natural environments are restorative, contributing to attentional recovery [88] and reducing mental fatigue. Kaplan [13] investigated the restorative benefits of the view from home by comparing window views in six low-rise apartment communities in Michigan (USA). Participants completed self-report questionnaires to measure satisfaction with their neighbourhood and well-being outcomes associated with attentional recovery and mental fatigue; participants also indicated the natural content of the view from their apartment windows. The study found that satisfaction and several well-being outcomes increased with the natural content in the view from home.

Natural environments vary in their complexity and species diversity and Han [67] found that natural environments also differ in their restorative potential. Importantly, this suggests that general comparisons between natural and urban settings may be deficient because they do not factor in the complexity of different natural environments. In a study from the UK, Fuller et al. [14] compared 15 urban green spaces and found that the restorative benefits to park users, as measured by self-report ability to reflect (a dimension of Attention Restoration Theory), increased with plant species richness in the green spaces. This highlights the importance of considering biological diversity and complexity when investigating the benefits of interacting with nature; this study was the only reviewed paper published from the biological sciences discipline, and the only study that specifically investigated the benefits of interacting with biodiversity, as opposed to an often loosely defined “nature”.

Urban environments can be very stimulating, requiring directed attention and negatively impacting cognitive function [88,89]. In urban areas, attention must be directed towards avoiding potential hazards and coping with noise and visual stimuli. This directed attention requires effort in order to inhibit or address these distractions and this can lead to mental fatigue, resulting in a lowered ability to concentrate and perform cognitive tasks [66]. There is good evidence to suggest that exposure to nature in urban and wilderness settings can improve cognitive function and performance [46,89].

Most studies of cognitive benefits used controlled experiments to measure cognitive performance. In a study of 38 students in Michigan (USA), Berman et al. [89] measured cognitive performance with a backwards digit span task, in which participants listen to a sequence of numbers and repeat them in reverse order. The results showed that cognitive performance was greater after students had walked through a tree lined arboretum when compared with a busy city street. The authors interpret this as evidence that the restorative properties of nature can improve cognitive function. Similarly, Hartig et al. [46] compared wilderness and non-wilderness vacations in southern California (USA) and found that proofreading, as a measure of cognitive performance, was significantly improved in the wilderness vacation participants for several weeks after the vacation, whereas the other two groups declined in proofreading performance over time. The authors conclude that a prolonged wilderness experience has restorative effects [46].

The restorative benefits of nature have also been studied by investigating preferences for different settings. In a survey of 140 Norwegian adults, Hartig et al. [90] found that natural environments were generally preferred settings for restorative experiences when compared with urban settings. In an earlier study, Herzog et al. [66] compared three settings ranging from urban to natural to investigate which might promote restoration. Participants were given two scenarios to accomplish either psychological recovery or reflection and were then shown a series of slides of natural, sport/entertainment and urban settings which they had to rank to determine the suitability of the location in achieving the scenario goal. Natural settings showed uniformly high restorative potential in comparison with less natural settings.

Studies have used self-report questionnaires and interviews with parents, guardians and teachers to gauge the effects of nature on children’s psychological well-being [62,91]. A study of children diagnosed with Attention Deficit Disorder (ADD) in the US tested whether indirect interactions with nature, such as exposure to nature during play, were related to attentional function [91]. Parents and carers of 96 children with ADD completed a questionnaire about the child’s attentional functioning after participating in leisure activities. Mean post-activity attentional functioning ratings were significantly higher for green outdoor activities as opposed to those in other settings, and the severity of ADD symptoms was significantly lower after playing in natural areas outdoors. It is unclear whether these psychological benefits are specific to children with ADD or applicable generally to all children.

Three studies tested for the cognitive benefits of indirect interactions with nature such as viewing images [89] and incidental interactions such as encountering indoor foliage vegetation [58]. Berman et al. [89] found that the cognitive performance of students, as measured by three directed attention tasks, was significantly improved after viewing pictures of natural scenes rather than urban areas. A study in Japan found that indoor foliage vegetation had a similar beneficial effect on cognitive performance, but only for male participants [58]. The female participants showed higher task performance than the males, but the presence or configuration of indoor plants had no additional effect on their cognitive performance. However, a related study showed that cognitive performance declined with increased density of indoor plants [92]. It is possible that indoor environments may not require as high levels of directed attention in comparison with outdoor environments, thus the restorative influence of nature in indoor environments may have less impact on cognitive function. Further investigation in this area would be fruitful and future studies might usefully employ a randomised, controlled experimental design that accounts for variables such as gender, age, plant configurations, plant densities and species diversity.

Incidental interactions with nature may positively influence the cognitive function of children. A longitudinal study in the United States correlated measures of vegetation around the home with measures of cognitive function to investigate the effect of moving from substandard low-income housing to new single family homes on 17 children [69]. There was a significant improvement in cognitive performance when the children moved to new homes surrounded by vegetation and natural elements. The author interprets this as evidence that presence of nature around the home is important for cognitive function in children [69]. However, no experimental control group was included, and although other possible explanatory variables (e.g., changes in housing quality) were analysed, it is unclear whether the presence of surrounding vegetation is the key variable influencing children’s cognitive performance, or what role other drivers such as the novelty of moving house or other features of the surrounding neighbourhood might play. Such multi-faceted analysis is of course difficult to achieve when one is relying on the goodwill of study participants.

In summary, the cognitive benefits of interacting with nature can be measured effectively using individual performance-based tests that are completed after interacting with nature in different settings. For this reason, objective and experimental research design has been employed by most studies investigating cognitive benefits. There is robust evidence to suggest that both intentional and indirect interactions with nature outdoors can improve cognitive function, although the evidence surrounding indoor interactions is more equivocal. Tightly controlled experiments are necessary to improve understanding in this area. As with psychological well-being benefits, much of the research into cognitive performance has made broad comparisons between green spaces and urban settings, and the specific effects of biodiversity on cognitive performance remain unknown.

Urban environments contain many potential stressors such as traffic, crime, dense crowds, and over-stimulation. There is evidence to suggest that interactions with nature may alleviate some of the negative physiological effects of such stressors. An experimental study in the Netherlands exposed participants to a stress-inducing activity and then measured cortisol levels following half an hour of either indoor reading or outdoor gardening [70]. While both activities led to decreases in cortisol levels, the decrease was significantly stronger for the gardeners, suggesting that gardening can relieve the physiological effects of stress [70]. However, because the control condition (indoor reading) is both passive and occurred in a non-natural setting, it is difficult to determine the relative contributions of the activity associated with gardening and the natural components of the environment in promoting stress reduction.

In an experimental study in China, Yamaguchi et al. [71] measured salivary amylase activity as a physiological indicator of stress in healthy males before and after exercise in both a forest and an urban environment. Enzyme activity was significantly reduced after exercise in forest environments and the authors conclude that the physiological effects of stress are reduced in forest environments. A related study in Switzerland found that a decrease in stress-induced headaches, as measured by self-report surveys, was significantly related to physical activity in parks [37]. Interestingly the effect was consistent for park and forest environments, despite the very different biological characteristics of those environments.

Indirect interaction with nature such as a window view or a picture of nature may also reduce the physiological effects of stress. A study in Taiwan used electromyography and electroencephalography to test participants’ responses to photographs of office environments with different configurations of indoor plants and natural views [65]. Physiological condition improved when viewing a picture of an office with a view of nature and indoor plants, suggesting a reduction in the physiological effects of stress.

Other studies of physiological benefits have investigated the relationship between nature and healing. In a retrospective study of cholecystectomy patients in a hospital in Pennsylvania (USA), Ulrich [9] found that post-operative healing, as measured by length of hospital stay, took less time for patients in a hospital room with a window-view of nature in comparison with patients with a view of a brick wall. Patients with a view of trees also required fewer strong painkillers, received fewer negative evaluative comments from nurses and had fewer postsurgical complications. In another study, Bennett et al. [43] discovered that an outdoor therapeutic camping trip reduced the probability of patient relapse for recovering substance abusers.

Maas et al. [10] conducted a broad scale epidemiological study drawing together self-report perceived general health from 250,782 people in the Netherlands with national land cover data. Green space coverage within 1 km and 3 km of participants’ homes was positively related to perceived health. A related study from the UK employed land use datasets and mortality data to examine the gender differences in relationships between urban green space and health outcomes [11]. Male cardiovascular and respiratory disease mortality rates decreased with increasing green space, with no significant relationship for women [11]. By contrast, in an unrelated study, Ulrich [44] found that the positive physiological responses of exposure to nature, as measured by heart rate and alpha amplitude while viewing images of nature, were significantly stronger for women. Gender is clearly a factor that should be carefully considered when designing research programs.

Studies on the effects of indoor plants in office environments have shown that their presence can improve health [68] and reduce the occurrence of illness [15,35]. For example, Fjeld et al. [68] assessed the effect of office foliage plants on symptoms of discomfort for 51 healthy office workers at a Norwegian oil company over a two-year period. Participants completed a survey with questions relating to physical symptoms such as itchiness and coughing. The results showed that the presence of plants in offices correlated with a reduction in dry skin, hoarse throat, coughing and fatigue, suggesting that the introduction of foliage plants into an indoor environment may reduce symptoms of physical discomfort and improve health. Related studies on the effects of indoor vegetation have found that the number and presence of indoor plants in an office [35] and a classroom [15] reduce the occurrence and frequency of time off through ill health.

Overall, research to date has identified a broad range of physiological benefits from interacting with nature. There seems to be a good balance between experimental and correlative studies, and most have utilised both self-report and objective data. Whilst the evidence for some of the identified benefits is fairly strong, confounding factors such as gender and age have not always been incorporated into the research design. Most studies of the physiological benefits of interacting with nature are descriptive, and there is little evidence suggesting how these benefits are delivered. Despite the use of a variety of different natural settings, little explicit consideration has been given to the importance of biodiversity itself (rather than simply “green”) in delivering improved physical function or health.

Some of the negative impacts of urbanisation include individual isolation, lack of social support, interracial conflict and increased incidence of crime and violence [93]. Our review suggests that provision and access to green space within urban areas may ameliorate or even reverse some of these social challenges and ultimately increase social cohesion [4,72]. Interactions with nature can facilitate social interaction in adults and children [62], foster social empowerment [62,74], enhance interracial interaction [16] and promote social cohesion and support [73]. There is a significant decrease in crime rates and violent behaviour in urban areas with surrounding green space or vegetation, in comparison with urban areas with limited greenery [25,63].

Much of the reviewed literature for this type of benefit focused on the role of group-oriented conservation projects (e.g., tree planting) and community gardens in delivering social capital benefits. A study based in rural Australia discovered that residents of a community who were involved with a land management group had a greater sense of community cohesion and were more willing to work toward improving their community than non-participating individuals [59]. This provides some indication that volunteering in conservation groups might increase social capital within the wider community, although evidence from studies of other forms of intentional nature engagement activities suggests caution in interpretation. For example, Kingsley et al. [72] found that participants described social support, connections and social networking as positive elements of participating in a community garden. Whilst this might indicate that community gardens encourage wider social cohesion and cooperation [72], the authors acknowledge that the evidence from the study does not demonstrate that the social benefits extended beyond the garden community. Similarly, Shinew et al. [16] found that inter-racial interaction among community gardeners in North America was enhanced in community garden settings but did not extend into the wider community.

Other studies have focused on the role of urban green space and vegetation in reducing violence and crime in urban areas. Studying 98 urban apartment buildings in the city of Chicago, Illinois (USA), Kuo and Sullivan [25] found that crime rates, as measured by police crime reports, were significantly lower in buildings surrounded by higher vegetation density. Inclusion of confounding variables in the analysis produced the same results; vegetation density remained a consistent predictor of crime rates. In a related study of public housing residents in Chicago, Kuo and Sullivan [63] found that occurrences of violence and aggression, as measured through self-reports from residents, were also significantly lower in buildings with higher densities of surrounding vegetation. This study also investigated potential mechanisms behind the identified relationship and found that aggression was correlated with attentional function and, specifically, mental fatigue. Thus, a potential pathway is that urban vegetation restores attentional function, which reduces the occurrence of violence and aggression in urban public housing, indirectly contributing social benefits to the broader community. This fascinating Chicago-based work has not—to our knowledge—been replicated in any other study system; it would be fruitful to discover whether these results generalise to other locations, not least because there are myriad social benefits that stem from reduced violence and crime in urban settings.

The potential social benefits of nature interactions have also been studied in other sectors of society. Maller [62] conducted interviews with school principals and teachers in Melbourne, Australia to determine the perceived social benefits for children participating in nature-based activities. The identified benefits included empowerment and social engagement [62]. Whilst suggestive of social benefits, the results are based on the perceptions of the educators; development of longitudinal studies that follow students over time would provide further useful insight into these potential benefits for children.

Research on the spiritual benefits of interacting with nature has been limited, with only three (5%) of the reviewed studies focusing specifically on these. Benefits included increased inspiration [42] and feelings of connectedness to a broader reality [47], both important for spiritual well-being [7]. It has been suggested that these types of benefits may also play an important role in positively influencing how people value natural environments [83] by inspiring a broader responsibility for the natural world [94].

Common elements among the multiple definitions of spiritual well-being include a sense of connectedness, a sense of purpose, a sense of awe and inspiration and faith in a larger reality [7,95,96]. These might or might not be connected to a specific religion per se; experiences of wilderness landscapes or specific natural features where the power of nature is apparent have been shown to invoke feelings of awe and inspiration. For example, a study of recreation programs in two separate wilderness areas in North America used on-site observations, journal analyses and interviews to examine wilderness experiences as a source of spiritual inspiration and well-being for women [42]. Participants reported on the expansiveness of landscapes and the power of nature as meaningful aspects of their wilderness experience. The authors interpreted this as evidence of spiritual inspiration and concluded that wilderness experiences are an opportunity for spiritual growth [42].

Viewing wildlife may invoke similar feelings of awe and inspiration, regardless of the environmental setting. In a study of tourists on two separate wildlife tours—in Spain and California—analysis of interviews and field journals revealed that viewing wildlife evoked a feeling of awe and wonder and created a temporal experience in which all concentration was focused on the moment [41]. This study suggests that wildlife-viewing events can promote a deep sense of well-being, leading to spiritual fulfillment.

One study investigated the occurrence of transcendent experiences in forest environments [47]. A transcendent experience is defined as one characterised by strong positive feelings of happiness and freedom, a sense of union with the universe or higher entity, absorption in the moment and a sense of timelessness [47]. This study, undertaken in Victoria, Australia, utilized a written questionnaire to prompt participants to recall a transcendent experience whilst in a forest environment. Participants included people who visit, live or work in forests. Two forms of transcendent experiences occurred in forests, one characterized by feelings of insignificance and the other by a sense of compatibility and familiarity. The authors interpret this as evidence of the spiritual value of forests [47].

Given the qualitative nature of the evidence, the lack of a single definition for spiritual well-being, small sample sizes and the limited number of studies, the evidence for spiritual benefits of nature experiences is not conclusive. There is a cultural bias towards Western society among the reviewed studies, despite the likelihood that the spiritual benefits of interacting with nature will vary significantly between cultures [76,97]. Also, the evidence for spiritual benefits is thus far largely limited to wilderness settings, which may have narrowed the range of benefits that have been studied. For example, feeding wildlife has been found to invoke a sense of belonging or connection to and, on a deeper level, a responsibility for the natural world [94]. In urban areas, where large natural areas are lacking, wildlife feeding may be an important mechanism for promoting spiritual well-being.

Interactions with nature can also produce tangible benefits. While large-scale industries such as agriculture and mining exemplify broad scale interactions that obtain tangible benefits through resource extraction or production, this review specifically focuses on individual interactions. Examples of these from amongst the 57 reviewed studies include food production from private domestic gardens [38,55], the impact of access to local green spaces on house sale prices [98,99,100], and the value of access to natural areas ascertained via willingness to pay for the experience [40].

In an early study on the benefits of gardening, a survey of gardeners in Michigan (USA) revealed that tangible benefits such as food production were rated as more important for plot gardeners than community or home gardeners [38]. The study also discovered that across all these gardeners other benefits, such as the gardening experience and sustained interest, were rated as more important than the tangible benefits. These results hint that while food production may be a tangible benefit, it may not be the primary motivation for gardening. In a recent study specifically focused on motivators for gardening amongst gardeners in Ohio (USA), producing food and herbs was one of the identified motivators for gardening but not the primary one [55]. Some disadvantaged neighbourhoods in developing countries largely depend on subsistence gardening as a source of food and income [101], thus cultural, regional and socio-economic differences may influence the relative importance of deriving tangible benefits from interacting with nature through gardening.

The economic value of access to natural areas can be calculated using hedonic valuation techniques, which estimate the influence of the natural area on the market price of another good, such as a house [98]. Morancho [98] used this technique to explain house prices in Castellon, Spain. Environmental variables included views of green space, distance from dwelling to nearest green space and the size of the nearest green space. Analyses revealed an inverse relationship between house prices and distance to nearest green space and that proximity is more important than size of the nearest green space [98].

Using the same hedonic technique, Bolitzer and Netusil [75] analysed sale price data for houses in Portland, Oregon (USA). Proximity to green space had a positive effect on home sale prices, but these effects were only significant for publicly accessible green spaces such as public parks or golf courses. While agreeing with Morancho’s [98] findings, these results highlight that the type of urban green space can be important in addition to its proximity. This may be because people place value on green space they can access, suggesting that inaccessible private green space may be of less value than a publicly available park, although further research on this potential distinction would be fruitful. In a related study, Pearson et al. [50] used the hedonic pricing technique to estimate the impact of the headland section of Noosa National Park, Australia, on land values in areas surrounding the park. In contrast to the other studies [75,98], while views of the National park increased property value, proximity had no significant effect [50]. Moreover, working in Baltimore, Maryland (USA), Troy and Grove [99] identified a negative effect of proximity to green space in neighbourhoods where crime rates were above a certain threshold, suggesting that green space can be viewed as a liability in some situations.

The economic value of recreation in natural environments can also be estimated at an individual level. In a study of three public beaches in South Carolina (USA), Oh et al. [40] surveyed 925 non-resident visitors to public beaches and found that participants were willing to pay on average an extra $6.60 per day for additional public beach access points in South Carolina. The identified willingness to pay an increased entry fee may be indicative of the economic value that people place on the benefits of visiting a beach.