The case for going outside

A considered reading of forty years of research on what natural environments do to depleted minds — and what the studies actually support, what they don't, and what the rise of the smartphone has done to the conditions under which any of it occurs.

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A walk in the park is one of the few pieces of received wisdom that the empirical record happens to vindicate. Forty years of research across attention science, environmental psychology, public health, and neuroimaging have converged on something not far from the cliché. Natural environments measurably restore something that built environments deplete. The science is more specific than the bumper sticker version of it, and considerably more interesting, but the broad direction is the correct one.

What follows is a careful reading of that literature: where the findings are strong, where they are weaker than they sound, and where the modern reader's experience of "going outside" diverges from the experience the studies measured. That last point — the gap between the laboratory walk and the everyday walk — has not received much attention in the popular writing on the topic, and is the part that connects most directly back to the rest of this site.

Two kinds of attention

The theoretical scaffolding for nearly all of the empirical work on this topic comes from a distinction made by William James in The Principles of Psychology in 1890, and elaborated a century later by Rachel and Stephen Kaplan, two environmental psychologists at the University of Michigan. James drew a line between voluntary attention — the effortful direction of focus toward something not inherently interesting — and involuntary attention, which is captured effortlessly by stimuli that are interesting on their own terms. The restoration framing — the idea that voluntary attention is a finite resource, drawn down by use and recoverable only by rest — comes mostly from the Kaplans rather than from James, who supplied the distinction without proposing a depletion model around it.

The Kaplans built the theory in 1989. In The Experience of Nature: A Psychological Perspective, and in a tighter restatement published by Stephen Kaplan in the Journal of Environmental Psychology in 1995, they proposed Attention Restoration Theory (ART). The argument runs, in essence, like this. Modern environments — particularly urban and built ones — make extraordinary demands on directed attention. Crossing a street, parsing a screen, ignoring an ambient stimulus, even holding a conversation in a noisy café: all of these tax the same limited cognitive resource that we use for sustained focused thought. Over the course of a day, that resource is depleted. The depletion has a name in the literature — directed attention fatigue — and a recognisable phenomenology: irritability, scattered focus, the difficulty of caring about what you are nominally doing (Kaplan, 1995; Kaplan & Kaplan, 1989).

Recovery, the Kaplans proposed, requires conditions that directed attention cannot supply itself. Specifically, it requires an environment that occupies the involuntary attention system gently and continuously — that is, that supplies what they called soft fascination. A river. A canopy of leaves. A city park. Stimuli interesting enough to hold peripheral attention without compelling it. While that involuntary system is occupied, the directed attention system is permitted, in effect, to power down and replenish.

The theory makes specific predictions about what should improve and under what conditions, and over the past twenty years a substantial empirical literature has set out to test them. The mechanism itself is inferred from task performance rather than directly observed, and the literature is best read as a body of indirect evidence whose convergence is the strongest part of the case.

A second theory, in the same direction

Before turning to that empirical work, a second theoretical strand deserves naming because it has produced its own evidence and is often conflated with ART in popular writing. In 1984, Roger Ulrich, then at the University of Delaware, published a now-famous short paper in Science. He had pulled the records of 46 patients recovering from gallbladder surgery at a Pennsylvania hospital between 1972 and 1981, and matched them by age, sex, and other surgical factors. The variable that distinguished the two groups was the view from their hospital bed: some had a window onto a small stand of deciduous trees, the others onto a brown brick wall.

The patients with the tree view recovered measurably faster. They left the hospital sooner, took fewer doses of strong analgesic medication, and received fewer negative evaluative comments in the nursing notes (Ulrich, 1984). The sample is small and the design is observational, but the effect is large and the matching is careful, and the paper became one of the founding studies of what is now called Stress Recovery Theory (SRT). Where ART argues that nature replenishes a depleted cognitive resource, SRT argues that natural settings damp the stress response — physiologically and affectively — through pathways that are not specifically about attention at all. The two accounts overlap in their predictions for ordinary outdoor activity, and in most real-world situations both pathways are probably operating together.

This duality is worth holding in mind. When researchers find that a walk in a park improves mood and cognitive performance, both theories predict it; their predictions diverge mainly in the more careful experiments that try to isolate one pathway from the other.

The first crisp experimental test

The single best-cited paper in the empirical literature on ART is Berman, Jonides, and Kaplan's 2008 study in Psychological Science. Their two experiments are worth describing in detail, because nearly every subsequent study borrows their basic structure.

In the first experiment, 38 university-student participants completed a backwards digit-span task — a standard measure of working memory and directed attention, in which the subject is read a sequence of digits and asked to repeat them in reverse. They then performed a deliberately fatiguing task to deplete directed attention further. Half were then sent on a 50-minute walk through the Ann Arbor Arboretum, a large wooded area; the other half on a comparable walk through a stretch of downtown Ann Arbor. They returned, performed the digit-span again, and the procedure was repeated a week later with the conditions reversed (within-subjects design). In the second experiment, the walks were replaced with viewing photographs of nature or urban scenes (Berman, Jonides, & Kaplan, 2008).

The results were what ART predicted. Working memory performance improved significantly more after the nature walk than after the urban walk; the effect was smaller but in the same direction in the photograph condition. Mood was also measured (using the PANAS), and shifted in the expected direction, but the cognitive effect held up when statistically controlling for mood — suggesting that the directed-attention improvement was not simply a downstream consequence of feeling better.

The design has held up well. It is within-subjects, with each participant serving as their own control a week apart, which removes the most obvious sources of confounding. The cognitive outcome is a working-memory task, which is less expectancy-sensitive than mood self-reports, though not immune to practice or experimenter effects; the within-subjects design partially addresses both.

The Berman paradigm has been replicated dozens of times with variations. Bratman, Daily, Levy, and Gross (2015), working with sixty participants on 50-minute walks around Stanford, found a similar pattern in a single study covering both ends of the outcome space: nature walkers, compared with urban walkers, showed both affective benefits (lower anxiety, lower rumination, lower negative affect) and cognitive benefits (improved working memory). Stevenson, Schilhab, and Bentsen (2018), in a systematic review of 42 studies published in the Journal of Toxicology and Environmental Health, Part B: Critical Reviews, found consistent low-to-moderate improvements in working memory and cognitive flexibility following nature exposure, with somewhat less reliable effects on attentional control specifically. The effects are not enormous. They are reliable, replicable, and in the direction the theory predicts.

The brain catches up

For roughly the first three decades after the original ART paper, the evidence for the theory was almost entirely behavioural. The mechanisms were inferred from cognitive task performance rather than observed directly. Two more recent papers have begun to close that gap.

In 2015, Gregory Bratman and colleagues at Stanford reported, in Proceedings of the National Academy of Sciences, an experiment in which 38 healthy participants were randomly assigned to a 90-minute walk in either a natural setting (a section of the Stanford foothills) or an urban one (a busy four-lane road in Palo Alto), with both walks matched for length at 5.3 km. Before and after each walk, the researchers used arterial spin labelling — a magnetic resonance imaging technique that measures cerebral blood perfusion — to assess activity in the subgenual prefrontal cortex (sgPFC), a region whose activity in both healthy and depressed individuals tracks closely with self-reported rumination. They also collected self-reported rumination scores using a standard scale (Bratman, Hamilton, Hahn, Daily, & Gross, 2015).

Both measures changed in the predicted direction after the nature walk and not after the urban walk. sgPFC perfusion decreased following the nature condition; self-reported rumination also decreased, though some of the rumination contrasts were marginal before the planned follow-up analyses. The convergence of a behavioural self-report and a regional neuroimaging measure on the same direction of effect is closer to suggestive convergent evidence for a rumination-related pathway than to a clean mechanistic demonstration of ART itself; ASL measures regional cerebral blood flow, not the cognitive resource the theory posits.

A complementary finding from Sonja Sudimac and colleagues at the Max Planck Institute in Berlin, published in Molecular Psychiatry in 2022, extended the imaging work to a different brain system. Sixty-three participants were scanned before and after a one-hour walk in either a forest on the outskirts of Berlin or along a busy commercial street. The relevant outcome was amygdala activation during a stress task. After the nature walk, amygdala activation decreased measurably. After the urban walk, it remained stable. The effect is consistent with SRT rather than ART specifically — the amygdala is more closely associated with the autonomic stress response than with directed attention — and it suggests that both mechanisms are real and operate concurrently (Sudimac, Sale, & Kühn, 2022).

A 2019 paper in Science Advances by Bratman and an unusually long list of co-authors — twenty-six researchers across the natural, social, and health sciences — set out to summarise the consensus position. Its core claim is modest and worth reading carefully: natural environments contribute to mental health through multiple, partially overlapping pathways, and the cumulative evidence is now sufficient to warrant incorporating mental-health benefits of nature contact into ecosystem service assessments and urban planning (Bratman et al., 2019). It is not the headline-grade finding that popular writing prefers, but the pluralism of the framing is closer to the actual state of the literature than any single-mechanism story.

The clinical edges

Where the effect sizes get genuinely large is in populations whose directed attention systems are already under unusual strain.

In a 2012 paper in the Journal of Affective Disorders, Marc Berman and colleagues replicated the original 2008 paradigm with 20 participants diagnosed with major depressive disorder. Before each walk, the researchers had participants briefly think about an unresolved negative autobiographical event, in order to induce rumination. After this induction, participants walked for 50 minutes in either a natural or urban setting, and then completed the backwards digit-span task. The cognitive effect of the nature walk, measured as a partial eta-squared, was 0.53 — a very large effect by the conventions of psychological research, and several times larger than the effects observed in the original healthy-population studies. The mood effects were also positive but uncorrelated with the cognitive effects, suggesting two genuinely distinct mechanisms (Berman, Kross, Krpan, Askren, Burson, Deldin, Kaplan, Sherdell, Gotlib, & Jonides, 2012).

A second striking effect comes from the work of Andrea Faber Taylor and Frances Kuo at the University of Illinois, working with children clinically diagnosed with attention-deficit/hyperactivity disorder. Their 2009 study, published in the Journal of Attention Disorders, took 17 children aged 7 to 12 with professional ADHD diagnoses and had each of them complete three 20-minute walks one week apart, in three settings: a city park, a downtown street, and a residential neighbourhood. After each walk the children completed the backwards digit-span task. The walks were guided to control for activity level and were administered in a single-blind procedure (Faber Taylor & Kuo, 2009).

Concentration improved most after the park walk, with effect sizes (Cohen's d) ranging from 0.52 to 0.77 against the other two conditions. The authors note, with appropriate care, that these effect sizes are comparable in magnitude to those reported for recent formulations of methylphenidate — a deliberately careful comparison that the popular reception of the paper has tended to flatten into a flat equivalence. The original framing is the right one: the effect is real, the sample is small, and a 20-minute walk is not a substitute for a clinical treatment. But the direction and the magnitude are consistent with the broader pattern. ART effects appear to be largest where directed attention is most depleted, and clinical ADHD is one of the contexts in which it is most reliably depleted at baseline.

A dose–response curve

A different kind of evidence comes from large observational studies that ask, in effect, what happens at population scale when nature contact varies across many thousands of people.

The most useful single paper in this genre is Mathew White and colleagues' 2019 study in Scientific Reports, drawing on the UK's Monitor of Engagement with the Natural Environment survey, a nationally representative sample of 19,806 English adults. The researchers asked how much recreational time each participant had spent in nature in the previous seven days, and modelled the relationship between weekly nature contact and self-reported health and subjective wellbeing, controlling for socioeconomic status, residential greenspace, urbanicity, air pollution, physical activity, and a long list of other plausible confounders (White, Alcock, Grellier, Wheeler, Hartig, Warber, Bone, Depledge, & Fleming, 2019).

What they found is a step pattern that resembles a familiar public-health dose–response shape. Below approximately two hours of nature contact per week, the survey detected no significantly elevated odds of self-reported good health or high wellbeing. At and above two hours, both outcomes lifted: the 120–179 minute group, for example, showed odds ratios of approximately 1.59 for good health and 1.23 for high wellbeing relative to people reporting no nature contact, with similar effects across the higher buckets and a plateau between roughly 200 and 300 minutes. The threshold did not depend on whether the time was accumulated in one long visit or several shorter ones, and the same broad pattern held for older adults and for those with chronic health conditions. The authors describe the 120-minute threshold as tentative; it is the inflection in this dataset, not a universal constant, and the design is cross-sectional and self-report.

Two hours a week is a low bar. Distributed across seven days it is seventeen minutes a day, which is the duration below which this dataset found no detectable association with elevated health or wellbeing.

The longitudinal cohort case

The strongest single piece of population-scale evidence comes from a 2019 PNAS paper by Kristine Engemann and colleagues at Aarhus University. Using Denmark's near-complete national registers, the authors linked the residential history of approximately 943,000 Danes to high-resolution satellite-derived vegetation indices, and then linked those records to subsequent psychiatric diagnoses through adolescence and into adulthood (Engemann, Pedersen, Arge, Tsirogiannis, Mortensen, & Svenning, 2019).

The headline finding is striking. After controlling for urbanicity, parental psychiatric history, parental age, and socioeconomic status, individuals who had grown up in the lowest-greenspace deciles were associated with up to 55% higher rates of subsequent psychiatric diagnosis through adolescence and into adulthood than those who had grown up in the highest-greenspace deciles. The effect was graded across the full range of childhood greenspace exposure, persisted across multiple sensitivity analyses, and showed a dose–response relationship that loosely mirrors the White et al. cross-sectional work over much longer time-scales.

Engemann et al. is correlational, not experimental, and there are plausible residual confounders that the authors cannot fully address. But it is the largest study of its kind, and the association is meaningful. Children who grew up with more greenspace around them were less likely to receive subsequent psychiatric diagnoses through their late teens and twenties, and the association held when the obvious alternative explanations were tested.

Where the evidence is thinner

A careful reader will, by this point, have noticed that the evidence is largely in one direction, and may reasonably ask where it is weaker. Three places.

First, the experimental effect sizes in healthy adults are not large. Stevenson, Schilhab, and Bentsen (2018), reviewing forty-two studies, found consistent improvements in working memory and cognitive flexibility, but the magnitudes were in the low-to-moderate range, with heterogeneity across measures and studies. The effect on attentional control specifically — the cognitive function ART most directly predicts will be restored — was less consistent than the effects on general working memory or cognitive flexibility. This is the kind of mixed picture that should produce some humility about the scale of the benefit, particularly relative to the more extravagant popular claims.

Second, the children's literature is more mixed than the adult literature. A 2021 systematic review in Environment International, by Mygind and colleagues, examined 223 studies of nature exposure and socio-emotional development in children under twelve. The associations between nature contact and improved cognitive functioning were relatively strong; the associations with mood, stress, and physical wellness were weaker, with study quality and bias risk affecting the overall picture (Mygind, Kurtzhals, Nowell, Melby, Stevenson, Nieuwenhuijsen, Lum, Flensborg-Madsen, Bentsen, & Enticott, 2021). The effect on Faber Taylor and Kuo's clinical ADHD sample is large; the average effect on socio-emotional development across all studies of all children is more equivocal.

Third, the mechanisms are not as cleanly separable as the theory implies. The paper that has done the most to clarify this is a 2017 expert review by Iana Markevych and colleagues in Environmental Research, which proposed a three-pathway framework for how greenspace affects health: harm reduction (lower exposure to air pollution, heat, and ambient noise), capacity restoration (the ART and SRT mechanisms discussed above), and capacity building (more physical activity, more opportunities for social interaction) (Markevych, Schoierer, Hartig, Chudnovsky, Hystad, Dzhambov, et al., 2017). Most studies, the Markevych framework points out, cannot distinguish these pathways. A person walking in a park is simultaneously breathing cleaner air, exercising more, encountering soft fascination, and possibly meeting a neighbour. ART might be doing some of the work; the other two pathways might be doing more. Disentangling them is genuinely difficult, and few studies have managed it cleanly.

The implication is not that ART is wrong. It is that the question "what is going on when nature exposure improves wellbeing?" probably has several correct answers operating in parallel, and that the strongest reading of the evidence is one in which several mechanisms contribute jointly to a real but compound effect.

A useful synthesis came from Terry Hartig and colleagues' 2014 Annual Review of Public Health paper, which argued that the literature should stop framing the question as a competition between ART, SRT, and physical-activity explanations, and start treating them as complementary pathways within a single ecological framework. That framing has held up well. By 2019, the Science Advances consensus paper led by Bratman was already adopting it (Hartig, Mitchell, de Vries, & Frumkin, 2014; Bratman et al., 2019).

The phone in the park

Here is the part that the broader literature on nature and health mostly does not discuss, and where the concerns of this site rejoin those of environmental psychology. It is also the part of this essay where the evidence is thinnest, and where I want to flag the distinction between what the studies show and what they suggest.

The mechanism ART proposes requires the soft fascination of a natural environment to occupy the involuntary attention system while the directed attention system rests. A leaf moves; you notice; you look back at the path; the mind drifts. The cognitive resource that has been depleted by hours of focused, effortful attention is permitted, finally, to recover.

A smartphone in active use does not look like that kind of stimulus. Its salience is intermittent, high-magnitude, and engineered to capture top-down attention rather than peripheral notice. There is direct experimental evidence, in laboratory and desk-based settings, that the mere presence of one's own phone reduces working-memory performance (Ward, Duke, Gneezy, & Bos, 2017), and that the receipt of a notification — even an unread one — degrades sustained attention (Stothart, Mitchum, & Yehnert, 2015). Both findings were discussed at greater length in the flagship essay on this site.

What the existing literature does not directly establish is that the same costs operate on a walk in a park. That extension is plausible — the proposed mechanism is competing salience and inhibitory load, which should not depend strongly on whether the participant is indoors or outdoors — but it is a hypothesis, not a settled empirical fact, and it deserves to be flagged as such.

What can be said with more confidence is that the field studies of ART have themselves treated the phone as a variable to be constrained. Bratman et al. (2015) did issue a smartphone to each participant — but only as a verification device: participants were asked to take ten photographs along the walk and the phone's GPS log was used to confirm the route. Sudimac, Sale, and Kühn (2022) similarly equipped participants with a phone, but instructed them not to enter shops or otherwise use the device, and used it primarily to play an alarm at the 30-minute turnaround point. Faber Taylor and Kuo's (2009) ADHD walks were guided and chaperoned. The original Berman, Jonides, and Kaplan (2008) walks predated widespread smartphone ownership entirely.

The pattern across the experimental literature is consistent: when researchers want to measure nature-based attention recovery, they constrain incidental phone use, even when they are happy to use the phone instrumentally for tracking. The outdoor walk that the studies measured is not the same walk as a contemporary outdoor walker typically takes, where the phone is unconstrained and signalling. It is reasonable to think the laboratory-style desk effects extend into the park, but it has not been clearly demonstrated, and the modern phone-pocket walker is in a condition the published effect sizes were not designed to describe.

The most defensible reading is that the published effect sizes describe a particular kind of activity — an outdoor walk in which the phone is either absent, restricted, or being used minimally and instrumentally — and that whether they generalise to the notification-permeable everyday version of the same activity is an empirical question the field has not directly answered. The translation from "go outside" to "go outside with phone in pocket on notifications-on" is not a free move.

Frances Kuo and William Sullivan's 2001 paper in Environment and Behavior is worth mentioning here as a much older but related strand. Studying 145 inner-city public-housing residents, the authors found that those living in more vegetated buildings reported less aggression and less mental fatigue than those in more barren ones, with the mental fatigue measure statistically mediating the aggression effect (Kuo & Sullivan, 2001). The finding stands on its own, and I cite it mainly to mark the lineage: cognitive depletion has measurable behavioural consequences, and the question of what modern conditions do to the restorative quality of green spaces is a continuation of the question Kuo and Sullivan posed twenty-five years ago.

What to actually do, given the evidence

Reduced to the simplest defensible recommendations, the literature supports the following.

1. Aim for two hours a week of nature contact, distributed however suits the week. This is the threshold below which the White et al. (2019) data show no detectable benefit, and above which the curve begins to rise. There is no further gain past about five hours; the plateau is meaningful, and it argues against treating nature contact as something that has to be optimised heroically.

2. It does not need to be wilderness. A city park works. The Berman et al. (2008) effects came from the Ann Arbor Arboretum, not the Sierra Nevada. Bratman et al.'s (2015) effects came from the Stanford foothills, which is not exactly remote. The empirical evidence is that the relevant variable is contrast with the urban built environment, not absolute wildness.

3. If you want the conditions the studies measured, constrain the phone. This is the most consequential and least-evidenced recommendation in this list. The field studies discussed above either restricted phone use, used phones only instrumentally for tracking and alarms, or predated smartphones altogether. Whether the published effects survive intact into a notifications-on, pocket-checking version of the same walk is a hypothesis grounded in the laboratory work on phone presence (Ward et al., 2017; Stothart et al., 2015), but it has not been directly tested in outdoor settings. Aeroplane mode, leaving the phone at home, or restricting it to map-and-alarm functions are the simplest ways to approximate the experimental conditions; they are also the recommendation with the largest plausible-but-untested upside.

4. If you have access to greener neighbourhoods, more time there is probably non-trivially good for the people in them — particularly children. Engemann et al. (2019), with its near-million-person Danish cohort, is the most defensible large-scale evidence on this point. The effect sizes are not transformative, but they are not small either, and they appear to compound across childhood.

5. Soft fascination is the operative concept, and you can recognise it. A path through trees provides it. A scrolling feed does not. A view from a window of a stand of leaves provides it. A view of a notification badge does not. The distinction is not romantic; it is the difference between a stimulus that occupies the involuntary attention system continuously and gently, and one that intermittently commandeers the directed attention system.

A note on the limits

It is worth distinguishing the confidence levels of the four kinds of evidence this essay has used, because they are not equivalent and the conclusion should not flatten them.

Acute experimental effects on attention and mood (Berman et al., 2008; Berman et al., 2012; Faber Taylor & Kuo, 2009): the strongest evidence base, with multiple replications and clear within-subjects designs. Moderate effects in healthy adults, larger effects in clinically depleted populations. This is where I would put the most weight.

Neuroimaging convergence (Bratman et al., 2015; Sudimac et al., 2022): supports the broader picture. ASL and fMRI show changes in regions plausibly involved in rumination and stress response after nature walks, but neuroimaging does not directly observe the attentional resource that ART posits, and effect sizes in regional activation studies should be read with the usual caution.

Observational dose–response (White et al., 2019): a useful guide to a public-health threshold, with a self-report exposure measure and a cross-sectional design. The 120-minute figure is an inflection in this dataset, not a universal constant; the direction and shape are the load-bearing parts.

Childhood greenspace cohorts (Engemann et al., 2019): the best large-scale evidence, but observational. Dose-response, controls for the obvious confounders, and a near-million-person sample push the finding past most plausible alternative explanations, but it remains correlational and the outcome is psychiatric diagnosis, not mental health broadly defined.

The phone-free hypothesis: the weakest of the four, an extension of laboratory desk-presence findings into outdoor walking that has not been directly tested. I think it is likely correct, but it has the lowest evidentiary status of anything in this essay and I have tried to flag it as such throughout.

The intersection of all four, taken with appropriate weights, is roughly this. Forty years of work has accumulated reasonable evidence that natural environments restore something in cognitive and emotional functioning. The mechanism is multiple and the average magnitudes are moderate. The simplest defensible recommendation — about two hours a week, somewhere with vegetation, with the phone constrained to the extent the rest of life permits — is consistent with what the literature directly supports, plus one extension that the literature does not yet directly establish.

The advice is unromantic and undramatic. Most of it the popular writing got to first, by intuition rather than evidence. The evidence, on a careful reading, has caught up with most of it, and is honest about the parts it has not yet caught up with.

Go outside. Spend a couple of hours a week in a park. Constrain the phone. The rest of this site is about what your phone has done to your attention and your sleep and your mood. This essay is about a particular kind of time — slow, unscheduled, outdoor, quiet — that appears, on the evidence, to do something close to the opposite. The two arguments are the same argument, viewed from opposite sides.

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References

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