Nature writers, myself included, have long promoted a walk in the wild to clear the mind and lift the spirits.

Having found something valuable - and free - we wish to make others aware of the pleasant, relaxing effect of immersing oneself in the world of plants and animals.

But we probably have another motive, too. If more people use and enjoy natural settings, the constituency for preserving them gets bigger and more effective.

We accept as a given that the natural experience is beneficial - many have said so, going back to the romantic poets and philosophers of ancient Greece.

But what evidence is there?

Frances Kuo, an environmental psychologist at the University of Illinois, sought an answer to that question. She and fellow researcher Andrea Taylor reported in the journal Children and Their Environments on a number of relevant studies in the field of child development, but all with significant limitations.

A few studies examined the results of programs that take youngsters into the wild and challenge them to work together to acquire the basic needs of food, water and shelter.

Participants reported that they were more self-confident, better able to concentrate, had better self-control and a more positive outlook. But self-reporting is too easily biased by personal beliefs or a desire to provide the expected answers.

Other studies - including one by Kuo and Taylor - extensively surveyed parents of children with attention deficit hyperactivity disorder. These found that the greener the play space of children with ADHD, the lower the parents rated the severity of symptoms.

Another strong suggestion of correlation, but perhaps more affluent families choose greener neighborhoods and send their children to less urban schools.

The researchers narrowed the focus, asking parents to rate the effects on their children's symptoms of activities in natural outdoor settings, indoors or at manmade outdoor locations. They found greenery yielded a consistent reduction in ADHD symptoms.

Reporting by parents, however, is also a subjective assessment that could be affected by bias. The parents may believe the natural setting is better and be inclined to find a confirming reduction in symptoms.

So Kuo conducted controlled trials, taking professionally diagnosed ADHD children on randomly assigned walks in three well-kept urban settings: a park, a neighborhood and a downtown area. Upon returning, the children were given a test of concentration that involved counting backwards.

Kuo found that the children who walked in the park could concentrate about as well as children without ADHD, far better than the other settings.

Writing last year in the Journal of Attention Disorders, Kuo said the size of the effect of the 20-minute park walks was comparable to that of the drug methylphenidate (Ritalin, for example) used to treat ADHD.

This is just one small study but well-designed. More will follow and science will prove, I think, what those of us who have spent time in nature have intuitively known.

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