Why would vegetation influence our mental well-being? For one thing, grass and trees provide a welcoming place for people to gather. In the hectic and crowded cores of cities, people need the little grove of chestnut trees outside their apartments where they can mingle in the shade and hear the hiss of wind in high trees. They need big public lawns where they can play together. They need the tiny sprouting plots of neighborhood gardens, where they can put aside the city's stress on time and the temporary in favor of growth and permanence.

Scientists suspect that green space also has a restorative effect on our voluntary attention, the kind of intense focus required to work or study, to ignore distractions and concentrate on the task at hand. Voluntary attention is like a mental muscle; we exercise it in nearly every aspect of our lives. It dictates how well we think and how we handle ourselves in difficult situations—whether we roll with the punches or fly off the handle. Living in a city with its relentless crush of noise and traffic, conflicts and demands, makes us "crabby and impulsive," Kuo says. Being in nature refreshes us by letting us give voluntary attention a rest and allowing us to surrender to involuntary attention: the effortless and often enjoyable noticing of sensory stimuli in our environment.

Kuo speculates that over the course of human evolution, there was selection for this response to the natural world. Our ancestors who found nature effortlessly engaging had an advantage. "They were the ones more likely to know where the berries could be found and where the critters hung out," she says. "When push came to shove in difficult environmental conditions, they were better able to survive."

In our modern era, with all its pressures, contact with nature in urban settings may be more crucial than ever. A park-rich metropolis helps us stay physically healthy and battle overweight and diabetes. Two big recent studies of people in populated urban centers in the Netherlands and Japan showed that those living in areas with easy access to green spaces where they could walk had significantly better health and lower mortality rates than those without. Health studies suggest that even relatively passive contact with nature lowers blood pressure and anxiety levels.