Homework has increased. Ditto scheduled activities.

And neighborhood covenants. "Just try to put up a basketball hoop, let alone a treehouse," Louv said.

Parents worry about Lyme disease from ticks, West Nile virus from mosquitoes, and, most of all, the chance, however slim, that some stranger will harm their child.

Zaradic knows all the parental impulses. Her children are 2, 7 and 11.

But, "out of some of this work, I've realized how important it is to have them play out in the backyard with the dog and just run around and do what kids do," she said.

Zaradic grew up in concrete-bound New York City. But most weekends, the family took to the Long Island beaches, where they went fishing, got mussels, and found "all these amazing things," she said. "It seemed like such a magical world."

She has noticed that her own children seem much different after their annual summer vacations, in an Adirondacks cabin without electricity.

"The kids come back so energized and enlivened, and at the same time calmer and more settled with themselves," she said.

Pergams and Zaradic hope to focus more studies on what happens if you do give kids a grounding in nature.

"It looks like that's the way to go, but there's no good data on it," Zaradic said. She hopes it will tell them what works, what doesn't.

She glanced up at the cloudy sky, tapped her boots against the ground and nodded. "We're getting to a point where I think it's pretty critical to turn this trend around."