Growing concern has led to a renewed interest in developing hands-on nature programs for children, with names such as "Ranger Ricks" and "No Child Left Inside."

For instance, the Schuylkill Center for Environmental Education in the city's Roxborough neighborhood has instituted nature programs for children as young as 2. "Nature Tots" come with their parents, and "we just take them out for walks," said Erin Johnson, program development manager. "Little excursions out to meadows and ponds to just look."

Among older kids, she can tell the ones who aren't familiar with the great outdoors. "They are constantly wondering what's going to get them. Even bugs. They're very scared of bugs."

Some become antagonistic, squishing tadpoles or insects.

Zaradic has a doctorate in ecology and evolution from the University of Pennsylvania and is a regional fellow with the Environmental Leadership Program, a national nonprofit that seeks to nurture environmental leaders. She and Pergams began studying how people relate to the natural world several years ago.

In their current paper, amid all the downhill data, they found but one tiny increase: People went from hiking once every 12.5 years to once every 10 years.

A recent 15-year trend analysis by the U.S. Fish and Wildlife Service echoes their findings. It shows declines of 16 percent among anglers, 11 percent among hunters, and 23 percent among those taking trips to watch wildlife.

National Park Service spokesman Gerry Gaumer acknowledges park officials don't see "as much of the family vacations as we used to," although he noted, "I don't see that there's any crisis as to visitation."

David Brown, executive director of America Outdoors, a national organization of oufitters, resorts and tour operators, said his group's research shows that primitive activities have, indeed, declined. Fund-strapped parks can't keep the trails open. Women don't like the bathrooms — or the lack thereof.

However, he said, demand is strong for "outfitted trips," especially near popular destinations.

"You would assume from the study that everything is crashing," he said.

But the Nature Conservancy's chief scientist, Peter Kareiva, based in Seattle, thinks the country is in the midst of a vast social change.

"I have kids," he said. "Almost everyone I knew growing up wandered around in the woods. There really is less of it."

Some say video games and TV aren't the only problems.

"When I talk to parents and kids, it's very clear there are other major things going on," said Richard Louv, author of a 2005 book, "Last Child in the Woods: Saving Our Children From Nature-Deficit Disorder."

One is access. "It's pretty tough to go out in the woods if it's been cut down."