"The average person in America used to go to the national parks every year. It was the iconic American family vacation. Now, there are less people doing that," said Patricia Zaradic, a biologist with the Environmental Leadership Program, Delaware Valley, in Bryn Mawr, Pennsylvania.

In prior research, Zaradic linked a sharp drop in admissions to the US national parks to electronic diversions, such as playing video games, surfing the Internet and watching television.

In the new study, which appears in the Proceedings of National Academy of Sciences, Zaradic and Oliver Pergams of the University of Illinois gathered data on 70 years of various nature activities. The group also looked at national park admissions in Japan and Spain.

They found that since the late 1980s, nature recreation has fallen by 18 to 25 percent on a per capita basis, depending on the type of outdoor activity.

"There's a real and fundamental shift away from nature, certainly here in the United States, and possibly in other countries," Pergams said in a statement.