NEW YORK - One in five American parents believe their children are spending too much time on the Internet, though most say the online activities haven't affected grades either way.
In a study to be released today by the University of Southern California, 21 percent of adult Internet users with children believe the children are online too long, compared with 11 percent in 2000.
Still, that's less than the 49 percent who complain their children watch too much TV.
About 80 percent of the children say the Internet is important for schoolwork, although three-quarters of the parents say grades haven't gone up or down since they got Internet access.
Forty-seven percent of the adults say they have withheld Internet use as a form of punishment.
Banning television is still more popular, reported by 57 percent of adults surveyed.
The study, meanwhile, found that although only 27 percent of cell phone owners use them for text messaging, photo transmitting and other nonvoice functions, the figure grows to 54 percent among those 18-24 and 45 percent among those younger than 18.
THE STUDY has been conducted most years since 2000. Over that time, researchers have seen Internet use grow to 78 percent, from 67 percent. Access at home increased to 68 percent, from 47 percent.
In one of the few surveys to look at why people are offline, the study found the lack of a working computer most often to blame.
Of the 22 percent of Americans who do not currently use the Internet, more than a quarter are former users who dropped out.
"Almost nobody drops out out of dissatisfaction," said Jeffrey Cole, director of USC's Center for the Digital Future. "The reason most people drop off is they change jobs or their computer breaks."
But more than half the former users have no intention of returning online, the most ever. Overall, 60 percent of nonusers have no plans to go online within the next year.
Mr. Cole said the numbers raise the prospect of a permanent subclass of nonusers.
"Internet penetration has largely plateaued," he said.
Americans 66 and older remain the most disconnected, with only 38 percent online. For all other age groups, at least 74 percent are online, with penetration hitting 99 percent for those 18 and younger, likely because most U.S. schools now have some form of Internet access.