One in five American parents believe their kids 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 Wednesday by the University of Southern California, 21 percent of adult Internet users with children believe the kids are online too long, compared with 11 percent in 2000. Still, that's less than the 49 percent who complain their kids 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.

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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 non-voice functions, the figure grows to 54 percent among those 18-24 and 45 percent among those under 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 non-users have no plans to go online within the next year.

Cole said the numbers raise the prospect of a permanent subclass of non-users.

"Internet penetration has largely plateaued," he said.

Americans 66 and over 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 under, likely because most U.S. schools now have some form of Internet access.

On average, users spend 14 hours a week online, compared with 9.4 hours in 2000.

Thirty-seven percent of home Internet users still have dial-up accounts, compared with 26 percent for high-speed cable modems and 24 percent for DSL. Eleven percent of Internet users go online through mobile devices — not necessarily exclusively — averaging two hours a week.

The study revealed little change in the effect on television. Thirty-six percent of home Internet users say they have spent less time watching TV since they started using the Internet, roughly the same as the 33 percent who said that in a 2001 survey.

Cole said the increased use of high-speed connections has a lot to do with that.

When people were on dial-up, they were accessing the Internet 20 or 30 minutes at a time — "generally time not spent watching television," Cole said. "Broadband changed all that. They are on 30, 40, 50 times a day for two or three minutes at a time. It's not a big bucket of time displacing television."

People may be paying less attention to television commercials, though, fitting in online use during program breaks, he said.

That said, 41 percent of veteran users — those online for more than nine years — say they have spent less time watching television, compared with only 23 percent among those who have joined the Internet within the year.

The study found nearly a quarter of online users — especially newcomers to the Internet — say they spend less time reading.

The telephone survey of 2,269 U.S. households was conducted in English and Spanish from February to April and included follow-up interviews with respondents to his previous studies. The study has a margin of sampling error of plus or minus 3 percentage points.