TV, movies, video games, and Internet use have serious consequences for children's health, according to a wide-ranging series of studies published in the April issue of Archives of Pediatric and Adolescent Medicine.

From obesity and social isolation to early sexual initiation and aggressive and violent behavior, 15 new studies link exposure to media images with a broad range of negative health, behavior and lifestyle issues in children and teens.

Moreover, the studies found that the harm begins early in the preschool years and continues through adolescence.

It's a "major public health issue," Archives editors Dimitri A. Christakis, MD, MPH, and Frederick J. Zimmerman, PhD, University of Washington, Seattle, say in a news release. They note that electronic media "are among the most profound influences on children in this country" and that "this intersects with many other issues that are critically important to child health, including violence, obesity, tobacco/alcohol use, and risky sexual behaviors."

WebMD took a closer look at three of the new studies.

Too Much TV May Hurt Kids in School

TV and First Sex in Young Teens

We hear about the sexual content of television. But there have been few scientific studies examining the effect of television on kids' sexual behavior, says M. Bruce Edmonson, MD, MPH, professor of pediatrics at the University of Wisconsin in Madison.

Edmonson's research team looked at detailed information gathered in a national survey of adolescents. Each teen was interviewed two times, one year apart. The researchers looked at data on some 4,800 teens who were younger than 16 and who said they'd never had sexual intercourse.

At the second interview, about 15 percent of the teens said they'd started having sex. That's not surprising, Edmonson says, as that's about the national average. But other factors influenced the decision of these young teens to have sex. A major factor: TV.

TV and the Role of Parents

Overall, those who watched more than two hours of television a day were 35 percent more likely to have had sex. But this much television had much greater effects on some kids.

When the researchers looked at kids who said their parents strongly disapproved of them having sex, those who watched more than two hours of television a day were 70 percent more likely to have sex. And if sex-disapproving parents didn't monitor their teens' TV viewing, more than two hours a day of TV upped a teen's odds of sexual initiation by 250 percent.

"Ironically, among kids whose parents strongly disapprove of them having sex, there is a mixed message," Edmonson tells WebMD. "Parents try to tell kids they don't approve, but TV gives them other messages. And by the time kids get to be teens, parents often give up on monitoring the programs they watch."

By the time children are in their teens, it may be too late to begin restricting the kinds of TV they can watch, Edmonson says.

"There are different ways to attack this problem," Edmonson says. "One way is for parents to get a little more aggressive about how much TV their kids watch -- probably at an earlier age, so that issue of parental control is resolved before the child is too old and the parents just give up."

Will this really help? Edmonson says he isn't sure -- but he's trying to find out.

"We don't really know if parents can monitor their kids' media exposure, even if they try," he says. "So that is the next phase of our research: to find out what parents should do and how they can do it. We don't know yet."

Edmonson warns against using the current study to make broad statements about the sexual content of television programs. Kids, he notes, are exposed to sex in all kinds of ways -- not only in television programs and television ads, but also through the Internet, video games, and old-fashioned print media.

"We cannot stamp all that out," he says. "But we are going to try to understand what obstacles parents face when they try to regulate what kids watch."

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A Lonely Spiral of Aggression

Kids who watch a lot of violence on TV may get sucked into a vortex of angry isolation, finds David S. Bickham, PhD, of Harvard's Center on Media and Child Health.

Bickham's data comes from detailed logs filled out by the parents of more than 3,500 children aged 6 to 12.

The findings reveal interesting patterns:

--The more time kids spend watching violent TV programs, the less time they spend with their friends. This isn't true for nonviolent programs.

--The more time kids spend watching TV with friends, the more time they spend doing other things with their friends.

What does it mean? Bickham thinks that TV viewing is something kids do with their friends. Violent TV programs are known to make kids more aggressive. When kids watch violent TV by themselves, their aggressive behavior makes it harder for them to have friends. So what do they do? They watch more TV -- becoming even more socially isolated, and even angrier.

"It says something when children choose to fill time with violent media," Bickham tells WebMD. "And we do know that more aggressive kids watch more violent TV. Because of the way we know violent TV affects kids, the best explanation is cyclical. The kid is angry and is drawn to these violent, stimulating shows, becomes more aggressive, and, because of that aggression, becomes more isolated and watches more TV."

This, Bickham says, may be where many bullies are born.

"Here we have violent, isolated kids, stewing and waiting for a moment to become aggressive," he says. "These are kids who are likely to become bullies, or be victims of bullies because they are isolated, and waiting for a moment when they can lash out."

It's a wake-up call for parents to monitor not just how much TV their kids watch, but what kinds of programs they watch, and with whom.

"Good, educational TV can be very positive," Bickham says. "This is not a death knell for TV. It is about what, specifically, children are watching. If we teach them violence, they are going to learn violence."

TV Time May Make Kids Bullies

TV: A Bad Babysitter

Very young kids aren't immune to the negative effects of TV, finds pediatrician Julie C. Lumeng, MD, of the University of Michigan in Ann Arbor.

Lumeng led a team that looked at more than 1,000 preschool kids enrolled in a U.S. study.

Her main finding: 3-year-olds were three times more likely to be overweight if they spent two or more hours a day in a room with a TV on.

"People say, 'Oh, but my child watches educational programs.' But we found the content of TV was not related," Lumeng tells WebMD. "You could be an upper-income family, with lots of educational toys in the room, watching educational videos. And your child is still at three times higher risk of overweight than kids who do not watch TV."

What's happening? Obviously, kids in a room with the TV on aren't outside getting more exercise. But they're also seeing TV ads. And TV ads on children's programming tend to sell high-calorie junk foods.

"It is not just absence of exercise, but TV commercials," Lumeng says. "Past studies have shown that the content of children's TV commercials is overwhelmingly about junk food. And if you show kids commercials, they ask for the junk food. So it may be the TV, even at this early age, is shaping their food preferences."

It's easy to say that parents should follow the advice of the American Association of Pediatrics: Don't let kids watch more than two hours of television a day. But that isn't easy.

"For a parent it is a struggle. When the TV is off, kids need more guidance and attention," Lumeng says. "Parents use TV as a babysitter. A lot of America's 3- and 4-year-olds are home watching TV. Maybe we need preschool programs to get the kids out of the house and exposed to less television."

Couch Potato Kids Pack on the Pounds

By Daniel J. DeNoon, reviewed by Louise Chang, MD

SOURCES: Bickham, D.S. Archives of Pediatric and Adolescent Medicine, April 2006; vol 160: pp 387-392. Ashby, S.L. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 375-380. Lumeng, J.C. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 417-422. Christakis, D.A. and Zimmerman, F.J. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 445-446. Jordan, A.B. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 446-448. Shifrin, D. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 448-450. Thompson, K.M. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 402-410. Wang, X. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 411-415. Brady, S.S. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 341-347. Bushman, B.J. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 348-352. Acevedo-Polakovich, I.D. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 354-360. Chamberlain, L.J. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 363-368. Fletcher, J. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 383-386. Barkin, S. Archives of Pediatric and Adolescent Medicine,April 2006; vol 160: pp 395-401. David S. Bickham, PhD, research scientist, Center on Media and Child Health, Harvard School of Public Health, Boston. M. Bruce Edmonson, MD, MPH, professor of pediatrics and director, pediatric diagnostic clinic, University of Wisconsin, Madison. Julie C. Lumeng, MD, assistant professor of pediatrics, University of Michigan, Ann Arbor.