Wired on Friday:Studies have shown that children three- to five-years-old who watch 'Sesame Street' are better able to recognise numbers, letters and shapes.

The digital divide used to separate rich from poor; now it separates parents from their children. Whether it's infants watching the new 24-hour Baby's First TV channel in the US, or teenagers instant messaging while they watch TV shows on their iPods, television is an enormous presence in the lives of children today.

The average American child spends three to five hours a day watching it. And they start their viewing careers much earlier than ever before. In 1961, the average child began to watch television at age three; today it is nine months.

Yet, for all the TV they are watching, much of what parents think they know about television's impact on their children is wrong. For instance, in the early 1970s it was common knowledge that television was bad for your eyes: My own parents were convinced that my bad eyesight was the result of sitting too close to the screen, and they, therefore, made me stay at least two metres from it. Today, most people know that television viewing does not cause vision problems, but a host of new myths have emerged, still ripe for debunking:

• TV makes children stupid. Actually, high-quality TV shows such as Sesame Street and Blues Clues improve children's cognitive abilities. Study after study has shown that children three to five years old who watch Sesame Street for an hour a day are better able to recognise numbers, letters and shapes than those who don't.

When 500 children who had participated in some of those studies were followed up as teenagers, those who had watched educational programmes as pre-schoolers had higher grades, were reading more books, placed more value on achievement and were more creative than those who had not.

• TV makes children violent. The real story is more complicated. In 1994 researchers reviewed hundreds of studies involving thousands of children and concluded that there was clear evidence that watching violence on TV makes children more aggressive. Similarly, pre-teens and teenagers exposed to sexual content on television are much more likely to engage in the kinds of activities they see on the screen.

But a study of more than 5,000 children also found that "pro-social" TV programmes make children kinder and more tolerant.

In fact, the linkage between good behaviour and watching good programming is as strong as the link between bad behaviour and bad programming. The problem is that children are increasingly watching shows with violence and sex instead of programming appropriate for their age.

• Educational videos make infants smarter. The names - such as Baby Einstein and Brainy Baby - suggest one thing, but the data suggest otherwise. According to a 2005 report by the Kaiser Family Foundation, no programme targeting children younger than two has demonstrated any educational benefit. Evidence from studies my colleagues and I have done suggests that early viewing (under the age of three) may be harmful to children's cognitive development. We found that children who watch TV before age three score worse on tests of letter and number recognition upon entering school than those who do not. And for each hour of television a child watches on average per day before age three, the chances that child will have attention problems at age seven increase by 10 per cent.

A 2005 University of Pennsylvania study found that even watching Sesame Street before age three delayed a child's ability to develop language skills.

• Sitting around watching television - instead of playing outside - makes children overweight. In fact, being a couch potato is not what causes obesity. Children sit around to read, too, but no one suggests that reading causes obesity. A 1999 Stanford University experiment found that when elementary school children watched less television, they did lose excess weight, however, reducing their television time did not make them more active. What that suggests is that television-watching itself - unlike other sedentary activities such as reading, block-building or working on art projects - encourages overeating. Snacking in front of the tube is a widespread habit (for children as well as adults) and the barrage of junk food advertisements only heightens that temptation. About 70 per cent of the ads children see on US television are for food products, and virtually none of them are for healthy choices.

A 2005 Harvard University study found that, on average, children eat about 170 more calories per day for each hour of television they watch, and all of those calories are derived from foods commonly advertised in television commercials.

• Television helps children to get to sleep. The opposite is true. In a 2005 study of more than 2,000 children, my colleagues and I found that the more television children watch, the more likely they are to have irregular sleep and nap patterns. As common as it is - about three-fourths of children had television as part of their bedtime ritual, according to a national survey - allowing kids to watch television because they can't sleep is part of the problem, not the solution.

• Children watch too much television. Actually, the bigger problem is what they watch and how they watch it. In what some consider the halcyon days of television, families used to gather around a single centrally located set and watched high-quality, family-centred programming together.

Nowadays, the typical US household has multiple television sets; family members (including young children) sit alone and watch programmes that too often are violent and sexualised. When parents watch with their children, the value of the best television programmes is enhanced - and the harm of negative programming can be curtailed.

• Dimitri A Christakis, a paediatrician and researcher at Children's Hospital in Seattle, is co-author of The Elephant in the Living Room: Make Television Work for Your Kids (Rodale). Author email: dachris(at)u.washington.edu