Studies document the potential negative effect of extensive television viewing, none of which should be surprising: The more time young children spend in front of the TV set, the greater the chance of developing attention deficit disorder/hyperactivity.
Less TV time means less aggression. More TV time means more obesity, not just because of the lack of activity but because kids eat more while watching.
We know that unsupervised TV means exposure to inappropriate material.
A study shows that third-graders with TVs in their bedrooms perform worse on standardized tests than those who don’t have them. Of course, in most countries it would be absurd to even imagine kids having their own TV sets. But it’s a common, unfortunate practice in the US. Read about this study here. If your child has a TV in the bedroom, take control and take it out. The longer the habit is indulged in, the harder it is to stop. (Some children claim they can’t fall asleep without the television on!)
If you have already curtailed access to TVs in the home, more power to you.
For evidence on lifestyle and ADHD see these research abstracts: