Some statistics and information from the 2008 book Parenting, Inc.: "A nationally representative study from the 1990s found that only 17 percent of children under one were watching television and fewer than half of children between the ages of one and two watched. In a 2006 study of 1,009 parents, 40 percent of babies were watching TV or DVDs/videos by three months; the average baby started watching videos at six mon…
In school we studied research that correlated how many of hours of TV a day a person watched, and life expectancy. As many people thought the more tv you watched your life expectancy fell. When you first hear that it sounds like TV can reduce your years on earth. When they took a close look though they found a confounding variable; health care. Those who watched more TV tended to be poorer, and thus had less healthca…
In this case we already know that the most important mediating variables for predicting outcomes on the relevant measures are parent-child talk and self-directed play, so it's very easy to see how each hour of watching TV causes increasing damage. I'm sure some of the gap is because of mere correlation, but it's also clear that TV watching itself is a huge part of the problem if only because it prevents the child from getting the types of interactions they need for their brains to develop properly. (whether or not the TV itself fries their dopamine system or whatever directly is still an open question.)