Brain Rules for Baby: How to Raise a Smart and Happy Child from Zero to Five

by John Medina

TV also poisons attention spans and the ability to focus, a classic hallmark of executive function. For each additional hour of TV watched by a child under the age of 3, the likelihood of an attentional problem by age 7 increased by about 10 percent. So a preschooler who watches three hours of TV per day is 30 percent more likely to have attentional problems than a child who watches no TV.

Just having the TV on while no one is watching—secondhand exposure—seemed to do damage, too, possibly because of distraction.

For every hour per day the children spent watching certain baby DVDs and videos, the infants understood an average of six to eight fewer words than infants who did not watch them.