This was recently reported in the Wall Street Journal. Here is the link:
A New View On TV
One of the big issues is what you stop doing or what you do less of when you watch TV. If it replaces something "bad," then TV can have benefits. For example:
"The economists found that television was especially positive for children in households where English wasn't the primary language and parents' education level was lower. "We don't exactly know why that is, but a plausible interpretation is that the effect of television on cognitive development depends on what other kinds of activity television is substituting for," says Mr. Shapiro, 28.
Growing up in the 1950s, Sonia Manzano, who plays Maria on "Sesame Street," was part of the first generation of children who watched television. Born in the South Bronx to Spanish-speaking Puerto Rican parents, she says that television "gave me a view of the world -- it gave me sort of a sense of what it was to be an American and what that was about.""
This describes the methods:
"The variation Mr. Gentzkow and Mr. Shapiro exploited was the timing of the introduction of TV into different cities. Television began taking off in the U.S. in 1946, after a wartime ban on TV production was lifted. But the Federal Communications Commission stopped granting new commercial television licenses from September 1948 to April 1952 while it made changes in allocating broadcast spectrum. There was a long lag between when some cities got television and when others did.
The economists then looked at results of a survey of 800 U.S. schools that administered tests to 346,662 sixth-grade, ninth-grade and 12th-grade students in 1965. Their finding: Adjusting for differences in household income, parents' educational background and other factors, children who lived in cities that gave them more exposure to television in early childhood performed better on the tests than those with less exposure."