But in 2002, the journal Pediatrics reported that preschool children with bedroom TVs were more likely to be overweight. In October, the journal Obesity suggested that the risk might be highest for boys. In a study among French adolescents, boys with a bedroom television were more likely than their peers to have a larger waist size and higher body fat and body mass index.

The French study also showed, not surprisingly, that boys and girls with bedroom TVs spent less time reading than others.

Other data suggest that bedroom television affects a child’s schoolwork. In a 2005 study in The Archives of Pediatric and Adolescent Medicine, researchers looked at the television, computer and video game habits of almost 400 children in six Northern California schools for a year. About 70 percent of the children in the study had their own TV in the bedroom; they scored significantly and consistently lower on math, reading and language-arts tests. Students who said they had computers in their homes scored higher.

Why a bedroom television appears to have such a pronounced impact is unclear. It may be that it’s a distraction during homework time or that it interferes with sleep, resulting in poorer performance at school. It could also suggest less overall parental involvement.

Another October study, published in Pediatrics, showed that kindergartners with bedroom TVs had more sleep problems. Those kids were also less “emotionally reactive,” meaning that they weren’t as moody or as bothered by changes in routine. While that sounds like a good thing, the researchers speculated that having a TV in the bedroom dampened the intensity with which a child responded to stimulation.

Another study of more than 700 middle-school students, ages 12 to 14, found that those with bedroom TVs were twice as likely to start smoking  even after controlling for such risk factors as having a parent or friend who smokes or low parental engagement. Among kids who had a TV in the bedroom 42 percent smoked; among the others, the figure was 16 percent.

“I think it matters quite a lot,” Dr. Epstein said. “There are all kinds of problems that occur when kids have TVs in their bedroom.”

So while many parents try to limit how much television and what type of shows their children watch, that may be less than half the battle. Where a child watches is important too.