The more television that a three-year-old watches, the more likely he or she is to behave aggressively, according to a U.S. study.

Just having the TV on in the background, even if the child wasn't watching it, was also linked to aggressive behavior although the relationship wasn't as strong, said the researchers.

"Parents should be smart about TV use," researcher Jennifer Manganello from the University at Albany, State University of New York, told Reuters Health.

"They should limit the time that children use TV, pay attention to the content of TV programs, and consider how TV is used throughout the home."

The study looked at 3,128 women from 20 U.S. cities who had a child between 1998 and 2000. While there was some diversity of education among the study participants, one-third hadn't graduated from high school.

Two-thirds of the mothers said their three-year-old watched more than two hours of TV a day, and the average viewing time for children was around three hours.

On average, the TV was on for about five additional hours on a typical day.

After accounting for factors known to be associated with aggressive behavior, such as living in a violent neighborhood or having a mother who suffers from depression, TV watching and household TV time were both still significantly associated with aggressive behavior, such as hitting others, having angry moods, being disobedient, and screaming a lot.

The American Academy of Pediatrics recommends no TV at all for children two and younger, and two hours a day or less for older kids, lead researcher Jennifer Manganello and her team from the Tulane University School of Public Health and Tropical Medicine noted in their report.

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