NEW YORK - 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 behaviour 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 behaviour, such as living in a violent neighbourhood or having a mother who suffers from depression, TV watching and household TV time were both still significantly associated with aggressive behaviour, 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.

There are a number of ways that excessive TV viewing could contribute to a child's degree of aggressive behaviour, the researchers add in their study published in the Archives of Pediatrics & Adolescent Medicine.

Children may see violence on TV, and time spent watching TV may mean less time for behaviours that help kids develop positively, such as reading or playing.

"We really don't know what's going on for certain," Manganello said, adding that future research was needed to look both at TV content and at what's going on in a child's home when the TV is on.

But Manganello said the findings show that parents have to consider the "overall TV environment" of the home, as well as how much TV their child is watching.