Experts have long known that children imitate many of the deeds - good and bad - that they see on television. But it has rarely been shown that changing a young child’s viewing habits at home can lead to improved behavior.

In a study published Monday in the journal Pediatrics, researchers reported the results of a program designed to limit the exposure of preschool children to violence-laden videos and television shows and increase their time with educational programming that encourages empathy. They found that the experiment reduced the children’s aggression toward others, compared with a group of children who were allowed to watch whatever they wanted.

“Here we have an experiment that proposes a potential solution,” said Dr. Thomas N. Robinson, a professor of pediatrics at Stanford, who was not involved in the study. “Giving this intervention - exposing kids to less adult television, less aggression on television and more prosocial television - will have an effect on behavior.”

While the research showed “a small to moderate effect” on the preschoolers’ behavior, he added, the broader public health impact could be “very meaningful.”

The new study was a randomized trial, rare in research. The researchers, at Seattle Children’s Research Institute and the University of Washington, divided 565 parents of children ages 3 to 5 into two groups. Both were told to track their children’s media consumption in a diary that the researchers assessed for violent, didactic and prosocial content, which they defined as showing empathy, helping others and resolving disputes without violence.

The control group was given advice only on better dietary habits for children. The second group of parents were sent program guides highlighting positive shows for young children. They also received newsletters encouraging parents to watch television with their children and ask questions during the shows about the best ways to deal with conflict. The parents also received monthly phone calls from the researchers, who helped them set television-watching goals for their preschoolers.

The researchers surveyed the parents at six months and again after a year about their children’s social behavior. After six months, parents in the group receiving advice about television-watching said their children were somewhat less aggressive with others, compared with those in the control group. The children who watched less violent shows also scored higher on measures of social competence, a difference that persisted after one year.

Low-income boys showed the most improvement, though the researchers could not say why. Total viewing time did not differ between the two groups.

“The take-home message for parents is it’s not just about turning off the TV; it’s about changing the channel,” said Dr. Dimitri A. Christakis, the lead author of the study and a professor of pediatrics at the University of Washington.

“We want our children to behave better,” Dr. Christakis said, “and changing their media diet is a good way to do that.”

Until she began participating in Dr. Christakis’s trial, Nancy Jensen, a writer in Seattle, had never heard of shows like Nickelodeon’s “Wonder Pets!,” featuring cooperative team players, and NBC’s “My Friend Rabbit,” with its themes of loyalty and friendship.

At the time, her daughter Elizabeth, then 3, liked “King of the Hill,” a cartoon comedy geared toward adults that features beer and gossip. In hindsight, she said, the show was “hilariously funny, but completely inappropriate for a 3-year-old.”

These days, she consults Common Sense Media, a nonprofit advocacy group in San Francisco, to make sure that the shows her daughter watches have some prosocial benefit. Elizabeth, now 6, was “not necessarily an aggressive kid,” Ms. Jensen said. Still, the girl’s teacher recently commended her as very considerate, and Ms. Jensen believes a better television diet is an important reason.

The new study has limitations, experts noted. Data on both the children’s television habits and their behavior was reported by their parents, who may not be objective. And the study focused only on media content in the home, although some preschool-aged children are exposed to programming elsewhere.

Children watch a mix of “prosocial but also antisocial media,” said Marie-Louise Mares, an associate professor of communications at the University of Wisconsin, Madison. “Merely being exposed to prosocial media doesn’t mean that kids take it that way.”

Even educational programming with messages of empathy can be misunderstood by preschoolers, with negative consequences. A study published online in November in The Journal of Applied Developmental Psychology found that preschoolers shown educational media were more likely to engage in certain forms of interpersonal aggression over time.

Preschoolers observe relationship conflict early in a television episode but do not always connect it to the moral lesson or resolution at the end, said Jamie M. Ostrov, the lead author of the November study and an associate professor of psychology at the University of Buffalo.

Preschoolers watch an estimated 4.1 hours of television and other screen time daily, according to a 2011 study. Dr. Ostrov advised parents to watch television with their young children and to speak up during the relationship conflicts that are depicted. Citing one example, Dr. Ostrov counseled parents to ask children, “What could we do differently here?” to make it clear that yelling at a sibling is not acceptable.

He also urged parents to stick with age-appropriate programming. A 3-year-old might misunderstand the sibling strife in the PBS show “Arthur,” he said, or stop paying attention before it is resolved.

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By CATHERINE SAINT LOUIS