"We found the more violent TV children see as preschoolers, the more likely they are to have anti-social behaviors -- acting aggressively, disobeying, getting in trouble -- at school age," said Dimitri Christakis, a lead author of the study, published in the scientific journal, Pediatrics.
"Cartoons are the main culprit," he told AFP.
"Most parents consider cartoons not threatening to their children because, after all, they're not real and they're just funny. But the truth is that preschool children don't distinguish between fantasy and reality the way older children and adults do. To them it's all very real.
"Precisely because cartoon violence is intended to be funny and depicts violence without real consequence -- even if people get blown up, they're black for a second and then return to normal -- it conveys the wrong messages about the effects of violence in the real world," Christakis said.
Neither preschool girls who watched violent programs nor children who watched non-violent or educational TV showed aggressive behavior later, the study conducted by researchers at the Seattle Children's Hospital Research Institute found.
The researchers used data gathered in a 40-year study of 8,000 US families to review what types of programs were watched by 184 boys and 146 girls between the ages of two and five and what behaviors they displayed later.
Programs classed as violent for the study included Power Rangers, Star Wars and American football matches.
Non-violent programs included Toy Story and the Flintstones, while Sesame Street and the Magic School Bus were among the educational shows.
"It's not just about watching TV, it's about what children watch," Christakis said.
"The take-home message to parents is that they have to be very mindful of what their children watch, and if they're careful about it, there's no harm in selective TV viewing.
"If they're not, there really are risks in terms of children's behavior," Christakis said.