Parents have wondered for years, and now we learn violence on TV translates into aggressive behavior in boys.  That's according to Seattle researchers set to release the findings of a study that spans years. 

The new study looked at specific types of children's programming.  It targeted pre-school age kids and then checked in with them again several years later.   

Seattle area researchers say many of the shows that children watch, from Power Rangers to aggressive cartoons, are violent and are having a serious impact on young boys. 

"Clearly we need to do more research to find out in fact of the effects are limited to boys and not to girls, but we did find it in boys.  We found a significant increase with violent TV," said Dr. Dimitri Christakis, University of Washington professor of pediatrics. 

Christakis is one of the researchers in the study published in this month's issue of Pediatrics.  They looked at the types of programming watched by 184 boys and 146 girls between the ages of two and five.  They then looked at those same children when they were seven to ten years old.  The results were astonishing. 

"For each hour of violent television that kids watched during the preschool age, their chances of having problems with aggression at school age were increased by almost four times," said Christakis. 

The increased aggression pertained only to boys not girls. Researchers say that's likely because they tend to choose more violent programs.   

Seattle parents like Jim and Deann Flack say they've always been fairly careful about what their  7-year-old son Nick watches, especially when it contains violence. 

"We make sure that we talk to Nick about the reality and unreality of such programming," said Nick Flack. 

Researchers say that discussion is vital but parents still need to be aware of the content of shows their kids are watching. 

"I think the important message for parents is what their children watch is very important and they should be very selective what their kids watch," said Christakis. 

So what constitutes violent behavior?  According to the report, it ranges from disobedience to extremely violent behavior. But Christakis says this report is not an indictment on TV watching for kids.  The study also found that educational TV for pre-schoolers had no effect and praises high quality learning shows.