Are children affected by violence on TV?

Nowadays we live in a society where the most of the people have a TV or a computer at home.
It can look like a good thing but the TV can cause serious problems to the children.
Now I'm going to explain why.
There are a lot of violence on TV and the children can watch it because the violence's programs are usuals on TV. When they watch this programes they can belive that it is a good thing and they can try to do the same.
For this reason there are terrible events like the columbine masacre witch happened a few years ago.
For all this reasons and more parents should control their children, by this way they couldn't watch violent's programmes and maybe the terrible events like the columbine masacre wouldn't be usuals among the population.