As part of a academic study I once wrote a psychology piece on young children exposed to a diet of violent television. Not ultra violent, but violence where the consequences weren't realistic. Such as cartoon violence, like Tom & Jerry, for example. Acceptable stuff still to this day, in many parts of the World. I proposed an experiment in which there would be TV's quietly showing cartoon violence while children were at play in a nursery environment. The children would be monitored by closed circuit cameras and the nature of their behaviours recorded. For a control I proposed another class where the TV background would be totally non-violent TV. This control group would also be monitored. Both groups would contain a cross section of nursery children.
This was in the era prior to the internet, so imagine my surprise when a lecturer came up to me one day and said she'd seen my piece and did I know that actual tests had been conducted in the USA, [I think] during the 1960's where the content of violent TV was monitored for its effect on young children in a controlled environment. In all the tests, the behavioural change in the children being shown the violent TV was statistically significant. In fact, in a lot of cases the children copycatted the behaviour even though there was little actual evidence that were they'd been even looking at the TVs. The control group children's behaviours did not change.
If I remember rightly, the change was so dramatic that the tests were cut short after protests were made by the parents, although I may have not remembered that last bit correctly. I think the experiment series has only been tried once. I might even look it up. There must be some stuff about it on the net.