One thing I find interesting about modern culture is that as violent and sexual imagery becomes more prevalent and easier to maintain than ever, actual violence and cases of sexual assault is trending down.
Studies have shown that almost from infancy, people have very little trouble distinguishing fantasy from reality, and adjusting their behaviour to suit - that is, at the moment. Lately, virtual reality has started to take off again in the form of the Oculus Rift and similar devices in development, and, having experienced it myself (I own one of the prototypes), I can say the feeling of
presence experienced through VR is very different from looking at a screen.
What I found most shocking about it wasn't the experience, but rather the memories of having tried it; in retrospect, my recollection of scenes from VR feels much the same as how I recall moments of my actual life, the only real difference being that I remember which parts were real and which weren't because of the absurdity of the scenes themselves. That said, things might change as the fidelity of virtual worlds increases and the feeling of presence becomes stronger.
I think if there's any consistent cause of violent behaviour in people, the strongest tie seems to be with trauma experienced in childhood.