I think that's a fairly generalized view of how the mind works...There are a bunch of psychological and social factors that play into it...
Like first of all, how in tune with reality is a person? Are they diagnosed with a personality or psychotic disorder? On the social side, were they taught what is and what is not socially or morally wrong? Someone who is taught that shooting people is bad is obviously going to be affected differently than someone who has not, and going back to mental disorders, one with schizophrenia is obviously going to react differently (and theoretically more negatively) than someone who does not...
You can't just say "Oh games will definitely change you," because that's not the case at all...
(And if anything, the military example is more an example of "familiarity"...Someone who has never fired a gun is holding something alien to them; they don't know what it will do when it fires, the kickback, etc...After putting yourself in the shoes of someone holding a gun, you are more familiar with it, and thus more confident to pull the trigger in real life...Morality is an impossible thing to judge, and thus can't be tested clinically...
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The military example comes from a study in which, they noticed human reluctance to fire in war situations and take a life. That trained soldiers, not people that have never fired a gun, still hesitated when targeting a human and taking a life. Then they started modeling targets to look like people, and found that people were finding it easier to target people. The mind altered some and the hesitation was gone. As they began to make the targets more realistic, even using computer characters with realistic death it became easier and easier. This is the mind changing.
There was another study that showed that behavior actually crystallizes and changes the how you process and think. The idea they were exploring was how patterns in behavior and how habits are created. I believe the media we take in has this effect on us.
I believe we do desensitize ourselves to whatever it is we expose ourselves to in life, and that doesn't mean it is always a bad thing, it is just habit. For example new parents desensitize themselves eventually to the sound their children crying or making crazy noises...but they can always tell when the baby is really in trouble.
And, I don't think this is a reason to ban all violence or sex, because you are right...exposure does not mean you are going to become a killer, a rapist, a pedophile, a racist, or whatever...and it doesn't even mean you are going to shift your moral compass and believe those behaviors are ok. What it does mean that behavior won't have the exact same effect on you as if you were never exposed to it. And that could be good or bad.
I do believe we should be careful what we expose ourselves to, and I think that even though we value the freedom of speech, and artistic expression we as a society and culture should place regulate in material to safe guard children and adults that do not want exposure. This regulation should never be forced or controlled by the government, but freely done by the industry.
And as the examples of the racist games existing and the child porn games existing, those are not created by the mainstream industry, nor would they even be accepted in anyway by the industry as a whole...those are fringe projects, and that type of media exists in all media forms and should not speak for what the industry is willing to accept.