Grrr.... I try to play one campaign of L4D while my wife gets little one to sleep.  She comes down, and five minutes later I'm getting lectured about how she doesn't like this and violent games cause aggressive behaviour and they should all be banned... Then I get presented with various research papers, and lists of incidents that have been linked.... 
 
I have to say in some individuals it probably does have an effect - but then would those people be pre-disposed to violent films or books instead?  Or going drinking and picking a fight?  I agree with restricting violent game access to children, but ... I find the idea that all violent games should be banned is absurd... 
 
The trouble is I find it very hard to find research that backs this up.  Generally most research is looking for problems - so they ARE biased... But at the same time... If there are causal links there are causal links.  Personally I think people have killing and murdering each other for centuries longer than games have been about... You could argue that violent games are a representation of a darker side of human nature.  Even Chess, a simple game of chess is simulating battle - pieces taking each other are effectively pretending to kill enemy troops....  Personally I don't think you can change human nature.  Prohibition didn't work in the states, and people all over the world take drugs - whether it's legal to or not - even at the risk of a death penalty in some cases.  This is because it's human nature to seek out experiences, to be taken away from where you are to somewhere else.   
 
We can all enjoy a nice round of 'Top Spin' but you will never get the adrenaline rush, the sheer excitement in playing a round of tennis - than using weapons to fight for survival.  Left 4 Dead, Gears of War..  You have to perform some fairly depraved acts to suceed at this games.... 
 
I guess the debate will rage on - and I will continue to be infuriated... Tomorrow night I'll opt for a quiet life and have a game of FIFA, just have to make sure I don't do any 'rough' tackles when my wife is watching :(