I take issue with the classification of World of Warcraft as a “violent game” in this study, but I suppose it’s more violent than Animal Crossing. The study, done at Middlesex University in the UK, had just under 300 people of all ages playing WoW for a couple hours, and found that players were more tired and calm after their gaming session.
It’s not a really in-depth study, but it’s a nice to have something to point to when ignorant people suggest that these games are just murder simulators that rile up kids and give them some unholy blood lust. With WoW, of course, the risk is more for loot lust.