How about both ? (Score 1)

There are plenty of studies that prove violent content has a direct impact on immediate behaviour, and behaviour in the next hours.

I think that poor gameplay is only an additionnal factor to violent behavior linked to video game use.

They should take 5 goups, one playing GTA 5, one playing a non-violent videogame with a decent gameplay, and one playing a regular game (cards, monopoly, etc...) to check the difference.

BTW, violent video-games have not been proven to exacerbate violence, but to diminish empathy, which can be equally dangerous.