Leeneia, here's another link to reports on the paper. It mentions different tribes out of the nine studied. Apparently the study went further than that and did involve more than a literature search. I'm trying to find the original paper, but presumably it was peer reviewed. On the other hand, (oops, accidental, but I'll leave it) the link in the wikipedia article references doesn't go anywhere, so it might have been one of those things where scientists release stuff unreviewed to the press as part of their grant bid process. It seems that the study was related to the number of lefties in sports being larger than the proportion in the non-sporting population.
I don't think the idea was to say that lefties were more violent, but that a society which was more violent would favour lefties.
I would imagine that handedness would show up in pre-literate societies in craft activities as well as hunting and fighting. Any sort of making activity would need a dominant hand.
But one thing does bother me about the study, more than the high numbers of lefties in bellicose groups. The pacifist societies had far lower numbers than would be expected, and I wonder if those societies also have a strong pressure not to be lefthanded, with individuals learning to use their right hand rather than be ostracised. Most lefthanders can learn to use the right, as was shown in the case of handwriting in schools before the schools became enlightened. I don't like to think about the other possibility.