I read Penny's link. It is based on only two tribes, one in South America and one in Africa.

Presumably the author searched the literature until finding two tribes out of all those available that supported his/her prejudice.

I was amused by the parellel to boxing. I doubt very much if these poeple engage in conflict by standing up, man-to-man, and demanding a fair fight. No, skulking in the underbrush and throwing a spear (blow gun, rock from a sling) is lot more like it. In that case, handedness doesn't matter at all.

To me the interesting question is why does the South American tribe show 22% left-handedness. (How exactly do you determine handedness in unlettered people, anyway?)