From what I read, people who are 100% left-handed are rare. You and I, Green Man, who use one hand or another for various tasks, are more common. However, we don't have a word for that.

We label a person lefthanded merely if she writes with her left hand. That's simplistic.

As for that tribe that's lefthanded and more violent, I don't believe it exists. Or if it does, we need to look at all the facts about it.

(Did you know that Eskimos do not have a large number of words for snow? Somebody made that up, and journalists have been quoting it for decades. It's probably the same for this convenient tribe.)