And in combat, the southpaw would have exactly the same setup as the right-handed majority, because that's the weapons that are around. Even in today's more enlightened world, people are still learning to write right-handed because they're more or less compelled to. In the world of ancient or medieval combat, lefties would be at a distinct disadvantage having to use stuff designed for right-handed warriors, or (and this has a natural selection angle) would avoid combat, which is why the characteristic hasn't been bred out. Mostly likely, since humans are naturally inclined to mortal struggle, they'd resort to sneaky left-handed dagger thrusts, basically becoming assassins.
There’s a medeival story about a Scottish clan, mostly lefthanders, who designed the castle for left handed fighting.
Can’t remember the name.
“In the world of ancient or medieval combat, lefties would be at a distinct disadvantage having to use stuff designed for right-handed warriors”
I would think that this point is obviously correct right up until the defensive formation breaks down (and it always does). Once one side ‘breaks’ that is where the killing really begins. And that may be where the left-hander gains an advantage. I don’t know this to be true, but I’m suggesting a reason that Left-Handedness managed to survive the tens of thousands of years of muscle-powered weaponry.