How much of this is just self-selection bias. Some body's are simply more adapted to the cold. I know people with passive metabolism of furnaces, my smart heate

How much of this is just self-selection bias. Some body's are simply more adapted to the cold. I know people with passive metabolism of furnaces, my smart heater doesn't run as often when the my significant other sleeps over. I've seen steam vaporing off some folks who aren't even working that hard in cold gyms. Me, I sweat profusely when exerting myself, would definitely be one of the first to die in a winter military campaign.