"...The rate of heat loss is relatively the same for any exposed part of the body, not simply the head..."

I just guessing here, but I suspect that most of us who buy into the hat thing do so on the assumption that, while outdoors in the cold, the ONLY part of the body that is exposed is the head, and possibly the hands. Most of us are wearing some form of clothing from the ground up, so if you are getting cold anywhere else, cover that exposed part with a hat, and you will feel warmer...