A dentist remarked to me today that whilst women tend to show their upper teeth when they open their lips, men tend to show their lower teeth. Never having thought about this before I found it remarkable, but convincing.
Clearly either men's teeth are higher in their heads than are women's in theirs, or men's lips are lower in their heads than are women's in theirs; or (of course) both.
It now troubles me that I do not know which.