User blog comment:NicoTDaddicted/You are not gonna believe this.../@comment-1815989-20120408043227/@comment-1874924-20120408154117

I've always assumed that Duncan's head shape (and, for that matter, the prototype Eva's) was meant to indicate a lantern jaw; but because of the way the mouths are placed for animation, the suggestion doesn't really work. (For those who don't know the term, "lantern jaw" means an undershot jaw, i.e. one where the lower teeth are in front of the upper teeth when the mouth is closed.)