http://scratchtap.com/word-compounding-strategies-as-visual-metaphor/
>>> It seems to me that the idea of visual metaphor may be behind our word-compounding strategy choices. I propose that words (at least nouns) written with a space are seen as most disconnected (by which I mean that the two compounding elements are perceived as having little to do with each other) or novel, whereas words written together as seen as most cohesive, and words written with a hyphen are somewhere in between.
Things may be written as two words when they are new concepts, then make their way to hyphenation and finally one-wordedness as they become more widespread. <<<
his example of [electronic mail] --> [e-mail] --> [email] is a good one.
i can imagine that when it was new, a Doughnut was often written as [Dough-nut] or with a Space