Beards, Razors, and History
I have been traveling, carelessly forgot to pack my electric razor, and so had to make do with a safety razor—an experience that suggested an interesting conjecture. A safety razor is easy to use on flat parts of the face such as cheeks and temples. It is harder to use and more likely nick you where the face is more curved, for example on the chin. I have never shaved with an old fashioned straight razor, but I assume the same pattern would hold.
The conjecture is that styles in men's facial hair are in part influenced by which parts of the face are harder or easier to shave. A mustache plus goatee, for example, leaves unshaven precisely the parts of the face that are hardest to shave, judged at least by my experience. If that conjecture is correct, the invention of the electric shaver ought to have resulted in some shift away from that style in favor of others.
Actually testing the conjecture would require a good deal of work, so I will leave that part of the project to others.