Beards, Razors, and History
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.