1 space after periods redux

I knew there was a reason why you're supposed to use just one space after a period when you're typing on a computer, but I couldn't remember what it was.

Turns out that, unbeknownst to me, my Downloads folder holds the answer, in a document entitled "Problems Noted on Student Writing":

This change occurred in academic writing as a result of the use of computers that have proportional fonts capable of adjusting the spacing as needed. This rule has been the standard in journalism and publishing since computers have been used for word processing purposes. 

Have I ever come upon a document I didn't download?

Sometimes I wonder.