http://www.slate.com/articles/technology/technology/2011/01/space_invaders.html
The article suggests the cause of the double space may be manual typewriters with their monospace font. But we all use proportional fonts these days.
Then again there’s someone else saying exactly the opposite (and backing it up with their own evidence): Why two spaces after a period isn’t wrong (or, the lies typographers tell about history)
…and when I’m composing a plain-text e-mail which is usually rendered in monospace?
Besides, if two-spacing is so common, why can’t typesetting software accommodate this and kern one-space and two-space the same?
Interesting article by someone else, stating exactly the opposite of the original article I posted (and backing it up with evidence)
http://www.heracliteanriver.com/?p=324