Did they though? When I read old books vs new books, I don't notice the difference at all. Especially with justification, spaces are already quite variable per line.
I don't think it aided reading at all. It was just a kind of random convention -- maybe somebody thought there was a logic to it -- that got dropped because it wasn't doing anything. One less rule to worry about.
The difference in reading speed only applies to people who are used to double-spaces, there is no difference in reading speed single-spaced vs. double-spaced for people who are conditioned to read single-spaced.
> Although the type of spacing following punctuation marks did not seem to have an effect on those individuals who type with one space after a period, those who type with two spaces after a period had greater reading speed when paragraphs were presented in the same way in which they type: with two spaces following periods and one space following commas.
I'd argue this data suggests the double-spaces are a waste of space that accomplishes nothing except slightly handicapping people who become accustomed to it and then read text without it.
Or maybe slightly boosting when reading double spaced? (I didn't read the article yet, my plane is just starting).
The habit was adopted for monospace typewriters precisely to try to match book typesetting.
Then the books dropped it, but it persisted in typing habits.
Then once Mac/Windows allowed anyone to use proportional fonts, we were taught to use a single space again.