Ever since the invention of movable type, the debate has raged: Are two spaces after a period better than one? The French said “Non!” from the beginning, using one space only. The British said “Aye!” and established their own two-space rule.
Now, three psychology researchers at Skidmore College are settling the debate with a study published in the journal Attention, Perception, & Psychophysics. Their study demonstrates that using two spaces makes reading “smoother.” In other words, your eyes spend a few milliseconds less on a period if it’s followed by two spaces rather than one. But before all you two-spacers out there start gloating like the annoying pestiferous bunch you are, here’s the caveat: The study also shows that it doesn’t actually make overall reading faster or your comprehension any better–unless you are a two-spacer to begin with.
The war of sentence spacing has been a long and tortuous one. Back in the 18th century, some printers used French-style single-spaced sentences and others the English double-space rule. It wasn’t until the mass printing era after World War II that most American books turned to single-spaced sentences to reduce costs and speed up production. The 1941 IBM Executive typewriter also introduced proportional spacing–which meant that each letter took only the horizontal space it needed instead of being monospaced. Things looked much better that way when using a typewriter, since it effectively eliminated the need for two spaces after every period. Today, computers and proportional fonts make the two-space rule absolutely useless–even while recalcitrant two-spacers continue to write eye-twitching emails using the English rule, claiming the rest of us rational people are wrong.
Skidmore College psychology researchers Rebecca L. Johnson, Becky Bui, and Lindsay L. Schmitt decided to remove the subjectivity from this longstanding debate by recruiting 60 students for a thorough test. First, they divided the students into groups–“one spacers” and “two-spacers”–by dictating a text for them to type into a computer. (Just 21 of them were two-spacers, for the record.) Next, the students donned eye-tracking devices and read through several text paragraphs that contained double spaces, single spaces, and random spacing after periods and commas. The computer tracked their eyes as they read to observe and analyze how their eyes moved involuntarily.
The results are pretty conclusive: Reading speed wasn’t slowed down with either single or double space after a period. The two-spacers experienced a marginal speed improvement while reading double-spaced sentences. Reading and comprehension was completely unaffected no matter the spacing rule they used. The paper argues, however, that readers spend “fewer milliseconds” looking at periods when there are two spaces after them, which they claim makes for a “smoother” reading experience. On the other hand, when they added an extra space after a comma, the reading speed diminished across the board. The eye tracking software demonstrated that our eyes have difficulty when they find extra spaces after a comma (which explains why my eye twitches every time I see one).
The lesson is that it doesn’t really matter if you use single space or double space after a period. It won’t affect the reading experience objectively. And if you insist on a double space, take extra care not to add one after commas–because that won’t just annoy the hell out of your readers, it’ll slow them down, too.