Ever since there has been typesetting and printed text, there has been a debate about spacing, particularly about the spacing after a period – should there be one space or two?
The first printers used two space sizes. A regular space was used for separating words, and a slightly larger space (called the emspace) was used for separating sentences. When typewriters gained widespread use, the emspace was replaced with two spaces, and this style of writing became the standard for decades. Around the mid-1900s, with the advent of more advanced typesetting, printing companies were able to achieve uniform spacing, and adding two spaces after periods fell out of fashion. Then, when proportional fonts were developed for computers and word processors, the double space (supposedly) served no purpose except when using monospaced fonts. Take a look at the following example:
Of more importance is that proportional text in magazines, books, and newspapers is usually both right and left justified. In such cases, the typesetting equipment ignores your space preference completely; it puts in just enough space between words and after punctuation to make the line lengths come out right.
Rebecca L. Johnson, Becky Bui, and Lindsay L. Schmitt, psychology researchers from Skidmore College, decided it was time for modern science to weigh in on the topic.
“Professionals and amateurs in a variety of fields have passionately argued for either one or two spaces following this punctuation mark,” they wrote in a paper published in the journal Attention, Perception, & Psychophysics. They cite dozens of theories and previous research – a 2005 study found two spaces reduced lateral interference in the eye and helped reading; a 2015 study found the opposite; and a 1998 experiment suggested it didn’t matter.
“However,” they wrote, “to date, there has been no direct empirical evidence in support of these claims, nor in favor of the one-space convention.” So they set out to get some.
They gathered 60 students to participate in the experiment. Their first step was to place them in front of a computer and have them type a short paragraph. This revealed how many spaces they used naturally. Twenty-one used two spaces, and the remaining 39 used only one.
Next, they clamped each student’s head into place, and used an Eyelink 1000 machine to record where their eyes looked as they read 20 paragraphs silently. The paragraphs were written in various styles: one-spaced, two-spaced, and even strange combinations like two spaces after commas, but only one after periods, or vice versa. Their conclusion? Two spaces were better.
Well, as Lifehacker’s Nick Douglas pointed out, there are some important caveats that affect the study’s conclusion. The major issue was that the subjects read paragraphs in Courier New, a fixed-width font similar to the old typewriters; it is only rarely used on modern computers, and it kind of undermines the purpose of spacing in the modern age. The major reason for using two spaces, though, according to the researchers, was to make the reading process smoother, not faster, and all subjects tended to spend fewer milliseconds staring at periods when a second space followed it.
What’s of greater importance is how the findings relate to reading speed and reading comprehension. Reading speed, the researchers found, improved only marginally. For “one-spacers,” speed increased about 5 WPM with one space. For “two-spacers,” speed increased about 9 WPM with two spaces. Bottom line: people liked what they liked, they were most comfortable with what they were used to, and the difference between them wasn’t very much anyway. Reading comprehension was not affected for any of the subjects, no matter how many spaces followed the period.
What would have been a more interesting study would be one that looked at proportional fonts, since those are the types most commonly encountered in our daily lives. It would also be interesting to look at the difference in readability between a both-justified text layout and a left-only justified one (one with a ragged right margin). With some exceptions, it has become increasingly common to use left-justified-only text because it keeps the spacing constant between words and therefore increases readability. For now, though, follow whichever style guide you want, just make sure you are consistent in its use, or that may interfere with readability itself.
 Drum, Kevin. (May 6, 2018). One Space or Two? That Recent Study Won’t Tell You. Retrieved from https://www.motherjones.com/kevin-drum/2018/05/one-space-or-two-that-recent-study-wont-tell-you/