In what may be one of the most controversial studies of the year, researchers at Skidmore College—clearly triggered by a change in the American Psychological Association (APA) style book—sought to quantify the benefits of two spaces after a period at the end of a sentence. After conducting an eye-tracking experiment with 60 Skidmore students, Rebecca L. Johnson, Becky Bui, and Lindsay L. Schmitt found that two spaces at the end of a period slightly improved the processing of text during reading. The research was trumpeted by some press outlets as a vindication of two-spacers' superiority.
For anyone who learned their keyboarding skills on a typewriter rather than a computer—and for the many who developed their keyboard muscle memory using software packages such as Mavis Beacon Teaches Typing—the double-space after the period is a deeply ingrained truth. While modern style, based on the fallacy that computer typography makes such double-spaces redundant and Paleolithic, has demanded the deprecation of the second tap of the space bar after a punctuation full-stop, many have openly resisted this heresy, believing that the extra space is a courtesy to the reader and enhances the legibility of the text.
I learned the two-spaces rule, but switched to one space after hearing more about it. I don’t have a strong preference. I think that one-space will win, because that’s the default in HTML (online content), even when you add an extra space.
For example, there are two spaces after this period: . <– but if you copy/paste it from the web page, there will only be one space there. The solution to adding an extra space there would probably not be very elegant from a coding perspective, as people would have to type
every time they wanted to insert two spaces in web page text.