“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 last week in the journal Attention, Perception, & Psychophysics.
They cite dozens of theories and previous research, arguing for one space or two. A 2005 study that found two spaces reduced lateral interference in the eye and helped reading. A 2015 study that found the opposite. A 1998 experiment that 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.”
I’ll circle back to read the full journal article shortly.1