I have always learned that, since the word processor supplanted the typewriter, that it was wrong to persist with the time-honored practice of two-space sentence separations.  But I never quite understood the argument for one-spacing.  Now a group of Skidmore College psychological researchers, in a study published in Attention, Perception and Psychophysics, prove reading is more efficient, albeit only slightly so, if sentences are more separated.  I might very well go back to two spaces after periods. (If you were wondering, two-spacing after commas was not beneficial, and indeed slowed down reading speed.) Ironically, the study was submitted to the journal two-spaced, but the journal’s proofreaders deleted all the extra spaces. 

Via Washington Post