Among men, a “humanist” typeface resulted in a 10.6% lower visual demand as measured by total glance time as compared to the “square grotesque” typeface. Total response time and number of glances required to complete a response showed similar patterns. Interestingly, the impact of different typeface style was either more modest or not apparent for women on these variables. (Reimer, B., Mehler, B. & Coughlin, J.F., 2012, p. 1)The study supports the theory that "humanist" typefaces are more readable. As instructional designers, this means that we should use this style when we want to reduce cognitive load. Specifically, we want to use a humanist san serif font, especially for eLearning projects.
So, what is a humanist typeface? One notable characteristic is that there is very little difference between thin and thick strokes. The apertures of the letters are more open, with ample kerning, making it easier for the eye to move horizontally across a line of text. For examples, visit: https://typekit.com/lists/humanist-sans-serifs
In contrast, grotesque typefaces have less variance between similarly-shaped letters (i.e., i and l, or m and n). The characters have tight kerning, meaning there is little space between letters. The lack of contrast between letters makes text written in grotesque typeface ambiguous. Deciphering the words contributes to high cognitive load. Examples: https://typekit.com/lists/grotesque-sans-serifshttps://typekit.com/lists/grotesque-sans-serifs
For a reliable, versatile, easy-to-read humanist sans serif font, consider Ideal Sans. You can read the description and find suggested pairings at http://www.typography.com/fonts/ideal-sans/overview/
Sources:
Reimer, B., Mehler, B. & Coughlin, J.F. (2012). An Evaluation of Typeface Design in a Text-Rich Automotive User Interface. MIT AgeLab White Paper 2012-12. Massachusetts Institute of Technology, Cambridge, MA. (pdf)
Badger, E. (2014, April 7) A remarkably small idea that could reduce distracted driving. The Washington Post. Retrieved from http://www.washingtonpost.com/blogs/wonkblog/wp/2014/04/07/a-remarkably-small-idea-that-could-reduce-distracted-driving/