Which Are More Legible: Serif or Sans Serif Typefaces?
http://www.alexpoole.info/academic/literaturereview.html

Poole, Alex
Alex Poole
2005

Abstract:

In 1998 when Times New Roman was still widely used on the web, my then boss made sure we always designed our medical web sites with Arial, as she hated the look of serif fonts on the web. Was it the case that sans serif fonts were more legible, or was it just a matter of taste? In an effort to get at the truth, I reviewed over 50 empirical studies in typography and found a definitive answer.