When I learned typography in design school, we always assumed that Serif fonts are slightly more readable than Sans-Serif fonts, since they have extra details that differentiate similar letters such as lower case l and upper case I (see what I mean?).
After graduating and working on digital products for several years I can safely say that turned out to be true, and that:
- Serif fonts are easier to read and distinguish between letters in a normal sized font size (15px and upwards).
- Serif fonts connote Reading, rather than Scanning. Reading is something you do with one body of text over long periods of time, Scanning is something you do with multiple bodies of text over fractions of a second and up to several seconds.
- Historically, Sans-Serif fonts render better on a screen since they are more geometrical and fit better on the rectangular pixel grid (rather than more organic shapes of a Serif font). That constraint (which is still prevalent, but less so since the introduction of Retinae screens), dictated the use of Serif fonts for smaller bodies of text on a screen and thus cemented the use of Sans-Serif fonts for screen based typography.
For example, these two lines of text are in Sans-Serif andSerif fonts respectively. Both are in 10px font size. The first is pretty light-weight, each letter is distinct, has ample white space around it and is definitely readable, while the other is heavier, with letters such as a, s, e and g (detail heavy and low white-space letters), somewhat blurry their inner white spaces almost completely blocked.
Serif fonts do make a comeback in recent years: they're beautiful, more distinct than Sans-Serif fonts and modern screens can easily accommodate their higher level of detail. Don't expect them to replace Sans-Serif fonts for system text other Scannable text, but I do hope to find them in more and more digital products.