Clear, legible fonts are supposed to be best for presenting information.

Not always...

I was reading about the "Cognitive Reflection Test", a short set of "brain teaser" questions that have intuitively appealing wrong answers. It turns out that presenting such problems in a "disfluent", degraded font actually results in significantly more correct answers! The effort in reading the text apparently encourages systematic and analytic, rather than quick heuristic, processing.

Try it for yourself:

(I deliberately tried to make the questions show up in a small, italicized font, in a very light gray on white, similar to the conditions in the study. Another group got normal-sized black-on-white text in the same typeface.)

Try to read the questions as is, without cheating by, say, selecting the text. Of course, the actual test subjects weren't given the background information I wrote!

The Cognitive Reflection Test: "Cognitive Reflection and Decision Making", Shane Frederick, Journal of Economic Perspectives 19(4), Fall 2005, pp. 25-42.

"Overcoming Intuition: Metacognitive Difficulty Activates Analytic Reasoning", Adam L. Alter, Daniel M. Oppenheimer et al., Journal of Experimental Psychology: General 136(4), 2007, pp. 569-576