I wrote a somewhat similar article about an year ago: http://www.adpushup.com/blog/how-typography-affects-readers/
Quoting from the article: An experiment conducted by Kevin Larson and Rosalind Picard looked at how typography affects mood and performance of the readers. It happened across two studies, where they took three kind of measurements: Relative Subject Duration (RSD), Likert Scale and through a cognitive task which was different for both studies.
There were twenty participants, half of them got the text with poor typography and half got good typography.
In the first study, the participants were given twenty minutes to read the passage, amid which (after 15 minutes) they were interrupted and asked to estimate how long they had been reading for. After the RSD data was recorded, they were asked to fill out the Likert preference questionnaire with 6 statements, where one of the statements was whether they found the text easy to read.
Next, they were given a cognitive task, which for the first study was a Candle problem.
The second study was very similar to the first one, except for the RSD data, they were interrupted 17 minutes into reading, and the cognitive performance task was changed to remote associates test.
Results - Participants with good typography underestimated their time by 3 minutes 18 seconds in the first study and by 5 minutes and 21 seconds in the second study. This shows that when a passage is well structured, the readers are more engrossed in the text. - With the Likert scale questionnaire in the first study, the results were not reliable. But in the second study, some scores were in favor of good typography. - For the Candle problem, out of the 10 participants who were shown good typography, 4 successfully completed the task, while 0 participants completed the task in the other group. - In the Remote associates task, the results were not statistically significant or reliable with the good typography group succeeding on 52% of the task at an average speed of 6395ms, and the poor typography group succeeding at 48% of the task at an average speed of 6715ms.
Even though the result was not reliable enough in the second study, the good typography group did perform better. This can be attributed to the fact that they were in a better mood before starting the task. Hence, they were better at problem solving.