In a 2008 study, Hyunjin Song and Norbert Schwarz found that when instructions for a task are presented in a font that is difficult to read, readers believe that the task will take significantly more time to complete and are significantly less motivated to attempt it.
This underscores the importance of making your presentations easy to understand, particularly when you are asking your audience to do something. (This principle applies to both your words and your visual aids—all parts of your presentation, really.) When the audience finds it difficult to understand your presentation, this will cause them to view what you’re asking them to do as more difficult, and they will be significantly less likely do it. On the other hand, if you make everything easy to understand, your audience will think what you’re asking them to do is easier, and they will be significantly more likely to do it.
Note: In a followup study in 2009, Song and Schwarz found that when the names of food additives or amusement park rides were hard to pronounce, they were perceived as riskier than those with names that were easy to pronounce.