In the fifth chapter of Thinking, Fast and Slow, [Daniel] Kahneman addresses the act of reading, and the impulse—even in what is normally thought of as the most cerebral of human acts—to fall back on System 1 [reading that involves making quick, unconscious assessments], to associate the ease of reading with the truth of what is read:
How do you know that a statement is true? If it is strongly linked by logic or association to other beliefs or preferences you hold, or comes from a source you trust and like, you will feel a sense of cognitive ease. The trouble is that there may be other causes for your feeling of ease—including the quality of the font and the appealing rhythm of the prose—and you have no simple way of tracing your feelings to their source.
Thus the context writing exists in and other aspects unrelated to the actual content are critical to the reception that writing receives. In addition to studies on the effects of different fonts on credibility, Kahneman also cites experiments that show the importance of the quality of paper (for printed materials), of the contrast between a font and its background, and of the presence of distractions that reduce the cognitive ease of reading. In short, environments that make it easy to read also make it easy to believe what is being read. Perhaps the most unsettling aspect of this mixture of context and content is that is it extremely difficult for you to separate the two.
So legibility and the absence of distractions are not just design niceties; when a reader chooses to move an article into an app like Instapaper, they are strongly increasing the odds that they will like what they read and agree with it. And since readers often make that relocation at the recommendation of a trusted source, the written work is additionally “framed” as worthy even before the act of reading has begun.