Are open access papers read by a larger readership than paywalled papers? One of the potential upsides of open access papers is that they can be accessed by anyone for free, including people who don't live or work in a university. As a result, one might think that non-"professional" researchers may access open access papers more than paywalled papers.

Is there any research/study/survey that tried to quantify to what extent open access papers are read by a larger readership  than paywalled papers?
This answers is not exactly what you ask for — you ask for readership, but most of the research has focused on citations. The two are, of course, related, and the answer seems to be a pretty clear yes. (In my field, it's practically the only way I find stuff from overseas, as many online repositories don't seem to have much international coverage except for a handful of big general journals but not the niche ones).


Gargouri, Yassine. “Self-Selected or Mandated, Open Access Increases Citation Impact for Higher Quality Research.” Public Library of Science ONE 5.10 (2010): n.p. Web.
This article indicates that citations are more common for open access articles, and that this is a result of higher availability and access (=likely readership) by researchers. [Link]
Lawrence, Steve. “Free online availability substantially increases a paper’s impact.” Nature 411 (2001): 521. Print.
This comes to the same conclusion — way back in 2001. The effect is found to be quite massive: “Averaging the percentage increase across 1,494 venues containing at least five offline and five online articles results in an average of 336% (median 158%) more citations to online computer-science articles compared with offline articles published in the same venue.” [Link]