Museums are paying to send stealth observers through their galleries. Based on what they see, the museums may rearrange art or rewrite the exhibit notes. Their efforts reflect the broader change in the mission of museums: It's no longer enough to hang artfully curated works. Museum exhibits are expected to be interactive and engaging. As well, many foundations and donors are requiring proof that their funding is well-spent, and the studies provide data to show a rise in traffic or exhibit engagement.

(Photo by Fabrizio Costantini for the Wall Street Journal)