Before she sang, Ms. Boyle seemed to be merely a frumpy 47-year-old unemployed church volunteer who lived alone with her cat, Pebbles, and had, she said, “never been kissed”

Now, after the video of her performance went viral, a flurry of commentary has focused on how we stereotype people into categories, how we fall victim to the prejudices of ageism or look-ism, and how we should learn, once and for all, not to judge books by their covers.

On a very basic level, judging people by appearance means putting them quickly into impersonal categories, much like deciding whether an animal is a dog or a cat. “Stereotypes are seen as a necessary mechanism for making sense of information,” said David Amodio, an assistant professor of psychology at New York University.