The usual difficulty is that it’s impossible to find, say, job seekers who are absolutely identical in every respect except race. As a result, it is difficult to conclude whether a white job seeker succeeded — and a black one didn’t — because of discrimination. While statistical techniques can adjust for some of these factors — education, geography and the like — no analysis can account for all of them.

But the new research allows for a clearer conclusion: It appears to have documented straightforward discrimination.

As a real-world experiment, it built on earlier “audit experiments,” as they are known in social science. Perhaps the most famous is a study by Marianne Bertrand of the University of Chicago and Sendhil Mullainathan of Harvard (who is a regular contributor to this column). In that earlier experiment, Ms. Bertrand and Mr. Mullainathan sent fictitious résumés to employers, finding that people with white-sounding names were more likely to receive a positive response than those with black-sounding names.

The new findings provide further indication of the many ways in which discrimination shapes the lives of African-Americans. What’s more, when it’s harder to get your neighborhood librarian to respond to a simple email about opening hours, it’s not much of a leap to imagine other interactions — dealing with a computer help desk, the front office at a school or just the dry cleaner — that go less smoothly.

Economists tend to group explanations of discriminatory behavior into two buckets: taste-based and statistical. If a librarian chooses not to respond because a person is black, that’s taste-based discrimination. In common speech, there’s a simpler label: racism.

Statistical discrimination, on the other hand, occurs when a librarian uses a person’s name or race as a marker for other characteristics. Perhaps an African-American-sounding name signals that a person is more likely to be poor. The librarian happens to be biased against poor people. In this case, race is being used as a statistic for inferring poverty, and it’s the perception of poverty that causes the discriminatory behavior.

But two pieces of suggestive evidence in this study point to the problem here as being straightforward, taste-based discrimination.