The study also found that racial disparities persisted when the testers wore business attire or dressed in army uniforms. For example, testers wearing army uniforms were allowed to ride free 97 percent of the time if they were white, but only 77 percent of the time if they were black.

This elegant experiment follows in a tradition of audit testing, in which social scientists have sent testers of different races to, for example, bargain over the price of new cars or old baseball cards. But the Australian study is the first, to my knowledge, to focus on discretionary accommodations. It’s less likely these days to find people in positions of authority, even at lower levels of decision making, consciously denying minorities rights. But it is easier to imagine decision makers, like the bus drivers, granting extra privileges and accommodations to nonminorities. Discriminatory gifts are more likely than discriminatory denials.

A police officer is an out-and-out bigot if she targets innocent blacks for speeding tickets. But an officer who is more likely to give a pass to white motorists who exceed the speed limit than to black ones is also discriminating, even if with little or no conscious awareness. This is one reason the Twitter hashtag #crimingwhilewhite is so powerful: It draws attention to the racially biased exercise of discretion by police officers, prosecutors and judges, which results in whites getting a pass for the kinds of offenses for which minorities are punished.

Racial discrimination is more likely in settings in which both decision makers and bystanders cannot easily observe how comparable nonminorities are treated. A restaurant is unlikely to charge Hispanics higher prices for a hamburger, because the victim could compare her bill to the price listed on the menu. But one-off accommodations where the decision maker retains substantial discretion don’t offer any easy point of comparison. My kids, who are white, have never been turned down when I asked if they could use a bathroom designated for “employees only.” After reading the Australian bus study, I wonder whether the same is true for minority families.

The bus study underscores this point. Drivers were more likely to let testers ride free when there were fewer people on the bus to observe the transaction. And the drivers themselves were probably not aware that they were treating minorities differently. When drivers, in a questionnaire conducted after the field test, were shown photographs of the testers and asked how they would respond, hypothetically, to a free-ride request, they indicated no statistically significant bias against minorities in the photos (86 percent said they would let the black individual ride free).