These results are arresting in part because they indicate that the form of racial bias at work here is simple affinity bias rather than stereotyping. NBA fans (of which I count myself one) are familiar with racial stereotypes from basketball's tv announcers: Black players are more likely to be described as "athletic," while white players are more likely to be described as getting by on "effort" or "basketball IQ." But if stereotyping were at work, one would not necessarily expect the race of the referee to matter. Indeed, one might think that the stereotype of white players as clever, hard-working oafs would lead to more foul calls: Regardless of the referee's race, he or she (yes there are some female refs) would likely be inclined to see the typical white player as committing clumsy fouls against the more-graceful typical black player. Yet the numbers don't bear that out.
Instead, the authors of the study find a simple matter of affinity. The white refs "like" white players more than they like black players, and vice-versa (though less markedly) for the black refs. These feelings appear to be quite deeply rooted and are almost certainly sub-conscious. The authors cite a 1988 study that found that referees call a larger number of fouls on teams wearing black jerseys! It's hard to imagine a professional referee saying to himself: "That looked like it may have been a foul. Yes, it must have been because the player's jersey is black."
What should we make of these findings? Here I'll give the last word to Price and Wolfers, who conclude as follows:
These results are striking given the level of racial equality achieved along other dimensions in the NBA and the high level of accountability and monitoring under which the referees operate. Thus, while the external validity of these results remains an open question, they are at least suggestive that implicit biases may play an important role in shaping our evaluation of others, particularly in split-second high-pressure decisions. That is, while these results may be of interest to those intrigued by the sporting context, we emphasize them instead as potentially suggestive of similar forces operating in a range of other contexts involving rapid subjective assessments.