WHEN a white police officer in Ferguson, Missouri shot an unarmed black teenager dead three years ago, the killing set off outrage across America over violence committed by police. (Barack Obama’s Department of Justice concluded that the officer acted in self-defence.) But with greater public scrutiny of racial disparities in the use of force, better-disguised forms of inequality soon came to light as well. In March 2015 the department published a report on law enforcement in the city, which found that Ferguson’s criminal-justice system seemed to focus more on generating income for the government than on ensuring public safety. Nearly a quarter of the city’s general revenues came from criminal fines, fees and court costs. Moreover, black residents paid a far greater portion of these expenses than either their share of the population or their share of total crimes committed in Ferguson would indicate. The investigators concluded that the police had displayed “unlawful bias” against blacks.

The city appears to have heeded the Department of Justice’s message: fines and fees are down 77% from their peak in 2013. However, Ferguson was unlikely to be a unique outlier, and other cities engaging in similar practices might well have continued outside of the national spotlight. A new paper by Michael Sances of the University of Memphis and Hye Young You of Vanderbilt University published this month in the Journal of Politics found that Ferguson was indeed more of a rule than an exception. After examining data on 9,000 American cities, they found that those with more black residents consistently collected unusually high amounts of fines and fees—even after controlling for differences in income, education and crime levels. Cities with the largest shares (98%) of black residents collected an average of $12-$19 more per person than those with the smallest (0%) did.

However, there was one subgroup of cities that bucked the trend: the relationship between race and fines was only half as strong in places whose city councils included at least one black member. This may be because black politicians are likelier than white ones are to respond to complaints from black constituents. Black councillors might also intervene to stop certain policies, like increasing court fees, from going into effect to begin with.

Part of the problem is that fines are a very effective method for cash-strapped governments to shore up their budgets without having to raise taxes or cut spending. As a result, the temptation to tell police departments to dredge up violations, no matter how petty, can be hard to resist. City judges tend to rubber-stamp these penalties. For example, in Peoria, Arizona, two people were jailed for not trimming weeds more than six inches tall. In Ferguson, a black man resting in his car after playing basketball in the public park was stopped by police and charged with, among other things, not wearing a seat belt in his (parked) car and making a false declaration after giving the officer a shortened name (like “Bob” instead of “Robert”). Such fines may fall disproportionately on the backs of black citizens, because they tend to be poorer and lack the resources to contest the penalties.

Despite the exhaustive controls the authors included in their study, the strong correlation they found does not demonstrate decisively that race is the ultimate cause of higher fines. However, it does put a very high burden of proof on researchers arguing that some other factor is responsible. Now that the pattern has been identified across the country, city governments that rely heavily on fines would be well-advised to consider more transparent sources of revenue, and ones that do not place an additional burden on a subset of residents who are already disadvantaged.