Four years ago, Congress decided to force down the hidden fees that credit card companies collect from their customers. It passed a law called the 2009 Credit Card Accountability Responsibility and Disclosure Act — a name chosen so the law would be known as the Card Act.

When Neale Mahoney, an economist at the University of Chicago’s Booth School of Business, set out to evaluate the effect of that law, he was confident he knew what he and his colleagues would find: It didn’t work.

“I went into the project with this sort of conventional wisdom that well-intentioned regulators would force down fees and that other fees and charges would increase in response,” he told me this week, comparing hapless rule makers to the carnival visitors playing the game known as Whac-a-Mole, where a mole springs up somewhere else as soon as one is knocked down.

But his expectation was wrong. The study came to a conclusion that surprised Mr. Mahoney and his colleagues: The regulation worked. It cut down the costs of credit cards, particularly for borrowers with poor credit. And, the researchers concluded, “we find no evidence of an increase in interest charges or a reduction to access to credit.”