“A SMILE is a curve that sets everything straight,” said Phyllis Diller, a comedian. But a dazzling one can set you back a lot of money, and dentists in North Carolina want to keep it that way. Just over a decade ago the North Carolina Board of Dental Examiners (NCBDE) noticed that many people were getting their teeth whitened at spas or kiosks in shopping malls. The procedure typically involves placing disposable strips impregnated with a whitening agent on a client’s teeth. The strips are deemed safe by the Food and Drug Administration and regulated as cosmetics. Salons charge as little as a tenth as much for this service as a dentist would. So in 2003 the NCBDE sent at least 47 cease-and-desist letters to teeth-whitening outfits, accusing them of practising dentistry without a licence and driving them out of business.

The NCBDE is composed mostly of practising dentists with an interest in eliminating competitors. Free marketeers have long complained about this sort of behaviour, which goes on in several states and many industries. In 2010 the Federal Trade Commission (FTC) agreed; and on October 14th the case reached the Supreme Court.

Hashim Mooppan, arguing for the dentists, claimed that a Supreme Court decision from 1943 gave the state dental board immunity from the antitrust provisions of the Sherman Act. Some of the justices sounded sceptical. Sonia Sotomayor complained that North Carolina has given a “group of private actors a pass on antitrust litigation”. Stephen Breyer mused that if the state were to let a group of wine merchants or truckers fix their own prices, “they might get out of hand.”

However, Justice Samuel Alito said he was “not attracted to the idea of federal courts looking at state agencies...to determine whether they are really serving the public interest.” Opening the door to this kind of supervision is “troubling,” he told Malcolm Stewart, the lawyer for the FTC. Justice Breyer asked whether a ruling against dentists policing dentistry would entail a new norm requiring bureaucrats, rather than brain surgeons, to regulate neurology. “I want a neurologist to decide it,” Justice Antonin Scalia said.

No one is suggesting that unqualified people should be allowed to offer dangerous services, however. Rather, the case hinges on whether state-sanctioned professional bodies may claim a monopoly over services that non-members could safely supply. A ruling that liberated non-dentists to whiten teeth would invite complaints against a host of other professional bodies. Hence the stampede of occupational therapists, masseurs, chiropractors and many others to warn of dire consequences if the court disrupts “a 150-year tradition of regulation by practising professionals”, as one brief put it. Others see it differently. Legalzoom, a firm that helps people write their own cheap legal documents using an online kit, has often been stymied by state bar associations.

Companies that act like cartels face stiff criminal penalties. Why, asks the Pacific Legal Foundation, a non-profit, should private parties acting under the government’s aegis be allowed to get away with the same thing? A decision is expected by June.