Last week, a federal judge handed down a major ruling against American Express, deciding that the brand-name card issuer had engaged in anticompetitive practices for more than a decade.

The court ruled that a part of American Express' contract with merchants known as a "nondiscrimination provision" had led to less competition, not more, and pushed consumer prices higher. That clause was intended to make merchants treat all credit cardholders similarly without favoring one card company over another.

Because American Express charges merchants a higher percentage of each sale than its competitors — broadly speaking, about 1 percent more of the sale than Visa and MasterCard — the court determined the practice led retailers to pass the higher cost onto all customers.

As a result, the judge contended, "a lower-income shopper who pays for his or her groceries with cash" or a debit card "is subsidizing, for example, the cost of the premium rewards conferred by American Express on its relatively small, affluent cardholder base in the form of higher retail prices."

The judge, Nicholas G. Garaufis of Federal District Court for the Eastern District of New York, declared that the result of his decision would be increased competition that would benefit the consumer: "The court expects that merchants will pass along some amount of the savings associated with declining swipe fees to their customers in the form of lower retail prices."

Judge Garaufis's heart may be in the right place. We all want more competition and lower prices. But if history is any guide, his ruling will lead to the opposite outcome.

The result, at best, will be that fees will come down for retailers but they will pocket the difference and simply end up with bigger profits. That's not necessarily a bad thing if you're a retailer, but it's clearly not the virtuous outcome for consumers that the judge seems to think his ruling will create.

Not only are retail prices likely to remain at current levels, but card companies like American Express, Visa and MasterCard may also end up raising their annual fees and lending rates because they will need to make up revenue lost from the merchant fees in order to pay for the myriad rewards programs they have created.

The evidence for such a pattern is well established: In 2010, Senator Richard J. Durbin, Democrat of Illinois, introduced the Durbin Amendment as part of the Dodd-Frank overhaul of the financial system. The amendment was meant to lower merchant fees for processing debit cards; retailers like Walmart and Home Depot argued that the banks were gouging them, pushing prices up for consumers.

So what happened after the legislation was put into effect?

"Most large retailers have seen significant cost reductions as a result of the Durbin Amendment, yet to date there is no evidence that those cost savings have been passed through to consumers," a study by the George Mason University School of Law said. Other academic research has come to the same conclusion.

It shouldn't be a surprise. Somewhat similar rules aimed at lowering fees were put in place in Spain and Australia, and the outcome was the same. "In Spain and Australia, the regulation of interchange fees (I.F.s) resulted in a transfer of costs from retailers to consumers," according to Europe Economics, a consulting firm. "Retailers' costs fell as they paid lower merchant service charges (M.S.C.s), but this cost reduction was not passed on to consumers in the form of lower retail prices."

Even Judge Garaufis seems to recognize t he possibility that his ruling won't have the intended effect. Buried in his thoughtful 150-page decision, he acknowledges that at the about three million retailers that don't accept American Express, prices haven't come down, nor have the stores tried to create competition among the card companies — for instance, by encouraging customers to use one card instead of another with higher costs, a practice called "steering."

"Amex correctly notes there is little evidence of widespread steering at these merchant locations," the judge wrote, "or that price competition among the networks has increased in the four years since Visa and MasterCard agreed to abandon their antisteering rules for the merchants operating these locations." (Visa and MasterCard used to have similar nondiscrimination provisions but got rid of them as part of a different settlement with regulators in 2010.)

American Express, of course, took issue with the judge's decision, saying it "would harm competition by further entrenching the two dominant payment networks, Visa and MasterCard. Only a small percentage of Visa and MasterCard holders carry American Express cards. By contrast, most American Express card members carry a competing card in their wallet," the company said in a statement last week.

There are other, perhaps more basic, questions about the ruling.

First, it is worth noting that retailers aren't required to accept American Express. Indeed, nearly three million retailers that accept Visa and MasterCard do not take American Express. When a retailer chooses to accept the card, it also accepts the terms of use, which outline that the merchant cannot engage in steering.

That makes sense. It would seem odd for a merchant to accept one brand of credit card and then encourage its customers to use a different one because the fees are cheaper — unless it was trying to piggyback on the brand of the more expensive card to get customers in the door in the first place.

"Amex charges some of the highest fees in the business, mostly because the bulk of its customers pay off their balances each month and thus deprive the company of the interest charges most credit-card issuers collect," Daniel Fisher recently wrote in Forbes, in a column that similarly questioned the judge's decision.

And, of course, if the retailer wants to drop the card, it is free to do so.

Judge Garaufis takes issue with that idea. He seems to think that American Express has such a strong position in the marketplace that virtually all retailers need to accept American Express. He cites a situation in 2004 in which Walgreen considered dropping American Express but ultimately chose to keep it, as well as another instance when Murphy Oil dropped American Express only to return to it later.

Those examples are real. But here's another: Just last week, a day before the judge's decision was published, the discount-retail behemoth Costco dropped its American Express co-branded card. The retailer won't accept regular American Express cards at its stores either. The card company lost its partnership with JetBlue last week, too. So much for the theory that merchants can't leave American Express.

Finally, Judge Garaufis assumes that if merchants who voluntarily accept American Express and its nondiscrimination policy were allowed to steer customers to lower-priced cards, those merchants would create special deals for consumers. The judge posits the possibility of "offering a 10 percent discount off the posted purchase price, free shipping, free checked bags, gift cards or any other monetary incentive for using their Discover card" over American Express.

That sounds great in theory, but if you think about it a little longer, you will realize quite quickly that no retailer is going to be offering 10 percent off to use a credit card with lower merchant costs. That's because the difference in fees is less than 1 percent.

American Express, as you might expect, is appealing the decision.