Before his conviction on Monday, Tom Hayes, a 35-year-old former UBS and Citigroup derivatives trader, argued he had merely been taking part in a widespread practice of manipulating the Libor benchmark of interest rates. Sentencing him to 14 years’ imprisonment, Mr Justice Cooke saw things differently. “The fact that others were doing the same as you is no excuse,” he said.

The past five years have seen the exposure of a litany of financial wrongdoing yet a dearth of criminal prosecutions, generating uproar that has now been heard. Hayes’s conviction shows that prosecutors and judges are serious about punishing financial crime — and, they imagine, thereby preventing it.

But their new zeal might not matter much. Everyone might be doing it, but not everyone can be prosecuted or jailed. Even if another dozen traders are sentenced, most of the people who manipulated Libor will go free.

Traders will process news of Hayes’s sentence through the lens of economics more than ethics. Yes, 14 years is a long time, but the probability of prosecution is low — arguably even lower than before, with so many resources dedicated to investigating past wrongdoing. You are less likely to be caught speeding after you see a car pulled over by a police officer than you were before.

And the evidence wielded by prosecutors provides a road map of what not to do. Hayes left a trail of electronic evidence. Lesson: do not ask a bank, in writing, to skew Libor “too low for the next few days” and then promise to “return the favour as when you need it”. You do not have to recognise the illegality of such requests to know that they might one day prove embarrassing.

Hayes co-operated with the government. Another lesson: keep your mouth shut. Hayes spoke to prosecutors on the record for 82 hours over several months. This may not have been wise. We have yet to learn the fate of other traders who allegedly manipulated Libor and refused to talk, but it is a fair bet they will one day congratulate themselves on their silence.

A conviction that leans so heavily on the defendant’s missteps is unlikely to be much of a deterrent. And the most important lesson from Hayes’s conviction is this: “Everyone’s doing it” is not merely an excuse, but also a temptation.

Manipulating Libor was not a criminal scheme of Hayes’s own conception. According to regulators, bankers at UBS were already submitting requests to move the bank’s Libor submissions up or down when he joined in 2006. Hayes told a close friend he found the practices unsettling, but felt he was not in a position to question what they were doing.

“I thought it was weird,” he said, “but that’s how they did it.”

Prosecution is a good way of deterring people from doing things that they understand to be crimes. But financial fraud is not like murder or bribery or even drug dealing. The transactions are complicated and can appear morally ambiguous. Bankers generally exploit information advantages and dodge regulations, and the lines between legitimate and criminal can be blurry.

At various stages, both Hayes and his employers seemed to see what he was doing as perfectly legal, even if morally wrong. Hayes did not hide his views about Libor, and he apparently even changed his Facebook status to show which way he wanted the benchmark to move. The year before he left UBS, the bank paid him nearly $2m; his boss cited his “strong connections with Libor setters in London. This information is invaluable for the derivatives books.” Hayes later left for Citigroup, which offered to more than double his pay and permitted him to risk up to $3m for every basis point that Libor moved.

No matter how many others were doing it, Hayes could have said no to the lucrative yet unethical behaviour of which he has been convicted. That would have carried costs and required courage. We hope that everyone — from our children to derivatives traders — will stand up to wrongdoing. Unfortunately, many traders who learn about Hayes’s conviction are more likely to go along with it.