Rewriting criminal laws as a means of deterring criminals may be a waste of time. That’s according to Colin S. Diver Distinguished Professor of Law Paul Robinson, who shares his findings in the forthcoming Oxford Journal of Legal Studies.

“ The assumption,” said Robinson, who co-authored “Does Criminal Law Deter?” with Princeton psychology professor John Darley, “is that if they pass or change a law it will have some influence in the real world.” But for that to happen, he says, three conditions have to be met.

First, says Robinson, the person you’re trying to influence has to know the rule. For the most part, he argues, they don’t. “The studies that actually talk to offenders and potential offenders suggest that they really don’t know the rules. They think they do, but they have it wrong more often than not.”

Even if they know the rules, potential offenders have to be in a mental state where they can use this knowledge to guide their conduct. There are lots of possible stumbling blocks here, says Robinson. The person may be intoxicated or on drugs, or in a situation where anger, passion or fear have the upper hand.

Robinson concedes that there’s an educated group—such as white collar offenders-—who are “really thinking and learning and keeping up with what the rules are.” But even this clear-headed subset is likely to trip over the next hurdle. For the threat of stiffer penalties to act as a deterrent, the potential offender has to conclude that the costs of committing the crime outweigh the benefits, and, says Robinson, people tend to discount the possibility of future consequences, “compared to the benefits, which are immediate and definite.”

Just as important, people rarely think they’ll be caught. “They see themselves as somehow better. Yes, they’ll say, ‘Joe got caught last month, but he’s stupid and I don’t make those kinds of mistakes.’ ”

Robinson and Darley combed the literature for proof of any permanent deterrent effect from setting criminal law rules and penalties—and came up empty handed. “The best you can get is a sort of temporary flash,” says Robinson, such as anti-drunk-driving campaigns where a PR blitz convinces the public that enforcement rates are high. “But even in those cases, it’s not lasting. As soon as the perceived enforcement rate drops off, you get a reverse.”

They did find evidence that crime rates drop with increased prison time. Those reductions are real, he allows, “but it looks like they can be explained by pure preventive detention. They’re in prison, so they can’t commit offenses. Deterrence isn’t playing a role there, incapacitation is.”