Illustration: Simon Letch Kleiman says we should be aiming for the minimum amount of punishment necessary to achieve the desired amount of crime control. Why? Because punishment is a cost, not a benefit. The benefit we're seeking is reduced crime, whereas punishment is a cost of achieving that benefit. Punishment comes at the expense of innocent taxpayers. Putting it another way, we should keep clear in our minds that the objective of punishment isn't retribution, it's deterrence. So how can we make our punishment more effective in deterring crime? Kleiman's thesis is that increasing the severity of punishment isn't cost-effective but making it swift and certain is. ''Theory and evidence agree: swift and certain punishment, even if not severe, will control the vast bulk of offending behaviour,'' he says.

Severity is not only a poor substitute for swiftness and certainty, he says, but also their enemy. That's because the more severe a sanction is, the less frequently it can be administered (prison cells are scarce) and the less quickly it tends to arrive. It shouldn't be hard for any parent to believe that threats of punishment are more effective if the child knows an offence will be punished and that the punishment will be immediate. The trouble with our crime control efforts is that many crimes go undetected and so unpunished - you have a pretty high chance of getting away with it - and, even when offences are punished, the punishment comes months or years later. Kleiman reminds us of the goal we should aim for: ''the perfect threat never needs to be carried out''. Achieve that effective deterrence and you've got the benefit of reduced crime without the cost of punishment. We could never reach that ideal but the closer we come the better. Kleiman's analysis, influenced by the insights of behavioural economics, is an advance on conventional economics, which assumes severity is an adequate substitute for the certainty of being caught and ignores the question of swiftness. All very logical but how do you put it into practice? Clearly, it would be impossible to make punishment of all offences swift and certain. But Kleiman observes our resources are spread very thinly. Instead, we should concentrate them, starting somewhere with a geographic region, a set of offences or a set of offenders.

You borrow resources from elsewhere and raise the certainty and swiftness of punishment until you reach the tipping point where offenders get the message and the rate of violation tips from being high to low. Because offending is subject to positive feedback - a violation rate that's high will tend to stay high, whereas one that's low will tend to stay low - you can then move the resources to the next priority area. The reduction in offences will have increased the resources available. Kleiman recommends focusing on those offenders subject to supervision in the community: people given community service orders rather than being sent to prison and prisoners let out on parole before their sentence is complete. ''As things stand, the community corrections system reproduces the flaws of the larger criminal justice system, having more rules than can be reliably enforced and imposing sporadic but sometimes severe sanctions,'' he says. A small set of rules, each clearly linked to the goal of reducing re-offending, adequate capacity to monitor whether those rules are being observed and a system of swift, reliable and proportionate penalties to back up those rules would perform much better.