Certainty vs Severity: What Actually Deters Crime

Why research suggests that the chance of being caught matters more for deterrence than the length of the punishment.

If expected punishment is the probability of being caught multiplied by the penalty, it seems that a government could deter crime just as well by doubling sentences as by doubling the chance of arrest. On paper, both double the expected punishment. In practice, researchers have found that the two are not equal.

The evidence

A large body of research in criminology and economics finds that the certainty of punishment has a stronger deterrent effect than its severity. The U.S. National Institute of Justice, summarising this evidence, has stated that the certainty of being caught is a vastly more powerful deterrent than the punishment itself. Studies of long sentences, including “three strikes” style laws in the United States, have generally found small deterrent effects relative to their high cost.

Why certainty wins

Economists offer several reasons.

The first is discounting. People, especially young people, tend to give much less weight to costs far in the future. The difference between a five-year sentence and a ten-year sentence happens years from now, so it may barely register at the moment of decision. Being caught, by contrast, happens soon.

The second is information. Most people do not know the exact sentence for a given crime, but they often have a feel for whether police in their area are likely to respond.

The third is that severity has diminishing returns. The later years of a long sentence may deter little extra crime, while costing the same amount to pay for.

Hot-spot policing

Many cities have tried hot-spot policing, which concentrates patrols on the small number of street corners where a large share of crime happens. Several randomised studies found that this reduced crime in those places, and often did not simply push it around the corner. It works by raising the perceived chance of being caught, not by changing any sentence.

Costs matter too

Longer sentences are expensive. Every extra year in prison costs taxpayers money and costs the prisoner and their family lost earnings and lost time together. If extra severity buys little deterrence, the same money might prevent more crime if spent on detection, or on programmes that reduce reoffending.

Assuming harsher is always safer

It feels intuitive that tougher sentences mean less crime. But because potential offenders discount the future and rarely know the sentence, much of the extra punishment does not change behaviour. Some longer sentences reduce crime mainly by keeping particular people off the streets, which is a different effect called incapacitation.

Key takeaways

Research generally finds the certainty of punishment deters more than its severity.
People discount distant costs, so extra years of a sentence weigh little at the moment of choice.
Most people know little about sentence lengths but can sense how likely they are to be caught.
Spending on detection may prevent more crime per dollar than spending on longer sentences.