Beware of common sense

Good intentions are not enough. If leaders and governments are serious about achieving their aims, they must base their actions on hard evidence.

YOU break your arm. At the hospital, the doctor tells you his team is going to inject iron nanoparticles into the broken bone and use electromagnets to realign it. Wow, you say, you've never heard of this method. "Oh, it's never been tried before," says the doctor. "But our hospital needs some publicity, and it sounds really impressive and high-tech, doesn't it?"

You would rightly be appalled if hospitals chose treatments this way. We expect medical therapies to undergo rigorous trials to ensure they are safe and effective. Yet we seem content to let our leaders conjure up policies based on what sounds good, rather than on what has been proved to work.

The effectiveness of policies in many areas, from education and crime to reducing greenhouse gas emissions, can be empirically determined. As in medicine, the best evidence comes from randomised controlled trials; better still, a systematic review of multiple randomised trials.

Admittedly, there are plenty of problems with evidence-based government. There are many aspects of government which the scientific method cannot be applied to and, even where it is applicable, it can be time-consuming and expensive. Trials have to be well designed and they often need to involve large numbers to produce robust results. Researchers also need to ensure trial results are directly relevant to policy-makers.

Proper trials are worth the effort, though. When they are carried out, they often reveal that policies and laws are having the opposite effect to that intended. "Common sense" and good intentions are no substitute for hard evidence. You might think, for instance, that scaring young offenders by showing them what prison life is like will discourage them from reoffending. In fact, randomised trials show that such schemes, long popular in the US, increase reoffending rates.