I know blogging is supposed to be all about instant analysis and reaction, but I'm afraid I've not got time for instant. So like re-brewed coffee here are a couple of thoughts about prison.

Bob Piper asks whether prison works and pretty quickly comes to the conclusion that it doesn't. I think it probably depends on what he means by works.

But before we get into that, let's take a look at the disconnect between what's happening – more people going to prison for longer – and what people think is happening – lenient sentencing. The graph on the right comes from the Prime Minister's Strategy Unit in a presentation they did back in 2003. I've no reason to think that things will have changed in the last few years, but I suppose they may.

So, back to Bob's question, "does prison work". If he's asking whether it reduces reconviction rates for those who serve time then the evidence isn't good. This Home Office research suggests:

reconviction rate within two years of discharge from prison has fluctuated around 53 per cent since 1989.

But then reconviction rates from all the interventions we're currently using in the criminal justice system don't seem to be doing much:

When overall reconviction rates for custody are compared with those for community penalties, there is no significant difference in the effects of the disposals on reconviction rates.

What if Bob were asking "do prisons reduce the level of crime?" Then the evidence seems less ambiguous. Here's Freakonomics author Stephen Levitt in one of his academic papers:

The theory linking increased imprisonment to reduced crime works through two channels. First, by locking up offenders, they are removed from the streets and unable to commit further crimes while incarcerated. This reduction in crime is known as the incapacitation effect. The other reason prisons reduce crime is deterrence—the increased threat of punishment induces forward-looking criminals not to commit crimes they otherwise would find attractive… The evidence linking increased punishment to lower crime rates is very strong…

…the increase in incarceration over the 1990s can account for a reduction in crime of approximately 12 percent for the Ž1st two categories [homicide and violent crime] and 8 percent for property crime, or about one-third of the observed decline in crime.

But hold on before you start building lots more prisons. It comes at a cost to us the tax payer. Here's the Strategy Unit in 2003:

The 30% increase in the UK prison population between 1997 and 2003, at a cost of £600m, is estimated to have reduced crime by around 5%, accounting for perhaps a sixth of the overall fall in crime.

The Home Office paper seems to be less generous:

the evidence suggests that the cost of achieving a reduction of around 0.6 per cent in crime through increases in the prison population alone would be about £380 million a year.

And Levitt himself says that hiring more police might make better sense:

a dollar spent on prisons yields an estimated crime reduction that is 20 percent less than a dollar spent on police

So prison seems to work, but at £37.5k a year per prisoner it may be a very expensive option.