We are continually being told that the causes of crime are both complex, built into the very structures of our society, and simple: it's all inequality, innit? Poverty, deprivation, righteous anger at the greed of the rich: fill in your own quotations from Polly Toynbee here.

However, there's another theory entirey: that much crime, if not most of it, is opportunistic. It takes place because those who would have more and are willing to get more through either violence or other illegality meet up with those who have the more and cannot defend it. Our problem is of course that we very rarely get the sort of natural experiment that we need in order to test which of these two is correct: or, if we are to be fair, or both could have some relevance, which explains the greater part of it. Rarely, but not never:

The Baltimore example is that over the period of the recent blizzards – when most potential victims were stationary, and not accessible to the police, the crime rate dropped.

For example, murders – of which there were 18 in the first 37 days of the year – dropped to 0 in 9 days.

Now it certainly isn't possibly true that inequality, poverty, deprivation or righteous anger dropped in those days of the snowstorms. It's also most certainly not true that policing had anything to do with it as they were as trapped as everyone else. No, we're rather left with our second explanation: the root cause of crime appears to be the opportunity to commit a crime. When that opportunity isn't there, nor is the crime.