Do police numbers really affect knife crime? Theresa May is partly right, but her government is still failing young people

But – and I don’t get to say this very often – Theresa May has a point. There is a link between police numbers and crime rates, but it appears to be weaker for violent crimes. Also, in general, the link is fairly weak and hard to tease out, and it’s likely that it’s not the most important factor here.

That said, it doesn’t let her off the hook.

First, it’s worth noting that any study of the links between crime and police is tricky. For one thing, the interaction is complex. If crime is going up, it makes sense for the government to employ more police, and vice versa. Then eventually the crime rate will start to fall again. So the data will show that increasing police numbers is followed by a fall in crime rates, but they’re not causing the fall: it’s just regression to the mean.

Worse than that, there are a million different factors behind crime rates, of which policing is only one, so it’s very hard to tease out the relationship at all.

Crime has, in general, been going down for the last two decades or so. That’s true in the UK and in most of the world. The ONS’s Crime Survey for England and Wales says that there were a little over 6 million crimes (not including computer misuse) in the year to October 2018, down from a peak of almost 20 million in 1995. Violent crime has gone from a peak of about 4.5 million, again in 1995, to about 1.4 million.

But it’s levelled off in the last couple of years. And what the ONS calls “lower volume but higher harm” crimes, such as homicide and offences involving knives or sharp instruments, have gone up: there’s only been data on knife crime since 2011, but in that time it’s gone up from about 30,000 to about 40,000.

When you’re dealing with rare events like this – 40,000 incidents in a year, in a population of 70 million, is a vanishingly tiny number – I’m tempted to ask whether there’s anything to explain at all. Sometimes numbers go up and down.

In the case of the 93% increase in under-16s getting stabbed, which is I think taken from hospital statistics, the statistics that Dispatches used aren’t available, but the most recent stats show a jump from 95 in 2012/13 to 163 last year, a rise of 72%. The year the Coalition Government came to power, before police numbers were cut, there were 159 under-16 stabbings. It hardly needs explaining; the numbers jump around a lot.

But Prof Will Jennings, a political scientist at the University of Southampton, and Prof Tom Kirchmaier, who lectures on crime and policing at the LSE, both tell me that the increase in knife crime is probably real. What they don’t say is that falling police numbers are necessarily the major factor behind it.

To be clear: they do both think that police numbers affect crime rates. Kirchmaier in particular points to a clever 2008 study by his LSE colleagues which looked at crime rates in inner-London boroughs in the weeks after the 2005 terror attacks. As it’s difficult to tease out cause and effect when it comes to crime, the idea was that in the wake of the atrocities, when police numbers in inner London were hugely increased but in the outer boroughs remained normal, there was a natural experiment. And it showed that crime went down when there were more police around.

Most studies find something like this. A review of the literature in 2011 looked at 13 studies, including the one mentioned above, and most of them reported a negative correlation between police numbers and crime rates. But the review is wary of drawing firm conclusions – it says that none of the studies on their own proves a causal link, and points out that most of the studies look at large, sudden increases in police presence, often in the wake of an atrocity, which are very visible.

Reductions, and therefore fewer patrols than normal, might not be noticed. The impact of police cuts and police increases could be “asymmetric”, the review says, and smaller, incremental changes may not have an effect. Cautiously, though, it suggests that there probably is a causal link.

Yet there are two things worth noting. First, the link isn’t huge – an “elasticity” of about -0.3, which is to say you get a 3% reduction in crime for every 10% increase in policing. That’s not insignificant, but police numbers in England and Wales have only gone down by about 15% since 2010, so it’s no use if you want to explain a 33% rise in knife crime since 2011.

Second, the review of the literature found that increasing your police force had different effects on different crimes. Property crime went down most – which makes sense, as some of those crimes are presumably carried out by people who are weighing the potential risks and benefits, and that calculation will be affected by seeing more police around.

But, the review said, violent crime was much less affected – which again makes sense. “Much violent crime … is conducted in the heat of the moment in pubs or on the street, or behind closed doors in the home”, it notes, so one would not “expect consideration or even awareness of potential police attention to come into play”.

So police numbers probably are important, but May still has a point if she’s saying that it’s not the dominant factor in the increase in knife crime. You could even argue that reducing police numbers was the right thing to do, given that crime was falling.

Let’s look at what the dominant factors could be. Some of them are out of the government’s hands: for instance, socioeconomic shocks often lead to crime, and although we didn’t see that in the immediate aftermath of the financial crisis, Jennings thinks it could be having a delayed effect.

But others are firmly in the government’s hands. Both Kirchmaier and Jennings said that a decade of cuts to social services and youth centres have probably contributed (although, again, it’s hard to tease out). Welfare cuts have led to a growth in youth homelessness, which makes young people more at risk of joining gangs.

And the Government’s continued cowardice over drugs leaves a huge industry in the hands of violent criminals – with gangs now recruiting “across county lines”, away from urban centres, and seeking new markets. A former Metropolitan Police chief has partly blamed school exclusions, driven by Government educational policy, for the rise in knife crime.

Theresa May has been either the home secretary or the prime minister for the last nine years. She’s probably right to say that the rise in knife crime isn’t clearly or directly linked to her cuts to police numbers. But that doesn’t mean it isn’t her responsibility.

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Tom Chivers is a science writer. His second book, How to Read Numbers, is out now.

TomChivers