The terrorist attacks of 7 July 2005 resulted in a very large redeployment of police officers to central London boroughs. New research by Mirko Draca, Stephen Machin and Robert Witt looks at the impact of this increased security presence on criminal activity in the weeks and months after the bomb blasts

Draca, M.

Machin, S.

Witt, R.

English

UCL Discovery

CentrePiece Autumn 20066Panic on the streets of LondonThe terrorist attacks of 7 July 2005 resultedin a very large redeployment of policeofficers to central London boroughs. Newresearch by Mirko Draca, Stephen Machinand Robert Witt looks at the impact of thisincreased security presence on criminalactivity in the weeks and months after thebomb blasts.CentrePiece Autumn 2006Increasing police resources isoften perceived as a primarytool for reducing crime – butthere is little hard evidenceshowing that more police do infact reduce crime. The mainreason for this is that it has been difficultto disentangle the causal relationshipbetween the two: higher crime usuallymeans more police and vice versa.As a result, it is difficult forgovernments and observers of publicpolicy to evaluate the effectiveness ofincreasing police resources in comparisonwith other possible causes of falling crimerates, such as increased incarceration ratesand improved economic conditions.This problem has led researchers toconsider the impact of unexpectedchanges in police deployment on crimerates. These types of changes are oftencharacterised as ‘natural experiments’ – aresearch design based on events, whichtries to reproduce the conditions of ascientific experiment without the pre-design that is available in controlledlaboratory conditions.For example, our research looks atpolice activity in central London in thewake of the 7 July 2005 terrorist attacks.Police deployment increased by over 30%in the six weeks immediately following thebombings, and the massive scale of thisresponse provides a good opportunity toexamine the causal relationship betweenthe number of police deployed and thecrime rate. Specifically, our research investigateswhat the heightened security presence didto criminal activity in the five Londonboroughs that were hit. The centralfinding is that crime fell significantly inthese places relative to outer London inthe six weeks immediately after theattacks. Furthermore, crime appears to behighly responsive to higher policedeployment – we calculate that a 10%increase in police deployment reduces thecrime rate by approximately 3%.7/7 as a natural experimentAs will be familiar to most readers,London’s public transport system wassubject to two waves of attacks in July2005. The first wave occurred on 7 Julyand involved the detonation of fourbombs. Three bombs were detonated onLondon Underground trains near thestations of Russell Square (in the boroughof Camden), Liverpool Street (TowerHamlets) and Edgware Road (Kensingtonand Chelsea). A fourth bomb wasdetonated on a bus in Tavistock Square,Bloomsbury (Camden).The second wave of attacks occurredtwo weeks later on 21 July and involvedfour unsuccessful attempts at detonatingbombs on trains near the stations ofShepherds Bush (Kensington andTable 1:Police deployment and crime rates before and after the 7/7 attacks, borough levelCentral London  171.41 190.34 +18.93 4.26 3.88 -0.38‘treatment’ boroughsOuter London  84.19 85.40 +1.21 2.20 2.16 -0.04‘control’ boroughsNote:The central London ‘treatment’ boroughs are Camden, Islington, Kensington and Chelsea,Tower Hamlets and Westminster.Police deployment is defined as total weekly hours worked by police staff at borough-level.Pre-period:8 July 2004 to 31December 2004Post-period:7 July 2005 to 31December 2005Difference Pre-period:8 July 2004 to 31December 2004Post-period:7 July 2005 to 31December 2005DifferencePolice deployment(Hours worked per 1,000 population)Crime rate(Crimes per 1,000 population)CentrePiece Autumn 20068Chelsea), the Oval (Lambeth) and WarrenStreet (Westminster), and on a bus inBethnal Green (Tower Hamlets). Thissecond wave of attacks caused muchturmoil in London.The bomb attacks took place in fiveboroughs in central London and we haveused these parts of the capital as the‘treatment’ areas that were most affectedin terms of a heightened securitypresence. We also need a set of controlareas against which we can benchmarkthe observed changes in the treatmentboroughs. We therefore use otherboroughs located in outer London as thecomparison group. Figure 1 shows the 32London boroughs, picking out the five inthe treatment group.Basic street-level policing of London iscarried out by 33 borough operationalcommand units, which operate within thesame boundaries as the 32 Londonborough councils plus one dedicated toHeathrow Airport. Our data areconstituted of daily police reports of crimefrom the London Metropolitan PoliceService before and after the attacks. They cover the period 1 January 2004 to31 December 2005, aggregated up fromward to borough level and from days toweeks over the two-year period. The impact of increasedpolice deploymentTable 1 reports police deployment andcrime rates before and after the attacks.Note that in this part of the analysis, wedefine the post-attack period as thewhole period from the 7/7 attacks untilthe very end of 2005. So what do thedata reveal?Before the attacks, the hours workedby the police per head of population werehigher in the central London treatmentboroughs: 171.41 hours per 1,000 peoplecompared with about half that in thecontrol boroughs. After the attacks,deployment levels remained much thesame in the control areas (85 hours per1,000 people compared with 84 the yearbefore) but rose very rapidly in centralLondon: by 19 hours per 1,000 people,an increase of 11% over the level in theprevious year.The patterns of change in the totalcrime rate are similar. As withdeployment, the crime rate was higher incentral London before the attacks at 4.3HillingdonHarrowBarnetEnfieldHaringey WalthamForestRedbridgeBarking and DagenhamHaveringBexleyBromleyGreenwichLewishamCroydon SuttonMertonKingstonuponThamesRichmondupon ThamesHounslowEalingBrent2.1.3. 4.City of London5.NewhamTREATMENT AREAS1 Kensington  &  Chelsea2.  City of Westminster3. Camden4. Islington5 Tower  HamletsMiles  0 2 4 6 8  10HackneyWandsworthHammersmith & FulhamLambethSouthwarkFigure 1:A map of London boroughsCrime fellsignificantlyin centralLondonrelative to outerLondon in the six weeksafter 7 JulyCentrePiece Autumn 20069crimes per 1,000 people compared with2.2 per 1,000 in the control boroughs.After the attacks, the crime rate changedvery little in outer London, but in centralLondon, crime fell very sharply to 3.9crimes per 1,000 people, a 7% fall in thecrime rate.Our basic analysis strongly suggeststhat increased police presence after theLondon bomb attacks was linked toreduced crime levels. But there are severalreasons why reaching this conclusion maybe premature. There may be missingfactors that determine crime, for example,the number of potential crime victimsmight have fallen after the attacksoccurred (with fewer people travellinginto central London) and this, rather thanincreased police presence, could havebeen the key driving factor.Our more detailed analysis deals withsome of these possibilities. For example,throughout the rest of 2005 after theattacks, police deployment in centralLondon was about 8.5% higher thanbefore. But levels of deployment wereactually much higher in the six weeksafter 7/7 at 34% higher. Between the endof August and December 2005, policedeployment levels fell back to their pre-attack levels.Crime fell significantly after 7/7 at anaverage of 6.3% over the remainingweeks of 2005. But again, as with policedeployment, there was significantvariation across the first six weeks and therest of the year. The data show a 12%lower crime rate in the six weeksfollowing 7/7 – the period when theheightened security presence occurred –and a smaller 4.5% lower level in theremaining weeks of the year.The respective timings of the increasein police deployment and the fall in crimevery much suggest that the increasedsecurity presence lowered crime. Whenthe estimates for police deployment andcrime are considered together, theysuggest that a 10% increase in policeactivity reduces crime by around 3%.The results so far show a significantcausal impact of police on crime. But itseems reasonable to think that particulartypes of crime are more likely to havefallen. We can thus look at different crimetypes to provide a reality check on ourresults. The results (reported in ourforthcoming CEP working paper) arereassuring. For public thefts and violentcrime (two types of crime where wewould expect increased police presence incentral London to have an impact), therewere significant reductions in crime afterthe attacks.What’s more, the effect was bigger inthe six weeks in which the increase inpolice deployment was concentrated,providing further verification of our mainhypothesis. Analysis of borough-specificeffects also shows that crime fellsignificantly in all of the affectedboroughs, with stronger effects evident inWestminster and Tower Hamlets.This article summarises ‘Panic on the Streetsof London: Police, Crime and the July 2005Terror Attacks’ by Mirko Draca, StephenMachin and Robert Witt, a forthcoming CEPDiscussion Paper.Mirko Draca is a research economist inCEP’s productivity and innovationprogramme. Stephen Machin is CEP’sresearch director. Robert Witt is at theUniversity of Surrey.Thesefindingssuggest thata 10%increase inpoliceactivityreducescrime byaround 3%

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