In addition to geographical and temporal displacement, offenders can switch targets, change their tactics, or change crimes.
Target displacement involves offenders shifting from newly protected targets to other targets. In 1970, when steering column locks became required in all new cars sold in Britain, thefts of new cars dropped from 20.9 percent of all cars stolen in 1969, to 5.1 percent in 1973. However, the overall theft rate of automobiles stayed roughly constant because offenders switched from the newer, protected, vehicles to older, unprotected, vehicles. This is one of the few documented cases where displacement wiped out most prevention gains, at least in the short run. Over a longer period, these devices appeared to have curbed theft for temporary use. As this case illustrates, it is easy for offenders to switch to very similar targets. Target displacement is less likely when the alternative targets are unlike the old targets.
Step 42 shows how geographical or temporal displacement can contaminate control groups. If the evaluation of a prevention effort uses a target control group, then a similar form of contamination can take place. Imagine a response to curb theft of purses from women over 60 years old in a shopping center. To estimate what the trend in elderly purse theft would be if nothing had been done, the theft of purses from middle-aged women of 45 to 59 is measured. If, unknown to us, the thieves displaced from the protected older women to unprotected middle-aged women, we would conclude that purse theft would have gone up without a response. When we compare this control target group change to the treatment group change we would mistakenly inflate the treatment effectiveness. A better control group might be even younger women shoppers (ages 30 to 44, for example), or even better, wallet theft of male shoppers. Though neither of these alternatives is perfect, they are improvements because one would expect far less displacement to dissimilar targets. Or, select another shopping district as a control area - in which case you would have to guard against geographical diffusion or displacement contamination, Steps 48 and 51.
Tactical displacement occurs when offenders change their tactics or procedures. They might use different tools to defeat better locks, for example. Or computer hackers might alter their programs to circumvent improved security. In medicine, some bacteria can mutate quickly so a drug that is effective against one form of the bacteria becomes less effective as mutant strains become more prevalent. One way of countering this is to use broad-spectrum treatments that are effective against a wide range of mutations. Similarly, "broad spectrum" responses protect against existing methods used by offenders and many modifications of these tactics. Broad-spectrum interventions require offenders to make big changes in their behavior that they may not be able to do. Paul Ekblom describes attempted tactical displacement following the installation of barriers in British post offices to prevent "over-the-counter" robberies; some offenders tried using sledge-hammers. This change in tactics was not particularly successful, however, and displacement was limited. These barriers are an example of a broad-spectrum intervention as they were able to defeat new tactics.
Switching crime type is another type of displacement you should look for. Offenders might switch from vehicle theft to vehicle break-ins, or carjacking. We sometimes evaluate responses to one type of crime by comparing the trend in a similar type of crime that did not get a prevention response. For example, we might select theft from vehicles as a control in the evaluation of a theft of vehicles intervention.
The same principles of contamination and protection apply to tactical and crime type displacement as we found with other forms of displacement. If the tactic or crime type is very similar to the tactic or crime type being addressed, then displacement could contaminate these controls. Dissimilar tactics or crime types are less likely to suffer contamination. But if they are too dissimilar it is not a useful control.
There is no perfect solution to this problem and compromises must be struck. The consequence is that it is often difficult to know if displacement is occurring and difficult to judge the effectiveness of the intervention. Compounding these difficulties is that multiple forms of displacement can occur simultaneously. Indeed, sometimes one form of displacement will necessitate another form as well. Target displacement may require a change in tactics, and if the new targets are not in the same places as the old targets, geographical displacement will occur, too.
You cannot find displacement unless you look for it. This means that you should examine a problem closely and imagine the most likely forms of displacement. Are there other opportunities for crime or disorder that are similar to the opportunities your efforts are trying to block? Will your offenders easily discover these opportunities? Looking for displacement opportunities prior to finalizing a response gives you two advantages. First, you can develop measures for detecting it should it appear. More important, you may be able to develop counter-measures that prevent displacement.
A study of target displacement: helmet laws and the reduction in motorcycle theft
In Germany (as elsewhere) the enactment of helmet laws was followed by large reductions in thefts of motorcycles. After the laws were brought into place in 1980, offenders wanting to steal a motorbike had to bring a helmet or they would be spotted quickly. The figure shows that by 1986 thefts of motorbikes had dropped to about one-third of their level in 1980, from about 150,000 to about 50,000. (The gradual decline probably reflects stronger enforcement and growing knowledge about the requirement.) This fact suggests that motorbike theft has a much larger opportunistic component than anyone would have thought. The existence of excellent theft data in Germany allowed researchers to investigate whether the drop in motorcycle theft had resulted in target displacement to theft of cars or bikes, other forms of personal transportation.
The other two lines show the national totals for car and bike thefts during the same years. These provide some limited evidence of displacement in that thefts of cars increased by nearly 10 percent between 1980 and 1986, from about 64,000 to 70,000. Thefts of bicycles also increased between 1980 and 1983, but by the end of the period had declined again to a level below that for 1980. Altogether, it is clear that at most only a small proportion of the 100,000 motorbike thefts saved by the helmet laws were displaced to thefts of other vehicles.
A little thought shows why this may not be surprising. Motorbikes may be particularly attractive to steal. They are much more fun to ride than bikes for the young men who comprise most of the thieves. Even if the intention is merely to get home late at night, a motorbike offers significant advantages, especially if the distance is more than a few miles. Motorbikes may also be easier to steal than cars since the latter have to be broken into before they can be started. Like bikes, cars also offer less excitement than motorcycles and they may require more knowledge to operate.
Source: Mayhew, Pat and colleagues (1989). Motorcycle Theft, Helmet Legislation and Displacement. Howard Journal of Criminal Justice 28:1-8.
- Ekblom, Paul (1987). Preventing Robberies at Sub-Post Offices: An Evaluation of a Security Initiative. Crime Prevention Unit Paper 9. London: Home Office.
- Webb, Barry (1994). "Steering Column Locks and Motor Vehicle Theft: Evaluations from Three Countries". Crime Prevention Studies, volume 2, Ronald Clarke. Monsey, NY: Criminal Justice Press. (Accessible at: www.popcenter.org).