Three principles of this manual are: (1) it takes more than offenders to create problems; (2) people cannot offend if there is no opportunity structure to support this behavior; and (3) altering the opportunity structures can dramatically reduce problems. It follows that responses focusing on only removing offenders have limited effects on problems. After some offenders are removed, there may be a decline in the problem for a short time, then either the old offenders return to take advantage of the opportunities, or new offenders start taking advantage of them. This is sometimes called perpetrator displacement. Natural replacement of offenders can be slow, particularly if the opportunities are obscure. But if someone discovered the crime opportunities in the past, others will rediscover them in the future. And if the old offenders were removed through imprisonment, some may return to take advantage of the opportunities upon their release.

New offenders attracted by opportunities might contribute to long-term crime cycles. Bank robberies in parts of the United States may be an example of this. For a few years there will be a large number of these crimes and then they will decline for several years, only to surge again later to start the cycle over again. One hypothesis for these cycles is that during peak robbery years, banks begin instituting a host of preventive measures and most offenders are caught and imprisoned. These efforts drive bank robbery down. After several years with few bank robberies, bank security becomes lax and the opportunities for bank robbery increase. Then new offenders start to take advantage of the lower security, beginning a new wave of robberies and prevention. This hypothesis draws attention to the fact that it takes more than enforcement to be effective, and prevention gains must be maintained to stay effective.

In fact, it is quite common to read descriptions of problem-solving efforts that begin with a description of failed enforcement efforts. In every situation either old offenders keep coming back or new offenders have replaced them. There are three ways in which new people are exposed to offending opportunities:

1. They are exposed to them through their normal daily routines. Police arrest young men stealing items from unlocked cars in a city center, for example, but unlocked cars with things in them remain there. Of the many people who use the city center on a daily basis, a few will notice these cars and try their hand at theft. If successful, some of these individuals will continue to steal from cars.
2. They are exposed to crime opportunities through informal networks of friends and acquaintances. People already experienced in taking advantage of an opportunity to commit crime or disorder may invite others in to help them or enjoy the experience. Since we are seldom 100 percent successful at removing all the offenders for long periods, there are usually many people around who can introduce new people to the opportunities.
3. They discover offending opportunities through recruitment. A criminal receiver may employ new burglars if the old ones can no longer supply him with goods. If prostitution is organized, then a pimp may recruit new prostitutes to fill the jobs left vacant by the former prostitutes. Gangs may bring in new members to replace old ones. It has been suggested that adult drug dealers, faced with stiffer penalties for drug convictions, started hiring juveniles to carry out the riskiest tasks because the penalties for juveniles caught with drugs were much less than for adults.

How do you find out if offenders are moving in? The most straightforward method is to compare the names of offenders associated with the problem before the response to the names of offenders associated with the problem after the response. If the names are different, then offenders may be moving in. The difficulty with this approach is that a complete roster of the offenders involved is seldom available. So it is not clear if the new names are really new offenders, or if they have been part of the problem for some time, but have only recently been discovered.

Offender interviews can also be helpful. Offenders may tell you when they became involved in the problem, how they became involved, and who else is involved. They can also provide information on tactical and other forms of displacement. However, offenders can be uncooperative and unreliable (Step 10).

Sometimes detailed examination of the methods used to commit crimes can provide insights into whether new offenders are involved. If the tactics are radically different than those used earlier, there is a possibility that new offenders are working. However, it is also possible that the old offenders have switched tactics.

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