One of the fundamental facts of criminology is that a small proportion of individuals commit a large proportion of crime. Data from Marvin Wolfgang's famous Philadelphia cohort study suggested that around 5 percent of offenders account for 40 percent of crimes. There are two explanations for repeat offending, the first of which is that impulsive individuals, with weak social attachments to others tend to get into trouble more frequently than less impulsive and more attached individuals. The second explanation is that people exposed to more crime and disorder opportunities take advantage of them and adjust accordingly (see Step 9). Both of these theories can be true. Impulsive individuals with weak attachments require routine exposures to crime opportunities to become repeat offenders.

Repeat offending can be detected by testing for the presence of the 80-20 rule (Step 18). This can be difficult in practice because offenders try to remain anonymous, so the data are seldom comprehensive, and may not even exist. Intelligence information can provide evidence of repeat offending, but the quality of this information is highly variable and seldom comprehensive about the offender population. We often know far more, and know it with greater validity, about places and victims than we know about offenders. Nevertheless, systematic interviews with offenders and their associates can reveal very useful information for understanding and addressing problems (Step 10).

Understanding repeat offenders' objectives and motives can help create prevention strategies. It can make a difference to the solution of a vehicle theft problem if the thieves want to have a good time riding around in a fancy car, to obtain transportation home after a late night of partying, or to sell it for cash to support a drug habit. It can make a difference to the solution of a graffiti problem if the offenders are marking gang territories, creating "public art", displaying their affection, or trying to terrorize local residents of a different religion, race, or ethnicity.

Successful offending can lead to more offending. This occurs in three ways:

• Offenders, like others, learn from doing. A successful crime teaches important lessons. This can lead to the offender attacking the same target again (see box). But offenders, like everyone else, can generalize. So they learn that they may be successful if they attack similar targets (see Step 29).
• Offenders learn from each other. Information can spread through individuals working in small groups, group breakup and new group formation. This underscores the need to understand offender networks. Police can use networks to spread information that enhances offenders' perceptions of risks or of the undesirability of the target or place. Part of the effort to reduce juvenile homicides in Boston, Massachusetts for example, involved highly targeted messages to gang members.
• Successful offending can erode prevention, thus making subsequent offending easier. A small break in a fence, for example, will become larger with use. If the influx of offenders and offensive behaviors is faster than the responses of guardians and place managers, then a small problem will become worse.

Many crime prevention techniques rest on the assumption of a credible threat (Step 40). Closed circuit television (CCTV) provides a deterrent threat to the extent that potential offenders believe either that someone is watching who will take action should they see misbehavior, or that offenders can be identified and arrested later based on CCTV recordings. This does not mean that there have to be many arrests, but a few well-publicized arrests can reinforce an important message. And the message may be powerful if it is communicated through offender networks.

When there is specific information that a few people are responsible for most of a problem, it can be productive to focus on these individuals. The Boston Police Department reduced homicides among young males by monitoring a relatively few gang members. Francis Cullen and colleagues suggest that probation and parole authorities should learn the specific circumstances under which each offender gets into trouble, then help offenders develop plans to avoid these circumstances, and finally monitor compliance with these plans.

Tackling repeat offending by removing facilitating environments can be effective. For example, in Staining, a village in England, a scrap yard served as a receiver for stolen vehicles, parts, and other loot from thefts. Many of the associated offenders were known. But despite police enforcement efforts this problem could not be resolved. The local constable was able to close the site using laws governing pollution and other environmental hazards. This substantially reduced crime in the village. Similarly, police in the United States often use civil laws to close down facilities that foster drug dealing, prostitution, and other crimes and disorder.

Conversely, creating crime opportunities to catch offenders can make things worse. In the late 1970s and early 1980s, a number of U.S. police departments experimented with "sting" operations in which they created fake markets for stolen goods, documented who sold such goods to them, and then arrested many thieves. A number of these operations were evaluated. There is no evidence that these operations reduced crime. There is some evidence that they may have increased crime by providing lucrative and convenient ways to sell stolen goods. Throughout this manual we have noted the strong influence facilitating environments can have on promoting criminal behavior. So one should be very cautious about creating artificial crime opportunities to round up unknown prolific offenders.

Information from repeat offenders and their confederates can be used to identify features of the environment that facilitate offending. Much of the early crime prevention implemented in convenience stores was developed from offender interviews (Step 9). In the early 1970s, the Lakewood, Colorado Police Department interviewed convicted burglars and learned a great deal about how they targeted dwellings and handled stolen goods. The Newport News, Virginia Police Department used offender interviews to help analyze thefts from vehicles. An important piece of intelligence they gained was that thieves targeted vehicles that the thieves believed contained drugs. More recently, when the Chula Vista, California Police Department interviewed car thieves they found that the thieves had a much simpler method for stealing cars than investigators had suspected. This alerted investigators to an unknown vulnerability of older cars of a particular make. Such information is available from no other source.

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