Evidence of its effectiveness:

Researchers, working with police, identified 34 crime hot spots. In half of them, authorities set to work—clearing trash from the sidewalks, fixing street lights, and sending loiterers scurrying. Abandoned buildings were secured, businesses forced to meet code, and more arrests made for misdemeanors. Mental health services and homeless aid referrals expanded.

In the remaining hot spots, normal policing and services continued.

Then researchers from Harvard and Suffolk University sat back and watched, meticulously recording criminal incidents in each of the hot spots.

The results, just now circulating in law enforcement circles, are striking: A 20 percent plunge in calls to police from the parts of town that received extra attention. It is seen as strong scientific evidence that the long-debated “broken windows” theory really works—that disorderly conditions breed bad behavior, and that fixing them can help prevent crime.

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Many police departments across the country already use elements of the broken windows theory, or focus on crime hot spots. The Lowell experiment offers guidance on what seems to work best. Cleaning up the physical environment was very effective; misdemeanor arrests less so, and boosting social services had no apparent impact.