Science doi:10.1126/science.1161405 (2008) The controversial ‘broken windows’ thesis holds that small signs of crime, such as smashed glass and graffiti, can lead to more of the same, or worse. This is the idea behind ‘zero tolerance’ policing. But strong, supportive evidence for the theory has been lacking. Kees Keizer and his colleagues at the University of Groningen in the Netherlands covertly observed fellow Groningers who were given the opportunity to steal an envelope that obviously contained a €5 note from a postbox. When the postbox was clean and tidy 13% took the bait; by contrast, 27% stole from a graffitied postbox and 25% from one with litter around it. Other tests showed that people are more likely to litter in the presence of graffiti or abandoned shopping trollies, and after hearing the crackle of illegal fireworks.