The northern Paris suburb of Villiers-le-Bel erupted in riots on Sunday evening, when two teenagers going at 70kph on a mini-motorbike hit a police car and were killed. France’s banlieues are notoriously sad and anomic. During three weeks of nationwide rioting in 2005, 9,000 cars were burned and 2,900 people arrested. So in recent days, people have professed surprise that more has not been done to fix matters. But this is naive. In theory, riots reveal social needs that can be met with public projects. In practice, they show investors, planners and bankers a place with a record of destroying capital investment.

What happens to a neighbourhood after a serious riot? The answer from the US has generally been: not much. It took Washington more than a generation to restore its black business districts to a fraction – and only a fraction – of the lustre they had when rioters burnt them after Martin Luther King Jr’s assassination in 1968. Other cities, from Detroit to Newark, never really recovered from that year’s riots. As horrifying as they appear at the time, riots are frequently more damaging than they look. They can push an area not just into crisis but beyond rescue.