One common denominator of successful bike programs around the world -- from Paris to Barcelona to Guangzhou -- is that almost no one wears a helmet, and there is no pressure to do so.

In the United States the notion that bike helmets promote health and safety by preventing head injuries is taken as pretty near God's truth. Un-helmeted cyclists are regarded as irresponsible, like people who smoke. Cities are aggressive in helmet promotion.

But many European health experts have taken a very different view: Yes, there are studies that show that if you fall off a bicycle at a certain speed and hit your head, a helmet can reduce your risk of serious head injury. But such falls off bikes are rare -- exceedingly so in mature urban cycling systems.

On the other hand, many researchers say, if you force or pressure people to wear helmets, you discourage them from riding bicycles. That means more obesity, heart disease and diabetes. And -- Catch-22 -- a result is fewer ordinary cyclists on the road, which makes it harder to develop a safe bicycling network. The safest biking cities are places like Amsterdam and Copenhagen, where middle-aged commuters are mainstay riders and the fraction of adults in helmets is minuscule.

"Pushing helmets really kills cycling and bike-sharing in particular because it promotes a sense of danger that just isn't justified -- in fact, cycling has many health benefits," says Piet de Jong, a professor in the department of applied finance and actuarial studies at Macquarie University in Sydney. He studied the issue with mathematical modeling, and concludes that the benefits may outweigh the risks by 20 to 1.

He adds: "Statistically, if we wear helmets for cycling, maybe we should wear helmets when we climb ladders or get into a bath, because there are lots more injuries during those activities." The European Cyclists' Federation says that bicyclists in its domain have the same risk of serious injury as pedestrians per mile traveled.