ONE spectacular Sunday in Paris last month, I decided to skip museums and shopping to partake of something even more captivating for an environment reporter: Vélib, arguably the most successful bike-sharing program in the world. In their short lives, Europe’s bike-sharing systems have delivered myriad benefits, notably reducing traffic and its carbon emissions. A number of American cities — including New York, where a bike-sharing program is to open next year — want to replicate that success.


So I bought a day pass online for about $2, entered my login information at one of the hundreds of docking stations that are scattered every few blocks around the city and selected one of Vélib’s nearly 20,000 stodgy gray bikes, with their basic gears, upright handlebars and practical baskets.


Then I did something extraordinary, something I’ve not done in a quarter-century of regular bike riding in the United States: I rode off without a helmet.


I rode all day at a modest clip, on both sides of the Seine, in the Latin Quarter, past the Louvre and along the Champs-Élysées, feeling exhilarated, not fearful. And I had tons of bareheaded bicycling company amid the Parisian traffic. 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.


Yet the United States National Highway Traffic Safety Administration recommends that “all cyclists wear helmets, no matter where they ride,” said Dr. Jeffrey Michael, an agency official.


Recent experience suggests that if a city wants bike-sharing to really take off, it may have to allow and accept helmet-free riding. A two-year-old bike-sharing program in Melbourne, Australia — where helmet use in mandatory — has only about 150 rides a day, despite the fact that Melbourne is flat, with broad roads and a temperate climate. On the other hand, helmet-lax Dublin — cold, cobbled and hilly — has more than 5,000 daily rides in its young bike-sharing scheme. Mexico City recently repealed a mandatory helmet law to get a bike-sharing scheme off the ground. But here in the United States, the politics are tricky.


SHAUN MURPHY, the bicycling coordinator of Minneapolis-St. Paul — which inaugurated its “Nice Ride” bike-sharing program this year — has been pilloried for riding about without a helmet. “I just want it to be seen as something that a normal person can do,” Mr. Murphy explained to the local press this past summer. “You don’t need special gear. You just get on a bike and you just go.”