Brandon Stanton, the photographer behind Humans of New York, creates images that prompt conversations about compassion, struggle, and humanity. Sometimes they’re sad, sometimes they’re funny, and sometimes they’re WTF — but rarely do they prompt the kind of snarly debate that swirled on Facebook in response to yesterday’s photo of a man and a kid riding a bike through the city.

Here’s a taste of what people had to say:

“Motorists have no respect for bicyclists. NYC especially. Dad needs to wear a helmet, & needs to have his vulnerable child wear one.”

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“Omg wa wa wa no helmets. How the hell did we all survive the 70s an 80s.”

“wah wah wah tell that to people who have permanent brain damage from falling off of a bike.”

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“noone wears helmets in The Netherlands. Kids go to school on their parents bikes. Sometimes 2 kids per bike.”

“What a foolish statement. NYC is not The Netherlands.”

It’s certainly not, dear commenter! “Dying while cycling is three to five times more likely in America than in Denmark, Germany or the Netherlands,” The Economist wrote in 2011. And the vast majority of cyclists who died in the U.S. were not wearing helmets.

Currently 21 U.S. states and Washington, D.C., have laws requiring helmets for kids under 18. Only Australia and New Zealand require everyone to wear a helmet all the time (and some of those cyclists are none too pleased about it). Most of Europe could not be bothered with any of this — they’re too busy riding everywhere, helmet-free, with multiple kids hanging tight on their back racks. And they are way, way, way safer than us.

So what’s the deal? Does riding with a helmet actually make you safer, or are you better off bare?

Americans bike far less than our friends across the pond. According to government data [PDF], not even 1 percent of all trips in the U.S. are taken by bike, compared with nearly 30 percent in the Netherlands, 18 in Denmark, and 10 in Germany. Still, bicycling is on the rise in the U.S.: Between 1995 and 2009, bike trips grew by 30 percent.

It logically follows that with more people on the road, more might end up in the emergency room — right? Nope! Cycling injuries fell by 37 percent between 1995 and 2011, and fatalities fell by 18 percent, according to Department of Transportation data.

Bike helmets no doubt have something to do with that. Head injuries are responsible for three-quarters of cyclist deaths, and helmets undoubtedly keep us safer in the event of a head-to-pavement collision — a 2009 study found they provide a 63 to 88 percent reduction in the risk of head and brain injuries in the event of a crash.

But helmets may not be doing quite as great of a job as we thought. In this month’s Bicycling magazine [PDF], Bruce Barcott looks at how modern helmets are designed to protect against skull fractures, but don’t do much for preventing traumatic brain injuries, such as concussion, which also sometimes go undiagnosed.

Most of us reflexively strap on helmets assuming they’ll protect us. But how well do they actually do the job? … As more people buckled on helmets, brain injuries also increased. Between 1997 and 2011 the number of bike-related concussions suffered annually by American riders increased by 67 percent, from 9,327 to 15,546, according to the National Electronic Injury Surveillance System, a yearly sampling of hospital emergency rooms conducted by the U.S. Consumer Product Safety Commission (CPSC).

Most helmet designs haven’t changed in decades to account for different types of crashes, Barcott explains — and only a newer kind of helmet engineered to deal with multiple types of forces would fully protect your brains from a bash.

There’s even a case to be made that helmets actually increase the risk of injury. Institute of Transport Economics researchers Aslak Fyhri and Ross O. Phillips “found that after having removed their helmets, routine helmet users cycled more slowly and demonstrated increased psychophysiological load.” In other words, helmet-wearers might be engaging in riskier cycling behavior than they might if they went bare.

Studies have also shown that drivers are more likely to give a wider berth to cyclists riding without a helmet, especially long-haired women. “We know from research that many drivers see cyclists as a separate subculture, to which they don’t belong,” traffic psychologist Ian Walker said in 2006. “As a result they hold stereotyped ideas about cyclists, often judging all riders by the yardstick of the lycra-clad street-warrior. This may lead drivers to believe cyclists with helmets are more serious, experienced and predictable than those without.”

So what if we eliminated all these variables and just forced everyone to wear helmets? Earlier this week Eric Jaffe at Atlantic Cities looked at how mandating the use of those head protectors actually works in practice, based on research led by Jessica Dennis at the University of Toronto: “[R]esearchers concluded that head injuries were decreasing across the country at a rate that wasn’t ‘appreciably altered’ by the new helmet laws. Other rider health initiatives — namely, public safety campaigns and the introduction of better bike infrastructure — rendered the contribution of helmet laws ‘minimal.'”

Given all this, I’m inclined to agree with this HONY commenter in response to the photos of the guy and his kid on the bike: “what you all should be saying is: WHAT?! NO BIKE LANE!?!”

Regardless of what your helmet will or will not do for you, it is quite clear that bike lanes and other infrastructure do make riders safer. But the other thing that has been shown to help — and this is Europe’s big secret to success — is more bikes on the streets.

Researcher Peter Jacobsen found in 2003 that there’s sheer and significant safety in numbers. Simply, the more pedestrians and cyclists on the road, the less likely they are to get hit by cars: “Since it is unlikely that the people walking and bicycling become more cautious if their numbers are larger, it indicates that the behavior of motorists controls the likelihood of collisions with people walking and bicycling. It appears that motorists adjust their behavior in the presence of people walking and bicycling.” Let’s call it the Amsterdam effect. (No no, the other Amsterdam effect.)

Ultimately the only thing that’s really going to make cyclists safer isn’t just better, more supportive infrastructure — it’s hitting a critical mass of ridership that may trigger a cultural change, Euro-style.

The good news is, if you build it, they will come and ride on it: Cycling growth is twice as big in cities that are building supportive infrastructure than in ones who are all just, “Good luck, wear a helmet.” (Of course, just because you build it doesn’t mean cars won’t still park in it — but there are solutions for that, too.)

Bike-share programs like the one just launched in New York City could also be a great way of boosting those numbers, but they have a helmet problem of their own. Some pols wanted helmets required, but Mayor Bloomberg said that might discourage ridership.

Will riders BYOH, go helmet-free, or choose not to ride because of all that hassle? If those programs could get enough new cyclists on the roads, the helmets may be beside the point altogether. Until then, though, I’ll still be buckling up.