Are e-cigarettes safe? The answer seems to depend on where you are. This week, the US Food and Drug Administration cracked down on e-cigarettes, a move that the vaping industry fears will put many companies out of business. On the other side of the Atlantic, a prestigious British medical group just last week endorsed e-cigarettes to help with quitting smoking.

Assuming that nobody has gone off the deep end, how is it possible that public health officials can look at the same body of evidence and come to such radically different conclusions? Answer: Priorities.

It is indeed true that e-cigs lack the black tar that makes cigarettes so bad for your lungs. But the kind of math that goes into calculating individual health effects doesn’t necessary apply when you’re talking about a group of people. To craft public health policy, you have to ask three questions. 1) How many smokers are trading cigarettes for e-cigs? 2) How many non-smokers are getting addicted to nicotine thanks to e-cigs, whose many candy flavors seem to teenagers? And 3) Which number is bigger?

Because e-cigs are so new, nobody knows the answer. One side will publish an analysis that says one thing, and the other will criticize their statistics. They go back and forth and back and forth. Neither side has enough data to draw irrefutable conclusions.

But neither side is budging—just look at their response to the FDA’s new rule to regulate e-cigarettes as tobacco products. “It makes no sense,” says Michael Siegel, a public health researcher at Boston University, who has been a proponent of e-cigs for harm reduction. “They’re essentially favoring cigarettes over e-cigarettes.” Siegel is referring to a particular quirk of US tobacco regulation: The 2009 Tobacco Control Act grandfathered in all products on the market before 2007, which includes virtually no e-cigarettes. E-cigs will now have to go through an approval process to get to market that old cigarettes did not.

To the other side though, the FDA’s rule is a bare minimum. “All they really do is assert jurisdiction,” says Stanton Glantz, a vocal anti-tobacco advocate who is a professor of medicine at UCSF. "The best evidence that this rule represents a step (albeit small) forward," he wrote on his blog, "is the fact that Mike Siegel has described it as "'a disaster for public health.'" Siegel has since responded with a comment on the blog. (Glantz was actually a mentor to Siegel years ago).

Glantz doesn’t understand what his colleagues in the UK are up to either. “I am completely dumbfounded—most of these people are friends of mine—people I'm still working with on other issues," he says. “There’s the most bizarre groupthink going on over there.”

On the other hand, anti-tobacco groups in the US have taken a hard line against e-cigs. I asked Erika Sward of the American Lung Association to compare the safety of e-cigs and cigarettes. “We should never try to compare something to the safety of a cigarette," she replied. "Nothing is more lethal on the market than perhaps a bullet." Harm reduction isn't even in the picture.

This rhetorical split is really because of the difficulties of public health messaging. What you’d really want to say, in a perfect world, is OK, you heavy smokers, you should vape instead of smoke (but if you can, eventually stop vaping, too). You non-smokers though, don’t start vaping because nicotine is bad for you, too! But public health messaging, which needs to reach as many people as possible, is not a place for nuance. You end up hitting people over the head with just one message or the other.

A few small things are different between the two countries: The UK has some restrictions about the size and nicotine content of e-cigs that the US does not yet have. And the smoking rate is slightly higher in the UK. The UK's tobacco policy officials are clearly more sympathetic to the idea of harm reduction; the National Health Service even offers e-cigs to people who want to quit smoking.

But in the end, we’re still talking about two pretty similar countries that even share the same language. What that means is the US and UK’s divergent policies have set up a natural experiment to answer the question of whether e-cigs get more people to quit smoking or more people to addicted to nicotine. Eventually the scientists will know who's right.