It’s something Americans hear after every mass shooting: The majority of voters support stricter gun laws. So why don’t these laws pass?

Two charts from the Pew Research Center, based on surveys of nearly 4,000 US adults this past spring, provide a key answer. They show, in short, that the people who oppose stricter laws are much more engaged in the debate than the people who support stricter laws.

Consider this first chart:

About 21 percent of gun owners have ever contacted a public official to express an opinion on gun policy, compared to just 12 percent of non-gun owners. And about 22 percent of people who want less strict gun laws have contacted a public official, while just 15 percent of people who support more strict laws have.

The differences are more pronounced if you look at contact in the previous 12 months. Gun owners are 80 percent more likely than non-gun owners to have contacted a public official about gun policy in the past year. And supporters of laxer gun laws are nearly 60 percent more likely than supporters of stricter gun laws to have contacted a public official over the issue in the same time span.

This difference in issue intensity also shows up in other areas. Take this second chart:

Again, the situation is skewed in favor of gun owners. About 28 percent of gun owners have contributed to an organization that takes a position on gun policy, while only 10 percent of non-gun owners have. That helps explain how a group like the National Rifle Association (NRA) has become so powerful, while there are no political equivalents — in terms of influence — on the other side.

One caveat to the charts: Based on Pew’s surveys, there are way more adults in the US who believe that gun laws should be more strict (52 percent) than those who believe gun laws should be less strict (18 percent). So the side in favor of stricter gun laws can afford to have a lower percent of its people contacting public officials.

It’s demonstrative, though, to focus on the 12-month data for this. As it shows, gun owners and supporters of laxer gun laws are more likely to have recently engaged a public official or political organization on this issue. That matters: If a senator gets phone calls every few months from gun owners and opponents of stricter laws, that’s going to make a bigger impression than calls from non-gun owners and supporters of stricter laws every year or so.

That’s especially true if you consider political party. Republicans are much more likely to oppose stricter laws, based on Pew’s data. So if a Republican senator mostly hears from GOP constituents, and these constituents are more likely to be really passionate about the issue since they are on the side that opposes stricter laws, that’s going to give the senator a very skewed perception of where voters are on this topic.

The charts demonstrate an issue that experts and political strategists have repeatedly raised to me in the past. As Republican lobbyist Grover Norquist said in 2000, “The question is intensity versus preference. You can always get a certain percentage to say they are in favor of some gun controls. But are they going to vote on their ‘control’ position?” Probably not, Norquist suggested, “but for that 4-5 percent who care about guns, they will vote on this.”

But what explains that difference in passion? Kristin Goss, a political scientist at Duke University and author of The Gun Debate: What Everyone Needs to Know, previously told me that it’s a sense of tangible loss. Gun owners feel like the government is going to take their guns and rights. Gun control advocates, meanwhile, are motivated by more abstract notions of reducing gun violence — although, Goss noted, the victims of mass shootings and their families have begun putting a face on these policies by engaging more actively in advocacy work, which could make the gun control movement feel more relatable.

The result is it’s more difficult for someone on the left to run on gun control, while someone on the right is forced to run on gun rights to avoid a revolt from a passionate base. That tilts the debate, regardless of how most of the public feels, in favor of doing nothing about guns.

For more on America’s gun problem, read Vox’s explainer.