How does change happen?

More to the point, in the wake of12 gun murders in the Navy Yard and a 3-year-old shot in Chicago: Why doesn’t change happen when the need seems so obvious?

It seems obvious that military-style weapons with no hunting or self-defense purpose should not be circulating. It seems obvious that people who hear voices and repeatedly fire guns in anger should be treated before they can buy more guns.

Yet nine months after the school shooting that was supposed to shift these propositions from obvious to undeniable, not much has changed, and no one expects much to change.

One reason is that even people who agree there is a problem disagree on targets for reform: handguns, semi-automatic weapons, multi-round magazines, waiting periods, gun shows, mental-health exemptions, gun locks, concealed carry and more. No single change will “solve” the problem.

The more important factor is that the opposition to reform is focused, passionate, unyielding and indefatigable.

You could argue that this principle of single-mindedness — call it the National Rifle Association Rule — can explain, in reverse, the greatest example of change achieved so far this century: the growing acceptance of gay marriage. Once advocates of gay rights coalesced around marriage (as opposed to, say, workplace rights) as their unitary goal, they were focused, passionate, unyielding and indefatigable. Many Americans are strongly opposed, but few are uniquely focused on stopping same-sex marriage.

Similarly, proponents of marijuana legalization focused on the politically sellable concept of “medical marijuana” and are now making gains on broader decriminalization. Anti-legalization forces, more diffuse, are on the defensive.

The “Dreamers,” too, might turn the rule to their advantage. Young immigrants brought to the United States as children and thus undocumented through no fault of their own — and who have a huge incentive to work the political process to achieve legalization — might succeed even if large-scale immigration reform languishes. It is hard to imagine the opposition remaining as intense and unblinking as the Dreamers could be.

But this is not the only route to change. Sometimes Americans evolve because they decide that what exists simply isn’t right. Prisoners are hardly a sympathetic constituency, but the country is moving away from tolerating rape as an inevitable side effect of incarceration. Young killers don’t generate sympathy, but a few determined advocates have helped move the country toward a view that no juvenile should be executed or locked away for life.

And intensely focused blocking agents don’t always win. Teachers unions have waged a fierce campaign against evaluations based on test results. The war is far from over — post-Bloomberg New York City may be in for a period of regression — but the general principle, accepted by Republicans and Democrats, won’t go away.

Teachers unions may be more open to reform because plenty of teachers who favor accountability, if fairly designed, will remain in the union. Gun owners who favor modest gun control are more likely to leave, or never join, the NRA than try to influence it from within.

But school reform is also succeeding because over time most people have accepted its logic, and it is hardly the only example of sensible policy eventually making a case for itself. Tobacco companies were, like the NRA, focused, unyielding and indefatigable in their opposition to any measures designed to discourage smoking. For many years they were maddeningly successful. But evidence and popular opinion eventually overwhelmed them.

Mothers Against Drunk Driving faced similar odds — first to overcome public apathy and ignorance and then in battling focused opponents such as bar owners and defense attorneys. Their fight, too, never ends, but on the general proposition they have prevailed. So have movements for seat belts and air bags, despite concentrated opposition from the car industry.

This is hardly a complete list, but it does suggest a couple of lessons. One is the power of the public-health argument. Americans believe that anyone 16 and older who can drive is entitled to a license, but we allow the state to put limits on that freedom in order to protect teens, discourage drunken driving and otherwise promote the general welfare.

Americans aren’t going to cede their right to own guns for sport and self-defense. But policies that focus on reducing the dangers of accidental shootings, gun suicides, crimes of passion and mass shootings — over time, with a public health focus, I think they have a chance.

And that is the other lesson: over time. Sometimes reform takes years or decades of slogging, seemingly hopeless effort. At an unpredictable moment, public sentiment teeters and then tips. What everyone knew was impossible is seen as having been, all along, inevitable.

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