It’s an iron rule of economics: If you want less of something, tax it. If you want more of something, subsidize it.

This iron rule captures a deep truth not just about money but about human nature — and we are seeing it play out in horrifying and dispiriting images of Baltimore under siege from within.

Of late, America has been offering a moral subsidy of sorts to rioting and looting.

We have been treating the nihilistic and destructive behavior of rioters and looters as though it is a legitimate expression of anger at alleged police misconduct. And so we are getting more of it.

If we were raising the cost of participating in this kind of illegal and destructive behavior — if we were taxing it, in other words — there would be less of it.

But authority figures in and out of government are finding it difficult to sort out the difference between legitimate protest and illegitimate destruction and separate the two.

That confusion is at the heart of the controversy that has swirled around Baltimore Mayor Stephanie Rawlings-Blake since Saturday night. She offered an earnest effort to do just that — but her own words betrayed her.

“I made it very clear that I work with the police and instructed them to do everything that they could to make sure that the protesters were able to exercise their right to free speech,” she said.

“It’s a very delicate balancing act. Because while we tried to make sure that they were protected from the cars and other things that were going on, we also gave those who wished to destroy space to do that as well. And we worked very hard to keep that balance and to put ourselves in the best position to de-escalate.”

It seems clear Rawlings-Blake meant to say she had told her city’s police department to make sure protesters were given an opportunity to express their anger at the unexplained death of Freddie Gray after he had sustained a spinal injury in police custody — that cops needed to balance a protester’s right to free speech with their responsibility to secure their safety and the safety of other Baltimoreans.

But what she meant to say and what she actually said were two different things. The words she actually spoke suggested the “balancing act” she had directed the cops to follow was between the rights of the protesters to protest and the rights of “those who wished to destroy.”

These words suggested that rioters and looters — “those who wished to destroy” — needed to be given “space to do that as well.”

This was an honest mistake, a moment of disastrous inarticulacy on the part of a worried and likely overwhelmed politician under intense pressure. She sought to clarify them later.

But it’s worth asking why she made the mistake in the first place. The answer, I think, is that Rawlings-Blake did not want initially to attack or criticize or express moral outrage at the vile conduct of the rioters and looters.

Why not? Primarily, one would hope, because her key goal was “to de-escalate” the situation, and she might have thought words of condemnation would do the opposite.

But there’s also the fact that since the deaths of Michael Brown and Eric Garner at the hands of police, pundits and community leaders and politicians alike have often failed to draw a bright-line distinction between proper protest on the one hand and large-scale social disruption and outright criminality on the other.

Those politicians and community leaders want police departments and police officers to feel the heat in order to provoke change.

It is their conviction that communities of color are being victimized, and that this victimization is the major moral stain society needs to address.

So they are loath, I think, to muddy the waters with excessive condemnation of lawless behavior, for fear the focus will shift away from the need for change.

As a result, it has often taken several days for America’s politicians to find their sea legs and say what they should have said the moment the trouble started. It took Rawlings-Blake two days to call the bad guys “thugs.”

And President Obama was briefed on Baltimore on Sunday, but it was Tuesday before he made a forthright statement: “There’s no excuse for the kind of violence that we saw yesterday . . . When individuals get crowbars and start prying open doors to loot, they’re not protesting, they’re not making a statement, they’re stealing.”

But the president also muddied the waters by describing the violence and looting as “counterproductive.” That choice of word itself suggests the criminals were somehow engaged in protest but lost their way or got out of hand.

By saying what he said, Obama followed Mayor Rawlings-Blake in offering an unconscious moral subsidy for the criminal disorder we have been watching for days now.

And so, rest assured, there will be more of it until our leaders understand that “no excuse” actually means “no excuse.”