Crowds broke into and looted some Washington stores after riots broke out in April 1968. (The Post Archives)

On Wednesday, I wrote a column on the causes of the 1960s riots, and why it is a mistake to look at them largely as a result of economic factors or other material conditions. I’d intended to include a section on what you might call the “technical” factors behind riots — the things that aren’t directly related to race, or class, or politics, or economics.

Alas, when my column hit double its assigned length, there was simply no space. So for readers who wanted to go Full Wonk, here’s the discussion of those other factors.

Before we get into that, though, it’s worth answering one common complaint from readers who said “You idiot! The riots were about racism!”

Well, of course. Persistent discrimination and economic hardship stoke rage, and also leave many people with little to lose by being arrested. Racism is central to any discussion of that decade’s disturbances.

But this argument misses the question I tried to address, which is why we saw so much rioting during the 1960s. Racism was not new in the ’60s. So while racism obviously contributed to the riots, it can’t explain why the vast wave of riots happened then, and not in the 1940s or the 1950s, when the racial caste system was even more rigid, and economic conditions even worse, for blacks.

The answer I proposed was “disrespect” — specifically, the disconnect between the economic and legal advances that blacks were making and the social disrespect that persisted.

But “respect” is a broad answer, encompassing everything from police stops to America’s consumerist status system. From the ground, we can see some more-granular factors that affected when and where riots happened. Not all of them can be influenced by policy. But all of them give us a richer picture of what happened in our cities during the 1960s.

Start with the weather. “It turns out that the probability of a riot after the assassination was strongly affected by the weather,” says Robert Margo, an economist who has studied the long-term effects of the riots that followed the assassination of Rev. Martin Luther King Jr. “If the weather was unusually bad, it lowers the probability of a riot, and especially a severe riot.”

That’s because the most fundamental ingredient you need for a riot is a crowd on the street. Two people smashing windows and throwing rocks at police are felons on the verge of arrest; a thousand people doing the same thing are a civil disturbance that is not easy to control. Still, most large crowds obviously don’t riot, so while good weather may be necessary, it is not sufficient on its own.

In 1978, sociologist Mark Granovetter suggested a model for thinking about riots — and a variety of other social phenomena, from new-product adoption to birth-control usage. His idea is that people often have desires that are in tension with each other, and a threshold for switching from one behavior to the other. Maybe you’ll be reluctant to try an IUD, or move to a new neighborhood, unless a certain number of the people around you have already done so. But once you see enough of the neighbors doing it, you’ll go along.

That makes sense in the context of rioting. The more people are engaging in the rioting, the lower the likelihood of getting caught. We humans also have a bit of the herd instinct — our perception of whether an action is influenced by seeing others around us doing it.

In his book, “Everything is Obvious: How Common Sense Fails Us” sociologist Duncan Watts asks us to imagine a crowd of 100 people. The first person is willing to riot all by themselves; they have a threshold of zero. The second person has a threshold of one (meaning they need to see one other person rioting before they will too), the second of two . . . and so on, all the way up to 99. It’s easy to see that this crowd is almost certain to riot, because as soon as the first person throws a rock, the second person will join in, and each new rioter will trigger the threshold of the next until the whole crowd is involved.

But what if you took that person with a threshold of one, and replaced them with someone whose threshold was two? A tiny change, but now when that first person throws a rock, and no one else joins them, the riot dies in the cradle.

While it’s only a model, this can help explain one of the few factors Seymour Spilerman was able to identify as contributing to the likelihood of a local race riot in the 1960s: the absolute number of blacks in the local population. Surprisingly, the percentage of the population didn’t matter, only the total number.

But Granovetter’s model helps us understand why that might be. In the context of repressive policing tactics, a higher absolute number of black residents meant more police abuses. And the higher the number of police interactions with minorities, the more likely it was that one would occur in the presence of a riot-prone crowd.

Building on that insight, we can think of rioting as something that only occurs when two unpredictable events happen to coincide: A precipitating incident that suddenly lowers people’s riot thresholds, and a nearby crowd with a sufficient number of people whose riot threshold was already low enough to make them willing to start smashing things when provoked. Which would make it difficult — maybe impossible — to predict exactly when and where a riot is going to break out, because so much depends simply on the composition of the crowd.

However, there were a number of factors in the 1960s that were probably acting to lower average riot thresholds, making riots more likely everywhere. High among them: The frustration of seeing the civil rights movement fall well short of King’s dream of a nation in which people would be judged “not by the color of their skin, but by the content of their character.”

Then there are demographics. The baby boomers were just coming of age. Larger populations of younger people meant any given crowd would contain more of those with relatively low thresholds to act. And, of course, young men are disproportionately the targets of the police activity that often give rise to riots.

Those particular youths, as Kevin Drum of Mother Jones has documented, were also often the victims of lead poisoning. Lead paint was poisoning children who lived in old, badly maintained homes — exactly the homes that economic discrimination and residential segregation had forced upon so many black Americans. It was also being used as a gasoline additive, poisoning the very air those children breathed — and again, people living in the dense urban cores were the worst affected.

People who are exposed to excessive lead as children can suffer permanent brain damage, especially to their impulse control. This is one of the most potent explanations of the crime wave that swept America from the 1950s to the 1990s — and potentially, also of the wave of riots in the 1960s.

Now add migration, which had swelled the black population of northern cities, and persistent residential segregation that crowded large numbers of blacks into smaller spaces where they became targets for police. And the Vietnam War, where blacks were especially likely to be drafted, and die, during the early years, which increased social unrest in those communities.

Finally, add television, which first penetrated many American homes — particularly poorer American homes — in the 1960s. Suddenly, anyone could see police aggression toward people like them happening a hundred miles away. And anyone could also see other people rioting, making it more likely they would reach their own riot threshold.

All these things may help us understand 1960s. But that still leaves us with one mystery to solve: Why did the riots stop? We still have television, good weather and structural racial inequality.  Yet civil disturbances today are smaller and rarer than they were 50 years ago, and even massive riots don’t seem to set off similar events in nearby cities, as they did during the 1960s.

But, of course, riot thresholds can be raised as well as lowered. The Vietnam War is long over, as is the baby boom. Lead has been banned in paint and gasoline, and we are making progress at scrubbing it out of the environment. And while opportunity is still lacking for far too many black Americans, the pervasive overt discrimination of the past no longer constitutes an absolute bar to black advancement.

Sociologist Seymour Spilerman also noted that during the 1960s, even in places where blacks could legally vote, they had relatively few black politicians or organizations — relative to their proportion in the population-based political structure — that could represent their interests and channel the anger into less-violent outlets.

We haven’t riot-proofed our society, but we seem to have made it more riot-resilient. We could make it more so by continuing to work on the things that lower riot thresholds — overly aggressive policing, the lead that lingers in our homes and soils, the inequality of opportunity that persists throughout our country. Dealing with those issues will be expensive and difficult, and it won’t happen quickly. But it’s the right thing to do — and better, by far, than praying for rain.