How dumb do Washington bureaucrats think you are? Really amazingly dumb. Now we have hard data to prove it.

Instead of polling Americans about Washington again, a pair of academics at Johns Hopkins tried something new — polling Washington about Americans. What they found was a combination of ignorance, contempt and disdain.

Survey data from the polled group — staffers from the White House and Capitol Hill plus career civil servants and the policy community of lobbyists and others who work closely with government from outside it — indicate that the functionary class thinks of itself as our betters. Our bosses, not our representatives. They see their own judgment as being far superior to that of the rest of us — the people whose wishes they are supposed to be carrying out.

The revelations in the new book “What Washington Gets Wrong: The Unelected Officials Who Actually Run the Government and Their Misconceptions about the American People,” by Jennifer Bachner and Benjamin Ginsberg, serve up the side benefit of providing a partial explanation for the rise of Donald Trump.

The DC insider class is much like the media class: Both misunderstand who the voters are and how they think. You could drive a Trump Train through that gap, and Trump just did.

The findings were revealing: By a huge margin, the bureaucrats said they knew better than the public what was right for the public. On Social Security, twice as many bureaucrats said they knew best. On crime, three times as many bureaucrats said their way was superior. On the environment, the ratio was almost four to one.

Reminder: As a group, these people enjoy calling themselves “public servants.”

As the authors hasten to acknowledge, professionalism is important. Lawyers and doctors really do know more about their professions than non-specialists. But lawyers and doctors don’t have such contempt for their clients. They consult with us, explain what they’re doing and why. That’s because we can go across the street to another provider if we don’t like what we’re hearing.

Washington is less and less interested in taking any kind of principles-based guidance from the people it purports to work for. An unresponsive blob, it grows larger each year, forever chewing up more of our tax dollars, creating new agencies, passing binding new administrative rules without any involvement from lawmakers.

Presented with simple multiple-choice quizzes, the bureaucrats failed badly. For instance, 65 percent of the DC insiders guessed that median household income is lower than it is in reality (about $52,000 a year). Almost four out of five respondents underestimated the percent of the population that is white (which is 78 percent of Americans). Sixty-four percent of those surveyed underestimated the cohort of Americans (age 25 and up) who have a high school diploma: It’s 85 percent. And 80 percent of respondents guessed that the rate of homeownership is lower than it is: 67 percent.

With all of this underestimating going on, it’s not surprising that Washington is constantly pushing urgent, potentially disastrous fixes (such as re-inflating the housing bubble by encouraging more and more Americans with sketchy credit ratings to buy homes) for imaginary ills. Renting your home is a perfectly acceptable way to live, and Fannie Mae shouldn’t be in the business of enticing renters of modest income to commit to large amounts of debt by obtaining mortgages.

Officialdom overestimates, by an average of 8 percentage points, the proportion of Americans who support increasing government spending in the areas of education, crime prevention, welfare and child care. This is blob bias: the mistaken belief that Americans want to direct more and more of our income to the blob so it can (fail to) solve more of our problems.

Occasionally, the blob gets a little too secure in itself and broadcasts to the world what it’s doing and how it thinks. Notoriously, in 2014, the architect of the Affordable Care Act and MIT professor Jonathan Gruber was revealed to have admitted that ObamaCare, which was passed against the wishes of the American people using a parliamentary trick, was designed in a deliberately misleading and “tortured” way in order to fool the voters, the Congressional Budget Office, even Capitol Hill. “Lack of transparency is a huge political advantage,” Gruber said. “Call it the stupidity of the American voter or whatever, but basically that was really, really critical for the thing to pass.”

Asked to estimate how much knowledge Americans have about various issues, the bureaucrats gave answers that were frankly contemptuous. According to government officials, 72 percent of Americans know “very little” or nothing about government aid to the poor. Eleven percent of officials think Americans know nothing — absolutely nothing — about science and technology policy, and another 60 percent think we know very little. Sixty-three percent of officials think we know little or nothing about environmental policy. According to the bureaucracy, 6 percent or less of Americans know “a great deal” about any of the nine issues sampled.

Trump has provided an outlet for Americans whose primary concerns were being downplayed by other candidates.

Trump, meanwhile, embodies a sense that not just DC elites but elites in general — cultural, media and academic figures included — are contemptuous of ordinary working Americans. He roared to the head of the Republican pack in a crowded primary season in large part because he was the only one who seemed authentically angry about the sense that the elites are happily building huge walls around themselves (while opposing a wall on the southern border).

Enforcing existing immigration law strikes DC elites as unnecessary or even undesirable (since economically insulated people employ, rather than compete with, illegal immigrants). But even if such a policy were necessary, it would still be impractical and/or racist, according to the elites.

Trump voters are disgusted by the suggestion that looking out for their own economic and cultural self-interests is horrible and racist when ordinary Americans do it, yet wonderful and open-minded when elites do it. So economic insecurity is closely tied to political correctness: People who feel victimized, or even threatened, by NAFTA or the presence of 11 million illegal immigrants think, “If globalism costs me my job, why should I be called a racist for noticing?”

Trump has provided an outlet for Americans whose primary concerns were being downplayed by other candidates. Washington won’t acknowledge that 71 percent of voters (according to an August Pew survey) think immigration is either a major problem or a moderately big problem. Fully 88 percent of voters think crime is a problem, including 45 percent who say it’s a very big problem. The vast majority of Trump supporters believe there is a link between illegal immigration and crime.

In the surveys conducted for “What Washington Gets Wrong,” there aren’t a lot of questions about crime and terrorism, but in one revealing chart, we learn that 54 percent of DC insiders think Americans know very little or nothing about proposed policies to deal with citizenship for illegal immigrants. Insiders think only 1 percent of Americans know “a great deal” about the subject. And most insiders (53 percent) think average Americans know very little or nothing about crime policy too. (Again, just 1 percent think we have “a great deal” of knowledge about the subject.)

As embodied by President Obama, who is constantly telling people that terrorists are no more likely to kill you than your bathtub, the political class thinks jihadism is under control. But 71 percent of the public calls terrorism either a huge problem or a moderately big one. Terrorism-focused voters support Trump over Hillary Clinton by a 20-point margin, according to a Washington Post-ABC News poll released two weeks ago.

More mundane examples of the arrogance of our ruling class are legion. When a congressman asked, on behalf of the taxpayers, the chief of the Consumer Financial Protection Bureau to talk about how much the agency’s new headquarters would cost (an estimated $125 million), director Richard Cordray replied snidely, “Why does that matter to you?” The blob wants your money to build itself grand palaces but gets snippy when you ask about the details.

In the past few years, agencies have created over 300,000 regulatory offenses, creating a thicket of rules that only specialists such as lawyers and lobbyists can negotiate. The principle, mentioned in the Declaration of Independence, that governments derive “their just powers from the consent of the governed” is being flouted: Not only do we not consent to every policy enacted in Washington, we often don’t even know about it until it’s law.

Take the citizens of Franklin Township, NJ, who discovered in 2011 that they couldn’t send a tractor over to remove a tree that fell into a creek and caused flooding. The feds had classified it as a “Class C-1 creek,” meaning federal permits were needed before any natural condition could be altered. So the flooding continued for 12 days, damaging many homes in the area, before the town was able to secure the federal permit.

City government, county government, state government — none of these can be trusted, apparently.

In Virginia, an 11-year-old girl who rescued a wounded woodpecker and carried it into a Lowe’s hardware store in a cage was confronted in the store by an agent from the US Fish and Wildlife Service. Later she received a notice saying she was being fined $535 and threatened with jail time for violating the Migratory Bird Treaty Act. The resulting bad publicity made the government back down.

City government, county government, state government — none of these can be trusted, apparently. More and more, the ruling class arrogates to itself even the most picayune decisions, which leads to your little girl getting menaced at Lowe’s. The more powerless we become, the angrier we become. Trump voters are frequently derided as frothing loons and haters, but there is plenty to be angry about.

Once a Clinton convinced the nation he felt our pain; today a Clinton who should be winning easily has had difficulty connecting with voters because she doesn’t feel their anger. In fact, as a Washington insider who ignores whatever rules she finds inconvenient but embodies the idea that ordinary Americans need lots of direction from elites, she’s a leading member of the class voters are angry with.