The solutions to the nation’s problems already exist somewhere out in the country; we just do a terrible job of circulating them.

For example, if you want to learn how to improve city schools, look how Washington, New Orleans and Chicago are already doing it. Since 2011 the graduation rate at Chicago public schools has increased at nearly four times the national average, to 77.5 percent from 56.9 percent. The percentage of Chicago students going to two- or four-year colleges directly after graduation increased to 63 percent in 2015 from 50 percent in 2006.

How is Chicago doing it? Well, its test scores have been rising since 2003. Chicago has a rich civic culture, research support from places like the University of Chicago and a tradition of excellent leadership from school heads, from Arne Duncan to Janice Jackson, and the obsessive, energetic drive of Mayor Rahm Emanuel.

Chicago has expanded early childhood education and imposed universal full-day kindergarten. After a contentious strike in 2012, Emanuel managed to extend the school day. But he and the other people who led this effort put special emphasis on one thing: principals.

We’ve spent a lot of time over the past few decades debating how to restructure schools. We’ve spent a lot of time trying to help teachers. But structural change and increasing teacher quality don’t get you very far without a strong principal.

Researchers from the University of Minnesota and the University of Toronto studied 180 schools across nine states and concluded, “We have not found a single case of a school improving its student achievement record in the absence of talented leadership.”

What do principals do? They build a culture. Researchers studied test scores from half a million students in 72 countries. They found that students’ mindsets were twice as powerful in predicting scores as home environment and demographics were. How do students feel about their schooling? How do they understand motivation? Do they have a growth mindset to understand their own development?

These attitudes are powerfully and subtly influenced by school culture: the rituals for welcoming members into the community; the way you decorate walls to display school values; the distribution of power across the community; the celebrations of accomplishment and the quality of trusting relationships.

Principals set the culture by their very behavior — the message is the person.

Research suggests that it takes five to seven years for a principal to have full impact on a school, but most principals burn out and leave in four years or less. Chicago has one of the highest principal retention rates of any large urban system, 85 percent. Principals are given support, training and independence. If you manage your school well for a couple of years in a row, you are freed from daily oversight from the central office.

But the big thing is transforming the role. Principals used to be administrators and middle managers, overseeing budgets, discipline, schedules.

Today’s successful principals are greeting parents and students outside the front door in the morning. That Minnesota-Toronto study found successful principals made 20 to 60 spontaneous classroom visits and observations per week.

In other words, they are high-energy types constantly circulating through the building, offering feedback, setting standards, applying social glue. In some schools, teachers see themselves as martyrs in a hopeless cause. Principals raise expectations and alter norms. At Independence Middle School in Cleveland, principal Kevin Jakub pushes a stand-up desk on wheels around the school all day.

When you learn about successful principals, you keep coming back to the character traits they embody and spread: energy, trustworthiness, honesty, optimism, determination. We went through a period when we believed you could change institutions without first changing the character of the people in them. But we were wrong. Social transformation follows personal transformation.