Since the economy began adding jobs after the Great Recession nine years ago, about 21.5 million jobs have been created in the United States, the second-best stretch of hiring in the nation’s history, second only to the 1990s.

But job growth isn’t being spread evenly across the land. Most of the new jobs have been located in a just a few dozen large and dynamic cities, leaving slower-growing cities, small towns and rural areas — where about half of Americans live — far behind.

Also read: U.S. adds 164,000 new jobs in July to keep unemployment near 50-year low of 3.7%

Along with climate change and racial justice, economic development is America’s biggest challenge over the next few decades. Inclusive growth is a must, or else our society will fall apart.

The problem: No one — certainly not President Donald Trump — has found the magic wand that will bring back jobs to rural and small-town America.

25 cities get most of jobs

According to a study titled “The Future of Work in America” by the McKinsey Global Institute released in July, 25 cities that are home to about 30% of Americans will capture about 60% of the job growth between 2017 and 2030, just as they did between 2007 and 2017.

Twelve are megacities (and their extended suburbs): Atlanta, Boston, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Philadelphia, Phoenix, San Francisco and Washington.

Another 13 are high-growth hubs in smaller cities: Austin, Charlotte, Denver, Las Vegas, Minneapolis, Nashville, Orlando, Portland (Ore.), Raleigh, San Antonio, San Jose, Seattle, and Tampa.

Smaller, fast-growing cities and a few privileged rural counties will also add jobs, while vast swaths of the South, Midwest and Plains will lose jobs. The New York metro area, home to 20 million people, added more jobs over the past year than all the small towns and rural areas — with 46 million people — did combined.

Trump’s appeal to the despairing

Anyone who’s been paying attention to the political map will recognize that the growth is mostly occurring in places that vote for Democrats, while the stagnation is mostly in places that vote for Republicans.

Trump has appealed to those who are the most fearful, the most resentful and the most despairing, but the situation hasn’t gotten any better since his election. Rural America is older, sicker, poorer and more dependent upon state aid than it was before.

Read:Tim Mullaney says Trump is pushing manufacturing toward a recession

The divide between the superstar metros and everyone else has widened further in the past year or two.

Over the past year, only 12% of 389 metro areas had any statistically significant job growth, according to an analysis of Bureau of Labor Statistics data by Aaron Sojourner, a former White House economist and now an associate professor at the Carlson School of Management at the University of Minnesota.

For most of this post-crisis expansion, about 80% of metro areas were experiencing significant gains.

Of the 47 metros that gained significant numbers of jobs over the past year, 21 were on McKinsey’s top 25 list. Most of the others are the smaller, fast-growing cities that were also singled out by McKinsey.

Meanwhile, the regional jobs data from the BLS showed that nonmetropolitan areas, which have about 18% of jobs now, accounted for just 5% of job growth over the past year through June.

Cities have problems, too

The cities have their own problems, of course. They have growth, but also lots of inequality. The gap between the richest family and the poorest is much wider than in the country.

Even as we try to revive the rural economy, we’ve got to figure out how to make living in the city more affordable. Housing is too expensive for the low-skilled workers who are needed. The education people need to rise to higher paying jobs is increasingly out of reach. And the traffic is a nightmare.

Cities are the greatest engine of economic growth ever invented. No matter what system is adopted — feudalism, capitalism, socialism, kleptocracy — cities are the locus of growth. Cities are where people go to buy and sell, to work and collaborate with others. Certain industries cluster in certain cities, attracted by the markets, resources, talent and support services that they need. Success breeds more success.

For centuries, rural areas could survive economically because it took a lot of labor to grow the food and extract the resources that the cities needed. But the industrialization of agriculture, mining, forestry and other extractive industries means that fewer jobs are needed in the country.

Globalization has pushed commodity prices down, so rural areas receive less revenue for what they do produce. Concentration of business ownership in large nationwide corporations means that the profits, which used to stay local, are now sent across the country to the owners and shareholders who live in cities.

This isn’t just a U.S. thing. Across the world, hundreds of millions of people are flocking to the cities for better opportunities.

Education, infrastructure, antitrust

It’s beyond the scope of a 1,000-word column to solve these problems. Luckily much smarter people than I have been working on potential solutions.

What can be done? Economist Jonathan Gruber and Simon Johnson’s new book, “Jump-Starting America,” suggests that the federal government ought to fund basic research facilities in smaller cities across the country, recreating the conditions that led to the creation of such tech hubs as Silicon Valley.

Read more: Building new tech hubs across America would jump start the economy

Economist Ken Rogoff gnaws on the issue in a recent opinion piece at Project Syndicate, arguing for greater anti-trust enforcement to allow startups in cities like his home town of Rochester, N.Y., to survive in place without being gobbled up by the Apples AAPL, +3.75% , Googles GOOG, +1.16% , and Microsofts MSFT, +2.27% of the world.

Democratic presidential hopefuls Elizabeth Warren and Bernie Sanders also give a big role for antitrust in their plans to revive rural America.

The McKinsey study suggests that communities that are being left behind ought to try almost everything: improved transportation to get residents to jobs, rural broadband, lifelong job training and much more.

The urban-rural divide must be bridged. We cannot write off one quarter of our people as a nation. Economically, we’ll do better if every can fully participate.

And politically, it’s essential. With rural residents overrepresented in our politics, we couldn’t ignore them if we wanted to. We need to find hope and opportunity for everyone. A house divided cannot stand.