A new concern is taking hold across the developed world: rapidly aging populations that scare policymakers.

Why it matters: Population growth is key to maintaining demand for housing, filling jobs yet to be automated, and paying into pension systems pressed by demographic realities and slowing economic growth.

• "You basically have a very large portion of mankind that is aging and then the workforce is shrinking. But I would say the G20 in particular are aging faster," said Organization for Economic Co-operation and Development secretary-general Angel Gurria. (AFP)

The big picture: Some countries — like the U.S. — are feeling these issues less thanks to immigration. Others — like China and Japan — are more restrictive, and feeling the bite in projected growth.

• As societies get richer and women get more rights, they work more and have increased access to contraceptives.
• While Europe and East Asia are already projected to lose population by 2050, the Western Hemisphere will mostly add people, and Africa's population is projected to soar, per UN projections.
• The developing world is predicted to bear the brunt of climate change impacts, including potential political instability, meaning many more people might want to move to developed countries.
• As we've seen in Europe and the U.S. in recent years, that isn't going well.

The bottom line: Earth has plenty of workers to do the jobs we need, just not in the countries where the jobs are right now. Fixing that mismatch is shaping up to be a central political challenge for the upcoming decades.

Go deeper: Special report... The aging, childless future

Interactive chart here.

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19 Democratic candidates visited the Iowa Democratic Party's Hall of Fame yesterday to deliver a five-minute campaign pitch to a throng of state party activists — and they hit the stage with a walk-out song of their own choosing, per TIME's Lissandra Villa:

• Michael Bennet: "The Rising," Bruce Springsteen
• Cory Booker: "Lovely Day," Bill Withers
• Steve Bullock: "Small Town," John Mellencamp
• Pete Buttigieg: "Never Giving Up," Batchelor
• Bill de Blasio: "Rudie Can't Fail," The Clash
• John Delaney: "I've Been Everywhere," Johnny Cash
• Tulsi Gabbard: "Ain't No Mountain High Enough," Marvin Gaye and Tammi Terrell
• Kirsten Gillibrand: "Good as Hell," Lizzo
• Kamala Harris: "Work That," Mary J. Blige
• John Hickenlooper: "Good Life," OneRepublic
• Jay Inslee: "Mr. Blue Sky," Electric Light Orchestra
• Amy Klobuchar: "The Bullpen," Dessa
• Beto O'Rourke: "Clampdown," The Clash
• Tim Ryan: "Old Town Road," Lil Nas X feat. Billy Ray Cyrus
• Bernie Sanders: "Power to the People," John Lennon
• Eric Swalwell: "Caught Up in the Country," Rodney Atkins
• Elizabeth Warren: "9 to 5," Dolly Parton
• Marianne Williamson: "Higher Ground," Stevie Wonder
• Andrew Yang: "Return of the Mack," Mark Morrison