The worldwide age divide makes migration — along with job creation in the global south — critical to balancing the world demographically, according to Rainer Münz, head of research and development at the Erste Group Bank in Brussels. Mr. Münz proposes what he calls a system of “demographic arbitrage,” with industrialized countries competing for talent from elsewhere. Even China, he maintains, will have to enter that race.

“A demographic arbitrage between aging societies with a shrinking work force and youthful societies would be good thing, if the whole thing could be managed,” he said.

Many politicians are making the opposite case. Just last week, Donald Tusk, the president of the European Council, told migrants not to come to Europe, which has sought to stanch the flow by offering development aid to the migrants’ home countries.

YET development aid can’t tamp down dreams. As poor countries prosper and their young become more educated, they are more likely to migrate. It explains in part why India has the largest diaspora in the world: In 2015, 16 million Indians were living outside India, double the number in 2000.