Putting a number to the problem, America's replacement fertility rate (the number of children per mother) needs to be 2.08. Our actual fertility rate is 1.87. 

Yet the population is growing, and that's almost entirely due to immigration. By the middle of the century, Pew researchers expect foreign-born Americans to account for almost all of the country's population growth.

Which brings us, at long last, to macroeconomics.

Population plays an enormous role in labor sector dynamics. Without enough workers to fill jobs employers have to compete for an increasingly scarce resource. While that can be good, it also can drive up inflation, raise consumer prices and cause the economy to stall out for lack of opportunity.

Even more big picture still, though, the economy contracts without a growing workforce.

"Labor force growth is one of the factors that allows the economy to grow," said Tim Duy, a professor of economics with the University of Oregon, "and we know that fairly low labor force growth really does reduce the speed limit of the U.S. economy. So I think that's certainly one factor that I consider, is that we certainly do need some immigration in order to grow."

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