Michael Barone writes here about the relationship between the record-low U.S. birth rate and economic growth.

Birth rates fell sharply during the Depression of the 1930s. They have fallen significantly since the housing collapse, from 69.3 in 2007 to 63.2 in 2011. The steepest decline in births since 2007 has been among Hispanic immigrants, who were also hit hard by housing foreclosures.

We don’t know whether this trend will continue. But if it does, the consequences will resemble the subtitle of Jonathan Last’s newly published book, “What to Expect When No One’s Expecting: America’s Coming Demographic Disaster.”

Last points out that our fertility rate — the number of children a woman has over a lifetime — has been below the replacement level of 2.1. Over time, a below-replacement-level fertility rate means population decline.

To see what that means, look at Japan. Its fertility rate is 1.4, its population is declining, and it has had essentially zero economic growth since 1990.

So who will pay taxes to cover all the entitlements?