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.
We are not in such a bad position, yet. Since the end of the recession in June 2009, quarterly GDP growth has averaged 2.1 percent.
The new normal?
That has left job growth way below the historic trend line. Four years ago, the incoming Obama administration's economists promised that we would be heading back up to the trend line, with unemployment down to a little above 5 percent now.
Instead, it was 7.9 percent in January, and that's with millions no longer even looking for work. Labor force participation is the lowest it's been since 1981.
The danger is that all this can come to seem the new normal. Low birthrates, as Last argues, can persuade others to want fewer children.
In the 1990s, Canada and Sweden faced economic crises similar to ours. In response, they sharply cut public spending. Their economies have done well since, and their governments have been running budget surpluses.
We did something like the opposite. The consequences could be enduring.