There is nothing like a recession to throw economists into a despondent mood. Much as happened in the late 1930s—when there was a fear of so-called secular stagnation, or the absence of growth due to a dearth of investment opportunities—many of my colleagues these days seem to believe that "sad days are here again." The economic growth experienced through much of the 20th century, they tell us, was fleeting. Our children will be no richer than we are. The entry of millions of married women into the workforce and the huge increase in college graduates that drove post-1945 growth were one-off boons. Slow growth...