Baby booms and busts are generally related to distinct, long-lasting changes in national fortunes. During periods of growth and employment, more babies are born; during recessions, the birthrate and fertility rate (how many babies each woman has in her lifetime) decline. During the Depression, for example, the fertility rate dropped precipitously. With unemployment around 25 percent and no sign of a recovery in sight, many families simply decided not to have children, or to postpone their family plans until the economy turned around.

"It was a very deep trough," says John Haaga, director of domestic programs at the Population Reference Bureau, a nonprofit demographic research institute. "The birthrate dipped below the replacement rate (2.1 children per family) for the first time ever."

That shift in the birthrate struck demographers and policymakers at the time as a sign of serious trouble. The idea that all cultures follow the same cycles of growth and decline had already entered the culture, largely through German historian Oswald Spengler and his 1918 book "The Decline of the West." And according to Haaga, many s