Fertility rates in Western countries had been trending down for more than a century, and so following World War II, demographers expected only a modest increase. What happened instead was the baby boom. Since then, social scientists have been arguing about the causes.
The best-known explanation comes from economist Richard A. Easterlin of the University of Southern California. He argues that the baby boom resulted from the unprecedented concurrence of three developments: an expansion of the economy, restricted immigration since the mid-1920s, and a relatively small cohort of new job seekers because of low fertility in the late 1920s and 1930s. This combination created unusually good job prospects for young people after World War II, and so feeling more prosperous than their parents, they married earlier and had more children.