The period from the end of World War II to the early 1970s was a golden era of American capitalism. $200 billion in war bonds matured, and the G.I. Bill financed a well-educated work force. The middle class swelled, as did GDP and productivity. The US underwent its own golden age of economic growth. This growth was distributed fairly evenly across the economic classes, which some attribute to the strength of labor unions in this period—labor union membership peaked during the 1950s. Much of the growth came from the movement of low-income farm workers into better-paying jobs in the towns and cities—a process largely completed by 1960.