Causes of the Great Depression The Great Depression was the worst economic slump ever in U.S. history, and one that spread to virtually the entire industrialized world. The depression began in late 1929 and lasted for about a decade. Many factors played a role in bringing about the depression; however, the main cause for the Great Depression was the combination of the greatly unequal distribution of wealth throughout the 1920's, and the extensive stock market speculation that took place during the latter part that same decade. The maldistribution of wealth in the 1920's existed on many levels.