The title of a 1930s article asked the question, "Stupidity or Hookworm?" In this article, the authors discuss research that attempts to answer the question of whether intestinal worms--namely, hookworm, whipworm, and roundworm--harm the mental performance of their hosts. After introducing the biology and epidemiology of intestinal worms, the authors present the historical background to the problem. They review research from the 1910s through the 1990s; there is evidence that high intensities of worms can affect mental performance, but not all dewormed children show improved performance. They discuss the mechanisms of how worms might affect the mind.