Whether Ashkenazi or Mizrahi Jews tend to have higher intelligence than other ethnic groups has been an occasional subject of scientific controversy. A 2005 scientific paper, "Natural History of Jewish Intelligence", proposed that Jews as a group inherit higher verbal and mathematical intelligence with somewhat lower spikes in spatial intelligence than other ethnic groups, on the basis of inherited diseases and the peculiar economic situation of Jews in the Middle Ages. Opposing this hypothesis are explanations for the congenital illnesses in terms of the founder effect, explanations of intellectual successes by reference to Jewish culture's promotion of scholarship and learning, and doubts about whether a group difference in intelligence really exists.