A Swedish researcher proposed a theory that links the production of shoes to the prevalence of schizophrenia: "Heeled footwear began to be used more than 1,000 years ago, and led to the occurrence of the first cases of schizophrenia . . . Mechanization of the production started in Massachusetts, spread from there to England and Germany, and then to the rest of Western Europe. A remarkable increase in schizophrenia prevalence followed the same pattern."34 Which one of these should make us hesitate before we agree that shoes must cause schizophrenia: confounding variables, people's behaviors affected by their awareness of participating in a prospective study, or people's faulty memories in a retrospective study?