This paper revisits a landmark study of the prevalence of mental illness in the state of Massachusetts conducted by Edward Jarvis in the 19th century. Jarvis drew an improper conclusion about the relationship between social class, ethnicity, and insanity, asserting that the Irish foreign-born had a higher prevalence of insanity in each social stratum. A reanalysis of Jarvis’ data shows that in both the pauper and independent social classes in Massachusetts, the prevalence of insanity was significantly lower among foreign-born persons than among native-born persons.

On the basis of his misperception, Jarvis constructed elaborate etiological theories. These theories made a strong impact on the mental health service policies of his day. The effects of incomplete examination of data on etiological theories and mental health policy in current times are highlighted in this article.