SummaryThis paper investigates the sources of the common Victorian belief that drunkenness was the most significant cause of insanity. While acknowledging the role of the Temperance Movement in promoting this belief, the paper argues that its development was rooted in the social conditions and prejudices of the time, as well as in the experience and reasoning of Victorian medical men. The asylums which reported the highest proportions of drink‐induced insanity generally lay in urban, industrial areas where drinking was perceived to be a serious problem and catered to a predominately working‐class clientele. The superintendents of the asylums were inclined to conclude from their experience that drink was the most common cause of insanity. Their tendency to do so was increased by medical uncertainty about the exact relationship between drunkenness and insanity, the habit of many doctors of reasoning in circular fashion, and their proneness to link theories of causation to traditional moral imperatives. These tendencies were evident in speculation about the causes of conditions such as paresis and ‘hereditary’ mental di