This thesis considers the response of one New Zealand institution, Tokanui Mental Hospital, to legislation and policies for 'mental deficiency' introduced during the first half of the twentieth century. Institutional reactions to these policies have been under examined in New Zealand. While psychiatric or mentally ill patients have been the subject of a number of New Zealand histories of the asylum, 'mental defectives' have often been overlooked. Yet during the early-twentieth century, 'mental defectives' were thought to be a source of a number of social problems, and the New Zealand government considered a range of measures aimed at limiting the spread and effect of mental deficiency in society. Policies for 'mental deficiency' were influenced by contemporary anxieties about crime, sexuality and hereditarism. As a policy of segregation was formally prescribed, more 'mental defectives' were committed to mental hospitals and other institutions than ever before. An understanding of the responses to this perceived problem also provides an insight into wider social policies in New Zealand in the first half of the twentieth century. This thesis argues that gender was a significant factor in the decision to commit mental defectives to Tokanui. Subsequent categorisation and treatment within Tokanui was also affected by gender. Official reports inform us about the policies that were in place, and historical materials from Tokanui show how these worked in practice. Most of the archives of Tokanui Mental Hospital have been unexamined by historians before now, and close analysis of patient cases also reveals more about institutional practices. The connection between Tokanui and neighbouring Waikeria Prison is also explored, in the context of contemporary fears surrounding mental deficiency and crime.