Historians have focused on early twentieth-century positive eugenics in New Zealand. In this article, I argue that the response came from a tradition of concern about heredity and white racial degeneracy, which extended beyond the British Empire. This article focuses on concerns about heredity at the Auckland Mental Hospital between 1850 and 1899, and contextualises these concerns in New Zealand mental hospital statistics from the late–nineteenth century. This article also considers Australasian, British, North and South American medical and immigration legislation history, and contrasts this with the legislation and medical discourses which formed part of a fear of heredity, racial degeneracy, immigration and mental illness in New Zealand.