Beginning in 1933, while working in New Guinea, Margaret Mead developed her so-called squares hypothesis. Mead never published its terms, though she made a brief comment on it in her autobiography, Blackberry Winter (1972), and the arguments found in Sex and Temperament in Three Primitive Societies (1935) and the research leading to Balinese Character (Bateson & Mead, 1942) bore its imprint. Beginning with William McDougall's distinction between temperament (innate predispositions) and character (learned organization of habit), Mead articulated a morphological approach to the interplay between biology and culture that yielded four primary and four intermediary personality types. Under specified but not inevitable circumstances, the conscious choices of a given people could render one or another of these types characteristic or predominantly stable within their population, giving each of the other types a definite relation to the dominant type and thereby the cultural ethos of its society. Persons of each type followed a developmental path specific to their type different both from that of other types and in its manifestations given the various relations of the individual's type to the dominant type. Mead's hypothesis was, therefore, a vision of the unity and diversity of a single human species as well as an approach to the differing psychological positioning of individuals in cultures. In examining Mead's hypothesis, this essay also takes up Mead's debts to several leading psychologists (McDougall, C. G. Jung, and Erik Erikson), and (provisionally) how her vision differed from that of Ruth Benedict.