Few studies of person perception have examined the development of human nature concepts, i.e., the understanding of what it means to be human. In an attempt to explore the development of what being human means within the framework of cognitive development theory, 55 boys and 60 girls in kindergarten, fourth, eighth, and twelfth grade were given a semi-structured questionnaire consisting of three open-ended items about their understanding of themselves and others. Responses were identified as external or internal concepts and then coded in ten more specific content categories. The results indicated that with increasing age self, persons, and human nature were conceptualized in increasingly differentiated and abstract terms. There was a marked increase in psychological or inferential concepts between the kindergarten and fourth grade level. At the kindergarten level, the concept of human nature was found to be less differentiated than self and persons. In contrast, at the fourth grade level, human nature conceptions were proportionately more internal than external, while self and person concepts were still more external. The results of the study suggest that with age there is an expanded diversity in human conceptions which include increasingly more depth than surface characteristics, more dynamic and transient than static characteristics. (LLL)