This article is a compilation of three articles written by Paul MacLean (refer to Footnote). Based on his research, MacLean states that there are common anatomical and functional characteristics among the brains of reptiles, mammals, and man. For educators, the most significant commonality is that of social behavior. Frequently, behaviors that are observed in the classroom, and believed to be learned, are actually inherited through the evolutionary process. To describe and illustrate this phenomenon, MacLean places it in the context of a model he terms the “triune brain”: the reptilian, the paleomammalian, and the neomammalian brain.