This ordering and examination of three broad functions of the human brain has been crucial to recent understanding of personality, culture and human motivation. It is pretty much taken for granted as a foundation for research and practice in the helping professions today, although it may not be so well known to the general population.
The first scientific paper on the tri-une brain was written by Paul D. MacLean and published in 1952, but it took about thirty years for this knowledge to inform thinking about human behavior and motivation.
The overall structure of the perspective is that there are three seats of knowledge in the brain: (1) the neo-cortex with its right and left hemispheres which is the seat of the “higher functions” such as language, imagery and reasoning, (2) the limbic cortex which includes the amygdala and septem and governs fight-flight responses and other emotional functions, and (3) a group of elements at the base of the brain clustered around the brain stem which process time and space awareness and other sensori-motor functions of the body. MacLean called this area “the R-Complex.” The neo-cortex is a late evolutionary development and found only in the primates. The limbic cortex is common to all mammals. The R-Complex is shared by reptiles, and hence is sometimes referred to as “the reptilian brain.”