The Triune Brain Theory, proposed by neuroscientist Paul D. MacLean in the 1960s, suggests that the human brain is structured in three distinct layers, each representing different evolutionary stages. The three parts are the reptilian brain, responsible for basic survival functions; the limbic system, which governs emotions and social behaviors; and the neocortex, associated with higher cognitive functions like reasoning and language. This model illustrates how these brain regions interact to influence behavior and decision-making. While the theory has been influential in psychology and neuroscience, it is important to note that modern research has revealed a more complex understanding of brain function beyond this simplified framework.