The triune brain theory is a model proposed by neuroscientist Paul D. MacLean in the 1960s. It 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 theory helps explain how different brain regions contribute to behavior and decision-making. While it simplifies the complexity of brain functions, it provides a useful framework for understanding the interplay between instinctual, emotional, and rational processes in humans.