Paul D. MacLean was an American neuroscientist best known for his work on the brain's structure and function. He developed the triune brain theory, which suggests that the human brain is composed of three distinct parts: the reptilian brain, the limbic system, and the neocortex. This model aimed to explain the evolution of the brain and its role in behavior. MacLean's research contributed significantly to our understanding of emotions and social behavior in humans and animals. He emphasized the importance of the limbic system in emotional responses and bonding, influencing fields such as psychology and psychiatry. His work remains influential in neuroscience today.