The triune brain refers to a hypothesis about how the human brain evolved and functions that was first proposed by neuroscientist Paul D. MacLean in the 1960s. It posits that the brain can be divided into three parts, called the reptilian complex, the paleomammalian complex, and the neomammalian complex, which originated sequentially in the course of evolution and are tied to progressively more advanced forms of thought. It is now considered disproved due to subsequent research in neurology, paleontology, and related fields, though it retains a presence in popular culture.
According to the triune brain hypothesis, the oldest and most basic part of the human brain is a group of nerve clusters called the basal ganglia, located below the cerebrum. This is called the reptilian complex, so named because this part of the triune brain was posited to have evolved in humanity's distant pre-mammal evolutionary ancestors. In the triune brain model, the reptilian complex governs primitive instincts such as aggression, dominance, and the fight-or-flight response.
The second part, the paleomammalian complex, encompasses the structures in the brain now referred to as the limbic system. This includes the amygdala, hippocampus, and hypothalamus, along with the cingulate cortex and parts of the cerebral cortex. MacLean argued that these structures governed the emotions and behaviors such as reproduction, parenting, and feeding. According to the hypothesis, this part of the brain first evolved among early mammals. MacLean was the first neuroscientist to identify the limbic system and its importance, and the concept is still widely used in modern neuroscience despite the subsequent discrediting of the triune brain hypothesis as a whole.
The third part, called the neomammalian complex, is the neocortex. The neocortex is a part of the cerebral cortex found exclusively in mammals. In the triune brain model, the neomammalian complex is the newest part of the brain to evolve and is responsible for higher mental functions such as language and abstract thought.
The basal ganglia are present in all vertebrates, and so their evolution probably substantially predates the emergence of reptiles. Similarly, brain structures included in the paleomammalian complex are not unique to mammals, and many non-mammalian vertebrates display the nurturing and child-rearing behaviors attributed to it. Sauropsids, a classification encompassing birds, reptiles, and dinosaurs, were all subsequently discovered to have brain structures similar in function to what MacLean called the neomammalian complex, which indicates that the evolution of these structures also predates the evolution of mammals. Sophisticated mental abilities once thought to be exclusive to mammals, such as toolmaking, are also present in some species of birds.
The idea of the triune brain also lost credibility due to greater understanding of the human brain. For example, brain damage in some areas classified as part of the paleomammalian complex can impair cognitive functions that are supposedly the sole domain of the neomammalian complex. This is difficult to account for in a model of the nervous system that attributes all higher mental functions to a single, specific part of the brain.