The triune brain was a once popular model of the evolution of the vertebrate forebrain and behavior, proposed by the American physician and neuroscientist Paul D. MacLean in the 1960s. The triune brain consists of the reptilian complex (basal ganglia), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. According to the model, the basal ganglia are in charge of primal instincts, the limbic system is in charge of emotions, and the neocortex is responsible for objective or rational thoughts.

Since the 1970s, the concept of the triune brain has been subject to criticism in evolutionary and developmental neuroscience and is regarded as a myth. Although it overlaps in some respects with contemporary understanding of the brain, the triune brain hypothesis is no longer espoused by comparative neuroscientists in the post-2000 era due to harsh criticism against it.

MacLean originally formulated his model in the 1960s and propounded it at length in his 1990 book The Triune Brain in Evolution. The triune brain hypothesis became familiar to a broad popular audience through Carl Sagan's Pulitzer Prize winning 1977 book The Dragons of Eden.

"Reptilian complex" (also known as the "R-complex", "reptilian brain" or "lizard brain") was the name MacLean gave to the basal ganglia, structures derived from the floor of the forebrain during development. The term derives from the idea that comparative neuroanatomists once believed that the forebrains of reptiles and birds were dominated by these structures. MacLean proposed that the reptilian complex was responsible for species-typical instinctual behaviours involved in aggression, dominance, territoriality, and ritual displays.

This consists of the septum, amygdalae, hypothalamus, hippocampal complex, and cingulate cortex. MacLean first introduced the term "limbic system" to refer to this set of interconnected brain structures in a paper in 1952. MacLean's recognition of the limbic system as a major functional system in the brain was widely accepted among neuroscientists, and is generally regarded as his most important contribution to the field. MacLean maintained that the structures of the limbic system arose early in mammalian evolution (hence "paleomammalian", with paleo- meaning old) and were responsible for the motivation and emotion involved in feeding, reproductive behaviour, and parental behaviour. ^{[citation needed]}

This consists of the cerebral neocortex, a structure found uniquely in higher mammals, and especially humans. MacLean regarded its addition as the most recent step in the evolution of the mammalian brain, conferring the ability for language, abstraction, planning, and perception. ^{[citation needed]}

The triune brain model argues that these structures are relatively independent from one another, but that they are still connected to each other in some form or another.

The model views different cognitive behaviors as caused by three different entities instead of one. The reptilian complex is said to control all of the instinctual and impulsive actions, while the neomammalian complex is responsible for keeping the primitive instincts constrained. An example is controlling the impulse of eating. It seems that if one is hungry, then that means the reptilian complex is commanding the body to eat. However, an individual has the rational choice not to eat when hungry, and this rational thought is said to be controlled by the neomammalian complex. The model thus suggest that these two (and three depending on the situation) structures are in a perpetual battle to control the body.