Throughout Brain Rules, Medina explores how evolution has shaped the human brain to be a remarkable processor capable of growth, memory, and symbolic reasoning. He suggests that the reason human brains are so unique is that they evolved this way; they found an advantage through complex processing capabilities. For each brain rule, Medina attempts to understand the evolutionary advantage. He recognizes that sleep—while leaving humans in a vulnerable state—helps to encode and process information that ensures later survival. He reveals how the cognitive preference for sight sensation is reflective of a need to survive in the wild, where humans took in billions of visual data points. In some cases, Medina is not able to determine the evolutionary value of the rule; for example, he cannot directly pinpoint the impact music has on cognitive function and emotion. However, the connection is undeniable, and the evolution of the brain is a story of intention.

Contemporary culture often functions in a way that conflicts with the evolutionary workings of the brain. Cultural norms often do not capitalize on the way brains are designed to function. One example of this is multitasking. Medina suggests that the brain is not wired for multitasking.