Instinct theory proposes that innate biological instincts, like animal behavioral patterns, primarily drive human behavior. These instincts are inborn, not learned, and are fundamental to decision-making and action. Just as animals rely on instincts for critical survival functions such as migration, nest building, and defense, humans are also believed to exhibit behaviors rooted in evolutionary needs. For example, the instinct to reproduce motivates sexual behavior, while territorial instincts may manifest as aggression, particularly in situations where personal space is perceived as threatened.

Psychologist William James played a significant role in formalizing the concept of human instincts. He identified a wide array of instincts, including emotions such as fear, anger, love, and shame, which he believed were integral in shaping human behavior. According to James, these instincts serve as unconscious guides for our actions. William McDougall expanded on this by actually identifying eighteen distinct human instincts, such as flight, repulsion, curiosity, hunger, self-assertion, self-abasement, parental, reproduction, acquisition, construction, herding, fear, hunting, gregariousness, submission, comfort, laughter, and curiosity (intellectual). These instincts, McDougall argued, form the foundation of a vast range of human behaviors, from basic survival actions to more complex social interactions.

Sigmund Freud approached instinct theory from a psychoanalytic perspective, suggesting that two primary forces — Eros and Thanatos — drive human behavior. Eros represents life instincts, focusing on survival, reproduction, and cooperation, while Thanatos, the death instinct, encompasses aggression and self-destructive tendencies. Freud highlights the dynamic interplay between life-preserving and destructive forces within human psychology.

Despite its insights, instinct theory has faced criticism for oversimplifying human behavior. It has been argued that instincts do not solely determine human actions but are heavily influenced by learning, cultural factors, and cognitive processes, which allow for adaptation beyond basic biological drives.