the relationship of person-society as an interactive system that defines and molds personal development. Personal interaction with other individuals, society, and nature create experiences in which self-identification is organized in relation to social environment. In other words, personality traits are a function of complex cognitive strategies used to effectively maneuver through social situations. Furthermore, according to the social-cognitive perspective, cognitive processes are central to an individual's unique expression of personality traits and affective processes. Through cognitive mechanism and social competencies, individuals interpret contextual situations to derive beliefs that guide their thoughts and behaviors, thus developing an enduring pattern of personality traits. Evolutionary Theory Humanistic Theory The evolutionary theory of personality development is primarily based on the evolutionary process of natural selection. From the evolutionary perspective, evolution resulted in variations of the human mind. Natural selection refined these variations based on their beneficence to humans. Due to human complexity, many opposing personality traits proved to be beneficial in a variety of ways. Primitive humans were collectivists due to tribe culture. The personalities of individuals within a tribe were very similar. The division of labor resulted in differentiation in personality traits in order to achieve a higher efficiency. Differentiation in personality traits increased functionality, therefore becoming adaptive through natural selection. Humans continued to develop personality and individuality through evolution. Lifespan Theory Classic theories of personality include Freud's tripartite theory and post-Freudian theory (developmental stage theories and type theories) and indicate that most personality development occurs in childhood, stabilizing by the end of adolescence. Current lifespan perspectives that integrate theory and empirical findings dominate the research literature. The lifespan perspectives of personality are based on the plasticity principle, the principle that personality traits are open systems that can be influenced by the environment at any age. Large-scale longitudinal studies have demonstrated that the most active period of personality development appears to be between the ages of 20–40. Although personality grows increasingly consistent with age and typically plateaus near age 50, personality never reached a period of total stability. Humanistic Theory Humanistic psychology emphasizes individual choices as voluntary actions that ultimately determine personal development. Individual personalities traits, although essential to the integrated self, are only parts that make up the whole of observable human experiences. Thus, personality development is articulated in terms of purposeful action geared towards experiencing mastery of free choice. Rather than compartmentalized elements of personality traits such as feelings, thoughts, or