Historically, researchers attempting to explain the increasing stability of personality with age have differentially emphasized intrinsic maturational processes versus exogenous/social processes. That endogenous processes underlie personality development has a long tradition within personality psychology (Barenbaum & Winter, 2008). For example, the early theories of psycho-sexual development (Freud, 1959/1908) focused on internal processes, and Allport (1937, p. 48) identified personality as relying on internal “psychophysical systems.” Some early proposed mechanisms implicated differences in body chemistry (Murray, 1938) and structure (Sheldon, Stevens, & Tucker, 1940). This perhaps intuitive belief has carried on with modern personality theories based on the relative influence or presence of neurotransmitters (Cloninger, 1998) and the structure of the reticular activation system within the brain (Eysenck & Eysenck, 1985). These works assume that intrinsic, specifically genetic, maturational processes are the leading causes of personality development. However, there are many personality theories that implicate exogenous influences as shaping personality development (McAdams & Olsen, 2010). Erikson (1963/1950) saw personality as developing in relation to success or failures with certain social challenges. Similarly, attachment theories posit that early caregiving experiences have a lasting influence on a multitude of aspects of an individual’s life (Bowlby, 1964).
