The assessment of personality stability and change over the life course is largely determined by theoretical beliefs. While some scholars have a more deterministic view and view human personality as quite predictable and stable over time (e.g. biogenetic determinism), other theorists view personality as fluid and highly malleable (e.g. behaviorism). The intermediate position assumes a bidirectional relationship between heredity and environment, which allows for both coherence and personality change. Furthermore, some theorists introduce a third element into the biosocial model: the individual (e.g. the transactional model of socialization). Assessing personality stability is difficult, given the lack of consensus on what personality is and how it should be measured even when viewed through a similar framework. Examining the two trait theories of Allport and Eysenck presents an intriguing juxtaposition on the definition, measurement, and performance of personality. Allport, a pillar of personality psychology, defines personality through individual traits, a relatively permanent characteristic, the existence of which can be verified scientifically through empirical methods or statistical analysis (Allport, 1931). He used a lexical approach to identify thousands of dispositions that can describe an individual and organized them into three main categories: cardinal (dominant), central (common), and secondary (unique) traits. Allport advises that although traits have a universal appearance, they should be examined and explained with caution and artistry. Interpretations of behavioral variance should consider both contextual circumstances and the possibility of coexistence of contradictory traits in an individual. He also suggested that the patterns and… the middle of the paper… personality, 8th edition. Belmont, CA: Wadsworth Publishing. Caspi, A., Roberts, B. W., & Shiner, R. L. (2005). Personality development: stability and change. Annual Review of Psychology, 56(1), 453-484. doi: 10.1146/annurev.psych.55. 090902.141913Costa, P. T., Herbst, J. H., McCrae, R. R., & Siegler, I. (2001). Personality in midlife: Stability, intrinsic maturation, and response to life events. Evaluation, 7(4), 365-378. doi: 10.1177/107319110000700405Eysenck, H. J. (1952). The effects of psychotherapy: an evaluation. Journal of Counseling Psychology, 16(5), 319–324. doi: 10.1037/h0063633McAdams, D. P., & Pals, J. L. (2006). A new Big Five: Fundamental principles for an integrative science of personality. American Psychologist, 61(3), 204–217. doi: 10.1037/0003-066X.61.3.204 Ryckmen, R. M. (2008). Theories of Personality, 9th ed. Belmont, CA: Wadsworth Publishing.