Personality is not completely consistent nor entirely at the mercy of the environment and change. Exactly what aspects of personality are consistent over time is impossible to answer. However, many theories have attempted to explain why personality is consistent and how it changes over time. Trait theories provide a useful means of measuring personality and can be complemented by the biological approach attributing genes as a possible cause of consistency. Alternatively, the psychodynamic theory maintains that the foundations of personality are rooted in childhood and parenting. In contrast, the cognitive approach argues that learning and the environment are the main factors contributing to consistency but also to change. Theorists like Walter Mischel take this theory a step further by arguing that behaviour is the sole product of changing circumstances and the environment; therefore personality is not measurable or consistent. Although these theories offer contradicting views, they all posses an element of truth and provide interesting insights into personality and its consistency over time. .
Because personality is such a vast topic, trait theorists break it down into quantifiable categories and characteristics for measurement. Trait theorists argue that words in the English language provide important information about personality, because any aspect of individual difference would be represented by some relevant words (Eysenck, 1994). Theorist Gordon Allport began with nearly 18,000 words to cover every aspect of personality. These words were gradually refined over time to form the commonly used Five-Factor model. Avshalom Caspi and his colleagues used this model to assess over 1,000 personalities in a longitudinal study. They found overall evidence pointing to consistency as opposed to change (Roberts, Caspi, & Moffitt, 2001). Similarly, Roberts and DelVecchio conducted a review on 152 longitudinal studies.
