Any useful measure of personality should have two features (Hogan, Hogan, and Roberts, 1996). First, the scores it gives should be reliable. Second, test scores should relate to nontest behavior. A personality measure that is unreliable or invalid has no practical utility. But don't people's personalities change over time? If so, this calls into question the reliability of personality test measures. Furthermore, people's behavior tends to vary from situation to situation. Doesn't this call into question the predictive validity of personality tests?

Longitudinal research has shown that personality remains consistent across adulthood. A six year longitudinal study by Costa and McRae (1988) found correlations between personality traits of 0.88. Conley (1984, 1985) found correlation coefficients of around 0.34 over a 45 year span. In a 50 year longitudinal study by Haan, Milsap, and Hartka (1986) found correlations averaging approximately 0.25.

The variability of people's behavior may still threaten the validity of personality measures. However, research by Caspi, Bem, and Elder (1989), Dudek and Hall (1991), and Roberts (1994) has found that personality measures have predictive ability over spans as long as 25 years.

Hogan, Hogan, and Roberts (1996) put forth a useful analogy: behavior is like the weather, and personality is like the regional climate. Although I may not be able to tell you what the weather will be like in Anchorage, Alaska on December 10, I can be nearly positive that it will be cold (which tells a lot about the potential weather, especially if you compare Anchorage to Honolulu, Hawaii). Personality is more of a pattern than an exact formula for predicting behavior; when measured correctly, these patterns are consistent, which leads to people differing from one another in noticeable and important ways. If personality does change, it changes gradually, while the more stable parts of our personality continue to affect our behavior in important ways.

References:

Caspi, A., Bem, D. J., & Elder, G. H. (1989). Continuities and consequences of interactional styles across the life course. Journal of Personality, 57, 375-405.

Conley, J. J. (1984). Longitudinal consistency of adult personality: Self-reported psychological characteristics across 45 years. Journal of Personality and Social Psychology, 47, 1325-1333.

Conley, J. J. (1985). Longitudinal stability of personality traits: A multitrait-multi-method-multioccation analysis. Journal of Personality and Social Psychology, 49, 1266-1282.

Costa, P. T., Jr., & McRae, R. R. (1988). Personality in adulthood: A six-year longitudinal study of self-reports and spouse ratings on the NEO personality inventory. Journal of Personality and Social Psychology, 54, 853-863.

Dudek, S. Z., & Hall, W. B. (1991). Personality consistency: Eminent architects 25 years later. Creativity Research Journal, 4, 213-231.

Haan, N., Milsap, R., & Hartka, E. (1986). As time goes by: Change and stability in personality over fifty years. Psychology and Aging, 1, 220-232.

Hogan, R., Hogan, J., & Roberts, B. (1996). Personality measurement and employment decisions: Questions and answers. American Psychologist, 51, 469-477.

Robert, B. W. (1994). A longitudinal study of the reciprocal relation between women's personality and occupational experience. Unpublished doctoral dissertation, University of California, Berkeley.

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