Optimistic Bias
William M. P. Klein

Optimistic biases are thought to represent one example of an array of self-serving beliefs that may influence behavior including the illusion of control (Langer, 1975 xClose
Langer, E. J. (1975). The illusion of control. Journal of Personality and Social Psychology, 32, 311-328. ), the better-than-average effect (e.g., Alicke et al., 1995 xClose
Alicke, M. D., Klotz, M. L., Breitenbecher, D. L., Yurak, T. J., & Vredenburg, D. S. (1995). Personal contact, individuation, and the better-than-average effect. Journal of Personality and Social Psychology, 68, 804-825. ), and the uniqueness bias (Goethals, Messick, & Allison, 1991 xClose
Goethals, G. R., Messick, D. M., & Allison, S. T. (1991). The uniqueness bias: Studies of constructive social comparison. In J. Suls & T. A. Wills (Eds.), Social comparison research: Contemporary theory and research (pp. 149-176). NJ: Erlbaum. ). People who overestimate their ability to control an outcome may engage in more risky decisions and behaviors (Klein & Kunda, 1994 xClose
Klein, W. M., & Kunda, Z. (1994). Exaggerated self-assessments and the preference for controllable risks. Organizational Behavior and Human Decision Processes, 59, 410-427. ). Perceptions of control and self-efficacy represent key components of many models such as Social Cognitive Theory (Bandura, 2001 xClose
Bandura, A. (2001). Social cognitive theory: An agentic perspective. Annual Review of Psychology, 52, 1-26. ) and the Theory of Planned Behavior (Ajzen, 1991 xClose
Ajzen, I. (1991). The theory of planned behavior. Organizational Behavior and Human Decision Processes, 50, 179-211. ), highlighting the importance that biases in control and efficacy beliefs might play in health behavior.

It is notable that optimistic beliefs and health threats seem to be only weakly associated with dispositional optimism (e.g., Goodman, Chesney, & Tipton, 1995 xClose
Goodman, E., Chesney, M. A., Tipton, A. C. (1995). Relationship of optimism, knowledge, attitudes, and beliefs to use of HIV antibody testing by at-risk female adolescents. Psychosomatic Medicine, 57, 541-546. ; Taylor et al., 1992 xClose
Taylor, S. E., Kemeny, M. E., Aspinwall, L. G., Schneider, S. G., Rodriguez, R., & Herbert, M. (1992). Optimism, coping, psychological distress, and high-risk sexual behavior among men at risk for acquired immunodeficiency syndrome (AIDS). Journal of Personality and Social Psychology, 63, 460-473. ), including when these optimistic beliefs are biased (Radcliffe & Klein, 2002 xClose
Radcliffe, N. M., & Klein, W. M. P. (2002). Dispositional, unrealistic, and comparative optimism: Differential relations with knowledge and processing of risk information and beliefs about personal risk. Personality and Social Psychology Bulletin, 28, 836-846. ). There is some evidence that dispositional optimism may interact with optimistic bias to magnify the detrimental effect of bias on information processing (Davidson & Prkachin, 1997 xClose
Davidson, K. & Prkachin, K. (1997). Optimism and unrealistic optimism have an interacting impact on health-promoting behavior and knowledge changes. Personality and Social Psychology Bulletin, 23, 617-625. ), although in this study optimistic bias was measured as a sum of risk estimates across several events without use of an accuracy criterion. Generally, people who are high in dispositional optimism (or a health-specific form of optimism) are more knowledgeable, less defensive in response to health information, and in better health (Aspinwall & Brunhart, 1996 xClose
Aspinwall, L. G., & Brunhart, S. M. (1996). Distinguishing optimism from denial: Optimistic beliefs predict attention to health threats. Personality and Social Psychology Bulletin, 22, 993-1003. ; Scheier & Carver, 1992 xClose
Scheier, M. F., & Carver, C. S. (1992). Effects of optimism of psychological and physical well-being: Theoretical overview and empirical update. Cognitive Therapy and Research, 16, 201-228. ), suggesting that dispositional optimism and optimistic biases may have opposing effects (Radcliffe & Klein, 2002 xClose
Radcliffe, N. M., & Klein, W. M. P. (2002). Dispositional, unrealistic, and comparative optimism: Differential relations with knowledge and processing of risk information and beliefs about personal risk. Personality and Social Psychology Bulletin, 28, 836-846. ).

Conclusions
Although much research has investigated the underlying causes and moderators of optimistic biases, less work has addressed how optimistically biased beliefs are related to health information processing, behavior, and physical health outcomes. Moreover, methodological problems make it difficult to determine how biases in risk perceptions influence these outcomes relative to other constructs in health behavior models such as attitudes and self-efficacy (Weinstein, 2005 xClose
Weinstein, N. D. (2005). Misleading tests of health behavior theories. Manuscript under review. ) as well as less traditional constructs like affect (McCaul & Mullens, 2003 xClose
McCaul, K.D., & Mullens, A.B. (2003). Affect, thought, and self-protective health behavior: The case of worry and cancer screening. In J. Suls and K. Wallston (Eds.), Social psychological foundations of health and illness. Malden, MA: Blackwell Publishers. ). In order to properly assess the impact of optimistic biases, it is important to use accuracy criteria that identify optimistic biases at the level of the individual. The increasing availability of risk engines such as the Harvard Risk Index (Colditz et al., 2000 xClose
Colditz, G. A., Atwood, K. A., Emmons, K., Monson, R. R., Willett, W. C., Trichopoulos, D., & Hunter, D. J. (2000). Harvard Report on Cancer Prevention Volume 4: Harvard Cancer Risk Index. Cancer Causes and Control, 11, 477-488. ) and the use of prospective designs should facilitate research taking this approach.