Optimistic Bias
William M. P. Klein
University of Pittsburgh

Optimistic bias is commonly defined as the mistaken belief that one's chances of experiencing a negative event are lower (or a positive event higher) than that of one's peers. The bias was first demonstrated by Weinstein (1980) xClose
Weinstein, N. D. (1980). Unrealistic optimism about future life events. Journal of Personality and Social Psychology, 39, 806-820. , who reported that a majority of college students believed their chances of events such as divorce and having a drinking problem to be lower than that of other students, and their chances of events such as owning their own home and living past 80 years of age to be higher than that of other students. Because a majority of individuals in a group cannot be above (or below) the mean unless the distribution is highly skewed, these findings represented a bias at the level of the group. Other terms representing the same construct include "unrealistic optimism," "illusion of invulnerability," "illusion of unique invulnerability," "optimism bias," and "personal fable." It is also possible to be optimistically biased by being overconfident about the objective chances of experiencing a positive event (or avoiding a negative event), irrespective of how one's chances compare with those of one's peers. As noted later, optimistic bias has been more frequently defined using the comparative definition above due to greater methodological ease.

Subsequent work has attempted to evaluate the prevalence of this bias as well as its determinants and moderators. Optimistic bias has been demonstrated across a wide variety of positive and negative events, with most work focusing on health problems such as lung cancer, HIV infection, and alcoholism (for reviews see Helweg-Larsen & Shepperd, 2001 xClose
Helweg-Larsen, M., & Shepperd, J. A. (2001). Do moderators of the optimistic bias affect personal or target risk estimates? A review of the literature. Personality and Social Psychology Review, 51, 74-95. ; Hoorens, 1993 xClose
Hoorens, V., & Buunk, B. P. (1993). Social comparison of health risks: Locus of control, person-positivity bias, and unrealistic optimism. Journal of Applied Social Psychology, 23, 291-302. ; Klein & Weinstein, 1997 xClose
Klein, W. M., & Weinstein, N. D. (1997). Social comparison and unrealistic optimism about personal risk. In B. P. Buunk and F. X. Gibbons (Eds.), Health, coping, and well-being: Perspectives from social comparison theory (pp. 25-61). Hillsdale, NJ: Lawrence Erlbaum. ). The bias appears in a wide variety of disparate samples including adolescents (Quadrel, Fischhoff, & Davis, 1993 xClose
Quadrel, M. J., Fischhoff, B., & Davis, W. (1993). Adolescent (in)vulnerability. American Psychologist, 48, 102-117. ), community residents of varying age and socioeconomic status (Weinstein, 1987 xClose
Weinstein, N. D. (1987). Unrealistic optimism about susceptibility to health problems: Conclusions from a community-wide sample. Journal of Behavioral Medicine, 10, 481-500. ), prostitutes (van der Velde, van der Pligt, & Hooykaas, 1994 xClose
van der Velde, F. W., van der Pligt, J., & Hooykaas, C. (1994). Perceiving AIDS-related risk: Accuracy as a function of differences in actual risk. Health Psychology, 13, 25-33. ), women marines (Gerrard, Gibbons, & Warner, 1991) xClose
Gerrard, M., Gibbons, F. X., & Warner, T. (1991). Effects of reviewing risk-relevant behavior on perceived vulnerability among women marines. Health Psychology, 10, 173-179. , and smokers (Weinstein, 1998 xClose
Weinstein, N. D. (1998). Accuracy of smokers' risk perceptions. Annals of Behavioral Medicine, 20, 135-140. ).

Optimistic biases are more likely to emerge for events that are controllable (Klein & Helweg-Larsen, 2002 xClose
Klein, C. T. F., & Helweg-Larsen, M. (2002). Perceived control and the optimistic bias: A meta-analytic review. Psychology and Health, 17, 437-446. ) and for which people have stereotypes of the typical person who experiences the event (Weinstein, 1980 xClose
Weinstein, N. D. (1980). Unrealistic optimism about future life events. Journal of Personality and Social Psychology, 39, 806-820. ). In addition, the bias is more likely to occur when people compare themselves with aggregated comparison targets such as "the average person" than with more individualized comparison targets such as a friend or even a randomly chosen person (Alicke, Klotz, Breitenbecher, Yurak, & Vredenburg, 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. ). People also hold similarly self-serving beliefs about their risk-related behaviors such as diet and alcohol consumption (Suls, Wan, & Sanders, 1988 xClose
Suls, J., Wan, C. K., & Sanders, G. S. (1988). False consensus and false uniqueness in estimating the prevalence of health-protective behaviors. Journal of Applied Social Psychology, 18, 66-79. ).

What causes optimistic bias? Some of the optimistic bias may be attributed to cognitive factors. When comparing their risk to that of others, people are egocentric in that they focus more on their own risk factors than on those of the peers to whom they are comparing (Chambers & Windschitl, 2004 xClose
Chambers, J. R., & Windschitl, P. D. (2004). Biases in social comparative judgments: The role of nonmotivated factors in above-average and comparative optimism effects. Psychological Bulletin, 130, 813-838. ); indeed, reducing such egocentrism seems to dampen the bias (Weinstein, 1983 xClose
Weinstein, N. D. (1983). Reducing unrealistic optimism about illness susceptibility. Health Psychology, 2, 11-20. ), and this egocentrism may lead people to be unrealistically pessimistic about rare positive events or common negative events (e.g., Chambers, Windschitl, & Suls, 2003 xClose
Chambers, J. R., & Windschitl, P. D. (2004). Biases in social comparative judgments: The role of nonmotivated factors in above-average and comparative optimism effects. Psychological Bulletin, 130, 813-838. ; Kruger & Burrus, 2004 xClose
Kruger, J., & Burrus, J. (2004). Egocentrism and focalism in unrealistic optimism (and pessimism). Journal of Experimental Social Psychology, 40, 332-340. ). People also seem to focus on base-rate information rather than individual risk factors when judging the risk of their peers (Klar, Medding, & Sarel, 1996 xClose
Klar, Y., Medding, A., & Sarel, D. (1996). Nonunique invulnerability: Singular versus distributional probabilities and unrealistic optimism in comparative risk judgments. Organizational Behavior and Human Decision Processes, 67, 229-245. ), and they may come to see any individual member of a group as discrepant because they compare individuals to general rather than local standards (Klar, 2002 xClose
Klar, Y. (2002). Way beyond compare: Nonselective superiority and inferiority biases in judging randomly assigned group members relative to their peers. Journal of Experimental Social Psychology, 38, 331-351. ).

However, there are also reasons to believe that optimistic biases derive from motivational causes such as a need to protect self-esteem because people engage in numerous strategies to protect these and related beliefs when challenged (e.g., Boney-McCoy, Gibbons, & Gerrard, 1999 xClose
Boney-McCoy, S., Gibbons, F. X., & Gerrard, M. (1999). Self-esteem, compensatory self-enhancement, and the consideration of health risk. Personality and Social Psychology Bulletin, 25, 954-965. ; Croyle, Sun, & Louie, 1993 xClose
Croyle, R. T., Sun, Y., & Louie, D. H. (1993). Psychological minimization of cholesterol test results: Moderators of appraisal in college students and community residents. Health Psychology, 12, 503-507. ; Gerrard, Gibbons, Benthin, & Hesling, 1996 xClose
Gerrard, M., Gibbons, F. X., Benthin, A. C., & Hessling, R. M. (1996). A longitudinal study of the reciprocal nature of risk behaviors and cognitions in adolescents: What you do shapes what you think, and vice versa. Health Psychology, 15, 344-354. ; Klein & Weinstein, 1997 xClose
Klein, W. M., & Weinstein, N. D. (1997). Social comparison and unrealistic optimism about personal risk. In B. P. Buunk and F. X. Gibbons (Eds.), Health, coping, and well-being: Perspectives from social comparison theory (pp. 25-61). Hillsdale, NJ: Lawrence Erlbaum. ; Kunda, 1987 xClose
Kunda, Z. (1987). Motivated inference: Self-serving generation and evaluation of causal theories. Journal of Personality and Social Psychology, 53, 636-647. ), making optimistically biased judgments highly resistant to change (Weinstein & Klein, 1995 xClose
Weinstein, N. D., & Klein, W. M. (1995). Resistance of personal risk perceptions to debiasing interventions. Health Psychology, 14, 132-140. ). Emotion also plays a role; for example, people are more likely to be optimistically biased when angry and less likely when fearful or sad (Lerner & Keltner, 2001 xClose
Lerner, J. S., & Keltner, D. (2001). Fear, anger, and risk. Journal of Personality and Social Psychology, 81, 146-159. ; Salovey & Birnbaum, 1989 xClose
Salovey, P., & Birnbaum, D. (1989). Influence of mood on health-relevant cognitions. Journal of Personality and Social Psychology, 57, 539-551. ).

Importance in health behavior models. Perceived vulnerability is a key component of many health behavior models including the Health Belief Model (Janz & Becker, 1984 xClose
Janz, N. K., & Becker, M. H. (1984). The health belief model: A decade later. Health Education Quarterly, 11, 1-47. ), Protection Motivation Theory (Rogers, 1983 xClose
Rogers, R. W. (1983). Cognitive and physiological processes in fear appeals and attitude change: A revised theory of protection motivation. In J. T. Cacioppo & R. E. Petty (Eds.), Social psychophysiology (pp. 153-176). New York: Guilford Press. ), the Precaution Adoption Process Model (Weinstein, 1988 xClose
Weinstein, N. D. (1988). The precaution adoption process. Health Psychology, 7, 355-386. ), and the Prototype/Willingness Model (Gibbons, Gerrard, & Lane, 2003 xClose
Gibbons, F. X., Gerrard, M., & Lane, D. J. (2003). A social reaction model of adolescent health risk. In J. M. Suls & K. Wallston (Eds.), Social psychological foundations of health and illness (pp. 107-136). Oxford: Blackwell. ). Based on these models, if people underestimate their risk of experiencing a negative health outcome, they will be less likely to take precautions to prevent that outcome from occurring. Thus, given that optimistic biases represent an underestimation of risk, it may be argued that such biases are maladaptive. However, a different line of research suggests that positive illusions such as the optimistic bias may be adaptive because they promote motivation and discourage placidity (Armor & Taylor, 1998 xClose
Armor, D. A., & Taylor, S. E. (1998). Situated optimism: Specific outcome expectancies and self-regulation. Advances in Experimental Social Psychology, 30, 309-379. ). The notion that underestimations of risk may either hinder or promote precautionary behavior may explain why risk perceptions are often only moderately predictive of behavior (Gerrard, Gibbons, & Bushman, 1996 xClose
Gerrard, M., Gibbons, F. X., & Bushman, B. J. (1996). Relation between perceived vulnerability to HIV and precautionary sexual behavior. Psychological Bulletin, 119, 390-409. ; McCaul, Branstetter, Schroeder, & Glasgow, 1996 xClose
McCaul, K.D., Branstetter, A. D., Schroeder, D. M., & Glasgow, R. E. (1996). What is the relationship between breast cancer risk and mammography screening? A meta-analytic review. Health Psychology, 15, 423-429. ). It is likely that optimistic biases have differential effects on health behavior depending on many variables that have yet to be identified, providing a fruitful area of future research.