Background . People overestimate the impact of health conditions on happiness, putatively because they focus excessively on resulting negative consequences while disregarding the impact of other unchanged aspects of life on happiness. However, typically, inferences about accuracy have been based on a confound of the viewpoint of judgments (Self/Other) with whether the respondent has the condition (Have/Not-have)—an important issue because people often judge themselves as different to others. This study measured Haves' and Not-haves' judged impact on happiness—for self and other—of several chronic health conditions, and whether ``defocusing'' respondents improved judgment.Method. 80 Haves and 80 Not-haves predicted the impact of health conditions on their own and others' happiness using a questionnaire, after some participated in a defocusing exercise. Haves also indicated their preferences for their health condition over other conditions.Results. Although Haves made more accurate forecasts than Not-haves, both overestimated the impact of health conditions on others' happiness— yet defocusing respondents prior to prediction had no effect. Haves were aware that Not-haves misjudge Haves' happiness, but underestimated the bias in Not-haves' judgments. Whereas Haves judged they were more happy than other Haves, Not-haves predicted they would be less happy than others if living with a health condition. Finally, Haves' preferences for health conditions exhibited an endowment effect. Conclusion. The existence of an impact bias is not attributable to the confounding of self/other and Have/Not-have in other studies. People who have a condition forecasted others' happiness more accurately, suggesting that experience of one condition helps in comprehending life with another.
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