Myside bias is the tendency to actively generate reasons favorable to one’s position and be reluctant to generate reasons unfavorable to one’s position. This study distinguished between quantitative myside bias (the number of reasons in favor of a position exceeds the number of reasons against it) and qualitative myside bias (where a person does not generate valid counterarguments to one’s most important myside reason). The study examines the relationship between myside bias and polarized thinking. In Study 1, university students (𝑛= 75) were asked to write both reasons for and against four topics, and rate their confidence in their choice of position. The results indicated that polarized thinking was intensified after writing the reasons, and qualitative myside bias may be positively related to this tendency. In Study 2, university students (𝑛= 130) were divided into (1) a control condition where they generated reasons as in Study 1, and (2) an experimental condition where they were asked to consider the falsifiability of one’s most important myside reason, and their qualitative myside bias and confidence in their position were compared. The results indicated that it was difficult for participants to refute their most important myside reasons. The tendency for polarization accentuation after writing reasons was reconfirmed. However, the possibility that polarized thinking is suppressed in students with reduced qualitative myside bias was indicated, and methods for reducing qualitative myside bias were discussed.
