This article describes a cognitive argumentation schema for written arguments and presents three empirical studies on the "myside" bias--the tendency to ignore or exclude evidence against one's position. Study 1 examined the consequences of conceding, rebutting, and denying other-side information. Rebuttal led to higher ratings of agreement and quality and better impressions of the author than when the same arguments excluded other-side information (i.e., exhibited the myside bias). In Study 2, claims had a significantly greater impact on agreement ratings and reasons had a significantly greater impact on quality ratings. When participants were given myside reasons supporting other-side claims, they acknowledged argument strength while making relatively minor changes in agreement. In Study 3, the authors found that a brief, theoretically motivated written tutorial was effective in improving undergraduate students' written argumentative essays by significantly increasing the precision of claims, improving the elaboration of reasons, and reducing the myside bias. (Contains 2 tables and 1 figure.)