Three studies examined the "myside bias" in reasoning, evaluating written arguments, and writing argumentative essays. Previous research suggests that some people possess a fact-based argumentation schema and some people have a balanced argumentation schema. I developed reliable Likert scale instruments (1-7 rating) for these constructs and conducted an evaluation of instrument validity and reliability. A myside bias in argumentative essays was predicted by the fact-based and balanced argumentation schema instruments using these individual-difference measures. Strength of opinion predicted the myside bias in generating reasons but not in writing argumentative essays. There was a weak but significant correlation between the myside bias in generating reasons and writing argumentative essays. In evaluating written arguments, the fact-based schema instrument predicted agreement and quality ratings for claims supported by factual but not nonfactual reasons. Ratings of the quality of rebutted arguments were predicted by the measures of individual differences in argumentation schemata. (Contains 9 tables.)