The content of argumentative essays is determined by multiple factors, but belief influences are understudied compared to topic knowledge and argument schema. We investigate how beliefs influence the inclusion of basic components in argumentative writing. A pre-screening survey identified believers and disbelievers in gun control effectiveness. In a subsequent laboratory session, subjects (N = 324) read a one-sided text that was either consistent or inconsistent with their beliefs. Subjects then reported their beliefs and wrote a 250-word argumentative essay explaining them. These essays were coded for the presence or absence of a claim, the number of reasons supporting the claim, the presence of a counterargument, text content, and other factors. 682 supplementary subjects provided approximately 10 ratings for each essay on several factors, including position, clarity, and consideration of both sides. Subjects who read a belief consistent text wrote essays that were more likely to contain a claim, more reasons, and text content. Subjects who read a belief inconsistent text were more likely to include an evaluative statement about the text and to consider both sides of the issue. Individual differences in belief change were related to the likelihood of stating a claim, the number of reasons, and likelihood of mentioning text content. Results suggest that beliefs influence the basic components of argumentation that are included in argumentative essays. Theoretical and practical implications of these findings are discussed.