The myside bias in written argumentation entails excluding other side information from essays. To determine the locus of the bias, 86 Experiment 1 participants were assigned to argue either for or against their preferred side of a proposal. Participants were given either balanced or unrestricted research instructions. Balanced research instructions significantly increased the use of other side information. Participants' notes, rather than search patterns, predicted the myside bias. Participants who defined good arguments as those that can be “proved by facts” were more prone towards the myside bias. In Experiment 2, 84 participants of high and low argumentation ability read a text called “More Than Just the Facts” designed to contradict this fact-based argumentation schema. For high argumentation ability participants, the intervention reduced the myside bias, but for low ability participants it increased. The roots of the myside bias are underdeveloped argumentation schemata leading to misconceptions about research and argumentation.