For the past decade the Model-Evidence Link (MEL) project has studied student reasoning around socioscientific issues including climate change, extreme weather, and the use and availability of natural resources. We use an instructional scaffold called the Model-Evidence Link (MEL) diagram in which students are provided with multiple models that explain scientific phenomena, one of which is the scientifically accepted model and one or two others that provide alternative explanations. Students evaluate lines of evidence that either support, strongly support, contradict, or have nothing to do with each of the models. During a MEL activity, students work in groups discussing the models, the lines of evidence, and the rationale for their choices. The MEL team developed a rubric with four categories to assess students' understanding of and reasoning behind the connections they make. The rubric features four distinct categories of evaluation: 1) erroneous, 2) descriptive, 3) relational, and 4) critical. In our current project, Evaluating Sources and Claims, we extend this analysis by recognizing that student reasoning may be motivated by accuracy (i.e., wanting to be correct in an explanation) or by a desired conclusion (i.e. wanting to support their existing belief). This is our Knowledge-Motivation Model. We are developing a three-tier instrument, the Reasoning About Socioscientific Issues (RASSI) measure. In the RASSI we provide a socioscientific claim about which students rate their agreement using a 1-6 Likert scale from "strongly disagree" to "strongly agree". We provide four justifications for students to explain why they choose the ranking they did. The plausible response options are grounded in common misconceptions students have and our Knowledge-Motivation Model. Last, we ask students to rate their answer, again on a 1-6 Likert scale from "not at all confident" to very confident. We share our on-going findings in this poster