Education researchers often compare performance across race and gender on research-based assessments of physics knowledge to investigate the impacts of racism and sexism on physics student learning. These investigations' claims rely on research-based assessments providing reliable, unbiased measures of student knowledge across social identity groups. We used classical test theory and differential item functioning (DIF) analysis to examine whether the items on the Force Concept Inventory (FCI) provided unbiased data across social identifiers for race, gender, and their intersections. The data was accessed through the Learning About STEM Student Outcomes platform and included responses from 4,848 students posttests in 152 calculus-based introductory physics courses from 16 institutions. The results indicated that the majority of items (22) on the FCI were biased towards a group. These results point to the need for instrument validation to account for item bias and the identification or development of fair research-based assessments.