In this essay, I explore how cognitive bias, rather than racial or ethnic bias, may inhibit the ability of school psychologists to advance social justice through research and practice. I begin with the argument that tests advance the cause of social justice because they define problems (i.e., disparities among groups) in objective, measurable terms. I then argue that, rather than doing the cognitively demanding work of collecting, evaluating and applying scientific evidence to enhance outcomes for all students, school psychologists default to less cognitively demanding strategies (specifically, confirmatory and conjunction biases) to guide how they define and respond to the problem of inequities among groups. I propose that research, practice, and professional education in school psychology would be more likely to advance outcomes for all students if we shift our focus from arguing what "could" cause disparities among groups to a focus on applying the theories and technologies derived from objective, measurable scientific research.