Callen was an assistant professor of political science at Harvard University, having previously been an assistant professor in the Department of Political Science at UCLA. His recent work uses experiments to identify ways to address accountability and service delivery failures in the public sector. He has published in the American Economic Review, Journal of Conflict Resolution, and the British Journal of Political Science. He is an affiliate at the Center for Effective Global Action (CEGA), an affiliate at the Policy Design and Evaluation Lab (PDEL), an affiliate at the Center for Economic Research Pakistan (CERP), alumni at Empirical Studies of Conflict (ESOC), and a principal investigator on the Building Capacity for the Use of Research Evidence (BCURE): Data and evidence for smart policy design project. He completed his PhD in economics at UC San Diego in 2011.