His research interests include international migration; citizenship and nationhood, with a focus on how immigration and citizenship policies intersect with and define boundaries of national belonging. Recent publications include: How the Federal Republic Became an Immigration Country: Norms, Politics and the Failure of West Germany’s Guest Worker System (co-authored with Karen Schönwälder). German Politics and Society VOL. 24, No. 3 (Fall 2006). Building Walls, Bounding Nations: Migration and Exclusion in Canada and Germany, 1870-1939. Journal of Historical Sociology VOL. 17, No. 4 (2004).