Karl is currently a Professor of Economics at the University of Sheffield. He gained his PhD in Economics from the Open University under joint supervision with Cardiff University. Previously Karl was employed at the University of Leicester 2001-2005 where he was initially a Lecturer before being promoted to a Senior Lecturer. He was appointed to a Readership in September 2005 at the University of Sheffield and was subsequently promoted to a Personal Chair in January 2009. From 2006-2010 Karl was an external Research Associate of the Institute for Social and Economic Research (ISER), University of Essex. He has published in a wide range of journals including: Journal of Money, Credit and Banking; Canadian Journal of Economics; Economica; Economic Inquiry; Economics of Education Review; Journal of Banking and Finance; Journal of Economic Behavior and Organization; Journal of Health Economics; Journal of Economic Psychology; Journal of the Royal Statistical Society; Labour Economics; Oxford Bulletin of Economics and Statistics; Oxford Economic Papers; and Review of Income and Wealth. He has also published policy related research in Fiscal Studies, Oxford Review of Economic Policy and The Lancet. Research interests include applied microeconometrics, household finances and labour economics. Karl was a member of the Grant Assessment Panel C of the Economic and Social Research Council (ESRC) from September 2016-2020. Currently Karl is an Associate Editor of the Bulletin of Economic Research.

He joined IZA as a Research Fellow in September 2009.