Are Prediction Markets Politically Biased? -- by Eric Zitzewitz

This version of the paper is preliminary; it will be revised following the 2026 midterms to include those elections and data from Polymarket. The author would like to thank the prediction markets included in the sample for making their data available for academic research. The pre-2007 data included in the sample was collected jointly with Andrew Leigh and Justin Wolfers for a project that never led to a paper. I have traded on prediction markets, but have no financial relationship with any of the markets mentioned in this work. The views expressed herein are those of the author and do not necessarily reflect the views of the National Bureau of Economic Research.