The results reflect only the authors’ views; the European Research Council is not responsible for any use that may be made of the information it contains. The authors are indebted to Dan Aisen and Ben Connault for access to data and for insight into the IEX system. The editorial team of this journal was exceptionally helpful in improving the exposition. The authors also benefited from comments by Peter Bossaerts, Peter Cramton, Tomasz Kozubowski, Pete Kyle, Natalia Lazzati, Kristian Lopez Vargas, John Musacchio, Axel Ockenfels, Ioanid Rosu, Nirvikar Singh, and Harold Widom and from participants at the 2018 Bonn MPI Experimental Finance Workshop, the 2018 Luxembourg Expbot Workshop, the May 2020 Australia–New Zealand economics workshop, and Economics Department seminars at the Universities of Essex, Maryland, Monash, and Virginia Commonwealth.