Pseudo-Placebo Effects in Randomized Controlled Trials for Development: Evidence from a Double-Blind Field Experiment in Tanzania Erwin Bulte, 1 * Lei Pan, 1 Joseph Hella, 2 Gonne Beekman 1 and Salvatore di Falco 3 1: Development Economics Group, Wageningen University, P.O. Box 8130, 6700 EW Wageningen, Netherlands 2: Sokoine University of Agriculture, Morogoro, Tanzania 3: London School of Economics, London, United Kingdom (*Corresponding author: Erwin.bulte@wur.nl) Abstract: Randomized controlled trials (RCTs) in the social sciences are not double-blind, so participants know they are “treated” and will adjust their behavior accordingly. Under some conditions this gives rise to so-called “pseudo-placebo effects,” which may bias assessment of impact. We implement a conventional economic RCT and a double-blind experiment in rural Tanzania (randomly allocating modern and traditional cowpea seed- varieties to farmers), and demonstrate that such pseudo-placebo effects can be large. For our case they explain the entire “treatment effect on the treated” as measured in a conventional economic RCT. Keywords: Randomized controlled trial (RCT), behavioral response, experimenter effect, placebo JEL Codes: C9, D04,