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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