Subject Rational Expectations Will Contaminate Randomized Controlled Medical Trials

27 Pages Posted: 7 Jul 2016 Last revised: 11 Dec 2017

See all articles by Gilles Chemla

Gilles Chemla

Imperial College Business School; CNRS ; Centre for Economic Policy Research (CEPR)

Chris Hennessy

London Business School

Multiple version iconThere are 2 versions of this paper

Date Written: June 2016


We develop a rational expectations model of placebo effects. If subjects in seemingly-ideal single-stage RCTs form rational beliefs about breakthroughs based upon personal physiological responses, mental effects differ across medications received, treatment versus control. Consequently, the average cross-arm health difference becomes a biased estimator of the mean non-placebo physiological effect. Constructively, we show: bias can be altered through choice of control; high-efficacy controls mitigate upward bias; and unbalanced panels may be preferred since bias approaches zero as treatment probability approaches zero. Consistent with experimental evidence, our theory implies outcomes within-arm and cross-arm differences can be non-monotone in treatment probability.

Keywords: Bias, Rational Expectations, RCTs

Suggested Citation

Chemla, Gilles and Hennessy, Christopher, Subject Rational Expectations Will Contaminate Randomized Controlled Medical Trials (June 2016). CEPR Discussion Paper No. DP11360, Available at SSRN:

Gilles Chemla (Contact Author)

Imperial College Business School ( email )

South Kensington Campus
London SW7 2AZ, SW7 2AZ
United Kingdom
+44 207 594 9161 (Phone)
+44 207 594 9210 (Fax)

CNRS ( email )

Dauphine Recherches en Management
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Centre for Economic Policy Research (CEPR)

United Kingdom

Christopher Hennessy

London Business School ( email )

Sussex Place
Regent's Park
London, London NW1 4SA
United Kingdom

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