, Volume 110, Issue 4, pp 437–442 | Cite as

Do double-blind studies with informed consent yield externally valid results?

An empirical test
  • Irving Kirsch
  • Michael J. Rosadino
Original Investigations


Subjective and physiological effects of caffeine were investigated via a 3×2×3 design that assessed independent and interactive effects of instructions (told caffeine versus told no caffeine versus not told whether beverage contained caffeine), actual beverage content (caffeine versus no caffeine), and time after ingestion (15, 30, and 45 min). Instructions affected altertness at 15 min after ingestion. Caffeine increased alertness at 30 min after ingestion and systolic blood pressure at 30 min and 45 min after ingestion. A highly significant instruction by drug interaction on tension was obtained at all measurement points, indicating an increase in tension only among subjects who knowingly received caffeine. Because people are generally informed of drug content in non-research settings, these data challenge the external validity of typical double-blind studies, in which subjects are informed of the possibility of receiving a placebo as part of the consent procedure.

Key words

Caffeine Placebo Expectancy Double-blind design Informed consent 


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Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • Irving Kirsch
    • 1
  • Michael J. Rosadino
    • 1
  1. 1.Department of PsychologyUniversity of ConnecticutStorrsUSA

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