EXPERIMENTAL RESEARCHsection of this chapter) is called a demand charac-
teristic. It occurs when research participants pick
up clues about the hypothesis or an experiment’s
purpose and then modify their behavior to what they
think the research demands of them (i.e., support
the hypothesis). Participants often do this to please
the researcher, which is why we often use mild de-
ception in the form of cover stories.
12.Placebo effect.The last type of threat to in-
ternal validity is the placebo effect.A placebois an
empty or nonactive treatment, such as a sugar pill in
medical research. It occurs when you give some par-
ticipants a placebo but they respond as if they have
received the real treatment. For example, you create
an experiment on stopping smoking for heavy smok-
ers. You give some participants a pill with an antini-
cotine drug to reduce their nicotine dependence and
others a placebo (empty pill). If participants who re-
ceived the placebo also stop smoking, then merely
participating in the experiment and taking something
that they believed would help them quit smoking had
an effect. The belief in the placebo alone may have
affected the dependent variable (see Table 3 for a
summary of internal validity threats).Demand characteristic A type of reactivity in which
participants in experimental research pick up clues about
the hypothesis and alter their behavior accordingly.FIGURE 4 Double-Blind Experiment: An Illustration of Single-Blind,
or Ordinary, and Double-Blind Experiments
Single-Blind ExperimentDouble-Blind ExperimentExperimenterExperimenter
Subjects Who Are Blind to True HypothesisSubjects Who Are Blind to True HypothesisAssistant Who Is Blind
to Details of TreatmentPlacebo effect A result that occurs when participants
do not receive the real treatment but receive a nonac-
tive or imitation treatment but respond as though they
have received the real treatment.