Memory Can Be Modified or Created by Suggestion • 223METHOD Presenting Misleading Postevent Information
The usual procedure in an experiment in which misleading postevent information (MPI) is pre-
sented is to fi rst present the stimulus to be remembered. For example, this stimulus could be a
list of words or a fi lm of an event. The MPI is then presented to one group of participants before
their memory is tested and is not presented to a control group. As you will see below, MPI is
often presented in a way that seems natural, so it does not occur to participants that they are
being misled. We will also see, however, that even when participants are told that postevent
information may be incorrect, presenting this information can still aff ect their memory reports.
The eff ect of MPI is determined by comparing the memory reports of participants who received
this misleading information to the memory reports of participants who did not receive it.An experiment by Elizabeth Loftus and coworkers (1978) illustrates a typical MPI
procedure. Participants saw a series of slides in which a car stops at a stop sign and then
turns the corner and hits a pedestrian. Some of the participants then answered a number
of questions, including “Did another car pass the red Datsun while it was stopped at the
stop sign?” For another group of participants (the MPI group), the words “yield sign”
replaced “stop sign” in the stop sign question. Participants were then shown pictures
from the slide show plus some pictures they had never seen. Those in the MPI group were
more likely to say they had seen the picture of the car stopped at the yield sign (which, in
actuality, they had never seen) than were participants who had not been exposed to MPI.
This shift in memory caused by MPI demonstrates the misinformation effect.
Presentation of MPI can alter not only what participants report they saw, but their
conclusions about other characteristics of the situation. For example, Loftus and Steven
Palmer (1974) showed participants fi lms of a car crash (● Figure 8.16) and then asked
either (1) “How fast were the cars going when they smashed into each other?” or
(2) “How fast were the cars going when they hit each other?” Although both groups
saw the same event, the average speed estimate by participants who heard the word
“smashed” was 41 miles per hour, whereas the estimates for participants who heard
“hit” was 34 miles per hour. Even more interesting for the study of memory are the
participants’ responses to the question “Did you see any broken glass?” which Loftus
asked 1 week after they had seen the slide show. Although there was no broken glass in
the original presentation, 32 percent of the
participants who heard “smashed” before
estimating the speed reported seeing bro-
ken glass, whereas only 14 percent of the
participants who heard “hit” reported see-
ing the glass (see Loftus, 1993, 1998).
The misinformation effect shows not
only that false memories can be created by
suggestion but also provides an example of
how different researchers can interpret the
same data in different ways. Remember that
the goal of cognitive psychology is to study
mental processes, but that these mental pro-
cesses must be inferred from the results of
behavioral or physiological experiments.
The question posed by the misinformation
effect is “What is happening that changes
the participants’ memory reports?” Different
researchers have proposed different answers
to this question. We will now describe three
explanations, one of which proposes that
MPI replaces old memories, another that
emphasizes the role of interference, and
another that is based on source monitoring.●FIGURE 8.16 Participants in the Loftus and Palmer (1974) experiment saw a fi lm
of a car crash, with scenes similar to the picture shown here, and were then asked
leading questions about the crash.
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