Loftus and Palmer (1974) conducted a classic experiment to investigate the effect of leading questions on the accuracy of eyewitness testimony.
Their sample consisted of 45 American students, who were divided into five groups of nine. All of the participants watched a video of a car crash and were then asked a specific question about the speed of the cars. Loftus and Palmer manipulated the verb used in the question, for example: “How fast were they cards going when they smashed / collided / bumped / hit / contacted with each other?’
They found that the estimated speed was affected by the verb used. For example, participants who were given the verb smashed reported an average speed of 40.5 mph, where participants who were given the word contacted reported an average speed of 31.8 mph, an overall difference of 8.7 mph.
The results clearly show that the accuracy of eyewitness testimony is affected by leading questions and that a single word in a question can significantly affect the accuracy of our judgements.
In a second experiment, Loftus and Palmer used a different sample of 150 American students, who were divided into three even groups. All the students watch a one-minute video depicting a car accident and were then given a questionnaire to complete.
One group was asked: “How fast were the cars going when they smashed into each other?”
Another group was asked: “How fast were the cards going when they hit each other?”
The final group (control) was not asked about the speed of the vehicles. One week later the participants returned and were asked a series of questions about the accident. The critical question was: “Did you see any broken glass?” 32% of the participants who were previously questioned using the verb smashed, reported seeing broken glass; 14% of the participants who were previously questioned using the verb hit, reported seeing broken glass; and 12% of the control group reported seeing broken glass.
There was no broken glass in the video clip and the participants who were questioned previously using the verb smashed, were significantly more likely to report seeing the broken glass, as a result of the earlier leading question. The verb smashed has connotation of faster speeds and broken glass and this question led the participants to report seeing something that was not actually present. Their memory for the original event was distorted by the question used one week earlier, demonstrating the power of leading questions.
Loftus and Palmer’s research has questionable ecological validity. On the one hand, questioning participants about everyday events like a car crash appears to be a genuine measure of eyewitness testimony. However, the participants in their research watched a video of a car crash and witnessed the events unfold from start to finish. In everyday reports of car accidents, witnesses rarely see the whole event; they are either involved in the event directly, or see a small part of the event happen in their peripheral vision. Therefore, their results to do reflect everyday car accidents and we are unable to conclude if participants involved in real accidents, who would have a stronger emotional connection to the event, would also be susceptible to leading questions in the same way.
A second weakness of Loftus and Palmer’s research is that their study lacks population validity. Their two experiments consisted of 45 and 150 American students. It is reasonable to argue that the students in their experiment were less experienced drivers, who may be less accurate at estimating speeds. Consequently, we are unable to generalise the results to other populations, for example, older and more experience drivers, who may be more accurate in their judgement of speeds and therefore not as susceptible to leading questions.
However, Loftus and Palmer’s research took place in a laboratory of Washington University and was therefore highly controlled. This high degree of control reduces the chance of extraneous variable, increasing the validity of the results. Furthermore, it is easy for psychologists to replicate their research, to see if the same results are achieved with a different population.
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