Co-witness information can have immediate effects on eyewitness memory reports.

When questioning a reluctant witness, investigators sometimes encourage the witness by providing information about what other witnesses have said. Three experiments were conducted to test the combined effects of such co-witness information and suggestive questioning on the accuracy of eyewitness memory reports. Experiment 1 was analogous to the experience of a witness who receives information from an interviewer or questioner about what other witnesses have already said, whereas Experiments 2 and 3 simulated the situation in which a witness receives information directly from a co-witness. In all three experiments, when participants received incorrect information about a co-witness's response, they were significantly more likely to give that incorrect response than if they received no co-witness information. This effect persevered in a delayed memory test 48 h after the initial questioning session in Experiment 3. Accuracy rates were lowest of all when incorrect co-witness information was paired with questioning that suggested an incorrect response. These results have implications not only for the immediate effects on the accuracy of witnesses' memory reports, but also for the impact that even one such inaccurate report can have on the manner in which a case is investigated by the police or other authorities.