Dateline: May, 2022, Issue 1

Are eyewitness memories distorted by talking with other witnesses?

Multiple witnesses often observe an incident and, on average, 86% engage in post-event discussion with their co-witnesses (Paterson & Kemp, 2006).

Witness communication can contaminate the memories of co-witnesses. Information suggested by one witness becomes, over time, part of other witnesses' memories and the other witnesses then remember seeing information which they only heard from another eyewitness. Co-witness communication leads to conformity in memories across witnesses.

A notable case of memory conformity was the investigation of the 1995 Oklahoma bombing incident. Three eyewitnesses had reported having seen the suspect come into the store they worked at to rent the vehicle that was used for the attack. Initially, two of the witnesses had correctly reported seeing McVeigh get inside the truck, but the third witness mistakenly believed that a second accomplice was present. After discussing the event with each other, all three witnesses agreed that a second accomplice had been present during the incident. The FBI believes this "accomplice" was an innocent person who rented a different truck the next day (Memon & Wright, 1999). The phenomenon of co-witness memory contamination is called the co-witness suggestibility effect.

Garry and colleagues (2008) investigated co-witness suggestibility by having pairs of participants sit together and watch a crime video projected on a screen. The participants watched the video with polarizing glasses that allowed, unbeknownst to the participants, two different videos to be projected onto the screen where each participant could only see one of the two videos. In both projected videos, Eric the electrician is seen wandering through an unoccupied house and helping himself to the contents, but eight details were different between the two videos. After watching the videos, each co-witnesses pair tried jointly to answer questions about what they observed, including questions about details that differed between the two video versions observed by each witness pair. Participants then individually completed a memory test about what they had observed. Participants reported seeing details they could only have heard about from their co-witness. Recall accuracy for information not discussed with the co-witness was 79%, but for information that had been discussed, individual witness recall accuracy dropped markedly to only 34%. Co-witness communication corrupted individual witness memories.

Ito and colleagues (2019) re-conducted this research in 10 different countries. Each of the 10 tests replicated the findings of Garry and colleagues: Individual witnesses reported on the final memory test a non-witnessed answer that their co-witness had stated during the memory test collaboration phase. Such co-witness suggestibility errors were especially likely when the witness had not disputed the co-witness's report during the collaboration phase. Individual recall accuracy for information not discussed and discussed was nearly the same as that reported by Garry and colleagues, with recall accuracy being substantially higher for non-discussed details than discussed details. The co-witness suggestibility effect was very strong with over 80% of the items of misleading information to which a participant was exposed being later recalled incorrectly by the participant. On average, participants were exposed to misinformation for 1.61 items of information and made 1.29 errors due to the fact they had been exposed to misinformation from the co-witness.

Gabbert and colleagues (2006) found that, in two studies in which co-witnesses had viewed different versions of the same event and then collaborated before being tested individually, the witness who mentioned a critical detail first was more likely to influence the other witness.

Witnesses have been found to be susceptible to co-witness influence for a variety matters, including when describing suspects (Loftus & Greene, 1980), making identifications (Zajac & Henderson), and attributing blame (Mojtahedi et al., 2020).

In sum, witnesses incorporate elements of other witnesses' memory reports into their own memory reports, even when that information contradicts what they themselves have seen. Co-witness communication leads to recall errors, memory corruption, and memory conformity.

Source Gabbert, F., Memon, A. & Wright, D.B. (2006). Memory conformity: Disentangling the steps toward infuence during discussion. Psychonomic Bulletin & Review, 13(3), pp. 480-485.

Source Garry, M., French, L., Kinzett, T. & Mori, K. (2008). Eyewitness memory following discussion: Using the MORI technique with a western sample. Applied Cognitive Psychology, 22(4), pp. 431-439.

Source Ito, H., Barzykowski, K., Grzesik, M., Gülgöz, S., Gürdere, C., Janssen, S.M.J., Khor, J., Rowthorn, H., Wade, K.A., Luna, K., Albuquerque, P B., Kumar, D., Singh, A.D., Cecconello, W. W., Cadavid, S., Laird, N.C., Baldassari, M.J., Lindsay, D.S., & Mori, K. (2019). Eyewitness memory distortion following co-witness discussion: A replication of Garry, French, Kinzett, and Mori (2008) in ten countries. Journal of Applied Research in Memory and Cognition, 8(1), pp. 68-77.

Source Loftus, E. & Greene, E. (1980). Warning: Even memory for faces may be contagious. Law and Human Behavior. 4(4), pp. 323-334.

Source Memon, A. & Wright, D.B. (1999). Eyewitness testimony and the Oklahoma bombing. The Psychologist, 12(6), pp. 292-295.

Source Mojtahedi, D., Ioannou, M. & Hammond, L. (1999). Intelligence, authority and blame conformity: Co-witness influence is moderated by the perceived competence of the information source. Joural of Police and Criminal Psychology, 35, pp. 422-431.

Source Paterson, H.M. & Kemp, R.I. (2006). Co-witnesses talk: A survey of eyewitness discussion. Psychology, Crime & Law, 12(2), pp. 181-191.

Source Zajax, R. & Henderson, N. (2009). Don't it make my brown eyes blue: Co-witness misinformation about a target's appearance can impair target-absent line-up performance. Memory, 17(3), pp. 266-278.