Case File 268 · Investigation Psychology

Why Should Witnesses Be Interviewed Separately?

By Murder Book Editorial TeamPublished by Gungrounds, the studio making Murder BookPublished 3 October 2026Updated 3 October 2026

Witnesses should be interviewed separately because people who discuss an event together tend to contaminate each other's memories. Research by Gabbert, Memon and Allan (2003) found that 71% of witnesses who talked about an event before being interviewed individually reported details that only their conversation partner had seen. Once a witness absorbs someone else's account, the borrowed detail can feel like a genuine memory, making it difficult or impossible to tell the two apart. Separating witnesses before their first interview is one of the simplest steps investigators can take to protect the independence of each account.

Key takeaways

Witnesses who discuss an event before being interviewed often incorporate details from each other's accounts into their own memories.
In a controlled study, 71% of witnesses who talked together reported details they could only have learned from their partner.
Memory conformity can happen because a witness trusts another person's account, wants social approval, or genuinely absorbs the new detail into memory.
Separate interviews preserve the independence of each account, making it easier to spot genuine overlaps and genuine disagreements.
Even well-meaning witnesses can unintentionally reshape each other's recollections through casual conversation.

What is co-witness contamination?

Co-witness contamination occurs when two or more people who witnessed the same event share information, and that sharing alters what one or both of them later report. The contamination can be subtle. A witness may not realise that a detail they recall vividly actually came from a conversation rather than from their own observation. Research on the misinformation effect has shown that post-event information from many sources, including other witnesses, can distort memory.

Gabbert, Memon and Allan's 2003 study in Applied Cognitive Psychology provided a clear demonstration. Pairs of participants watched a video of the same event, but each person saw a slightly different version containing unique items visible only to them. When pairs discussed the event before recalling it individually, the majority reported details that belonged to their partner's version, not their own. A control group that did not discuss the event showed no such crossover.

How does memory conformity differ from deliberate agreement?

Memory conformity is not the same as two people agreeing on a story. It describes a process in which one person's recall genuinely changes after exposure to another person's account. Wright, Self and Justice (2000), in a study published in Memory, found that participants who discussed an event with a co-witness often adopted the other person's details without awareness that they were doing so.

Two mechanisms can drive this. Informational influence occurs when a witness concludes that the other person's version is correct and updates their own memory accordingly. Normative influence occurs when a witness goes along with the group to avoid disagreement. In both cases, the outcome is the same for investigators: the accounts are no longer independent. For a deeper look at how witnesses reshape each other's recall, see Can Witnesses Change Each Other's Memories?.

Why does independence matter for an investigation?

Independent accounts are valuable precisely because they have not been filtered through someone else's perspective. When two witnesses separately describe the same detail, that convergence carries weight. When two witnesses describe the same detail because one heard it from the other, the convergence is meaningless.

Investigators who compare witness accounts rely on the assumption that each account reflects what that person actually saw, heard or experienced. If witnesses have already pooled their information, an investigator may mistake borrowed details for corroboration. This can strengthen a flawed lead or mask a genuine disagreement that would have prompted further questions.

The problem extends beyond factual details. Research on memory conformity has found that witnesses can also influence each other's confidence. A witness who was uncertain about a detail may become confident after hearing a partner state it firmly, even though nothing about the underlying memory has changed.

Best-practice interviewing guidelines in several countries advise separating witnesses as soon as practical after an event. The aim is to take each person's account before outside information can alter it.

The cognitive interview, developed by Fisher and Geiselman, is one widely studied approach that emphasises interviewing witnesses individually and using open-ended recall before any specific questions. A 2010 meta-analysis by Memon, Meissner and Fraser, published in Psychology, Public Policy, and Law, examined 65 experiments and found that the cognitive interview consistently increased the amount of correct information witnesses reported, with only a small increase in errors.

More recent research by Vredeveldt and colleagues has explored whether there are circumstances in which a structured joint interview might add value after individual interviews have been completed. Their work suggests that pairs of witnesses can sometimes correct each other's errors when interviewed together under controlled conditions. However, the recommended sequence is always to conduct individual interviews first, so that each witness's independent account is preserved before any collaborative recall takes place. For more on how the timing and method of recall affect what witnesses remember, see How Does Time Affect Eyewitness Memory?.

What can go wrong if witnesses are not separated?

Several documented problems can arise:

Blended accounts. Witnesses merge what they saw with what they heard from others, producing a composite memory that no single person actually experienced.
False confidence. A hesitant witness becomes certain after hearing a confident partner, even if the partner was wrong.
Lost disagreements. Genuine differences between accounts, which could have pointed investigators toward gaps or errors, are smoothed over before anyone records them.
Contaminated identifications. If one witness describes a suspect's appearance and another witness absorbs that description, a later identification may reflect the conversation rather than an independent observation. Research on why eyewitness identification fails highlights how fragile recognition memory can be.

Does casual conversation count?

Yes. Contamination does not require a formal debrief or a deliberate attempt to align stories. Everyday conversation at a crime scene, in a waiting room or on the way to a police station can be enough. Paterson and Kemp (2006), in a study in Applied Cognitive Psychology, found that witnesses who overheard another person's account were influenced by it, even when they were not actively participating in the discussion.

This is why practical guidance for first responders often emphasises physically separating witnesses as quickly as possible and asking them not to discuss what happened until they have each given their own account.

How does Murder Book explore witness interviews?

Murder Book: The Art of Interrogation is an upcoming narrative interrogation game from Gungrounds, an independent studio in Zagreb, Croatia. Development screenshots show an evidence board linking statements from different sources. Players examine unreliable stories, look for narrative gaps and contradictions, and weigh whether details across accounts are truly independent or might reflect contamination. The game's press kit names Avoidance and Narrative Gaps among the behaviours players assess, both of which become harder to evaluate when accounts have already been compared outside the interview room.

Murder Book is in development for PC (Steam) and mobile, with release currently planned for the end of Q2 2027.