Case File 261 · Investigation Psychology

What Is Source-Monitoring Error?

A source-monitoring error happens when you remember a piece of information correctly but misremember *where* it came from. The detail is real; the source is wrong. You might recall a fact but not whet

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

A source-monitoring error happens when you remember a piece of information correctly but misremember where it came from. The detail is real; the source is wrong. You might recall a fact but not whether you saw it, were told it, read it, or imagined it. In everyday life this is harmless, like being sure a friend told you something you actually read online. In an investigation it matters a great deal, because a witness can genuinely remember a detail while being mistaken about whether they witnessed it or picked it up afterwards, from another person, the news or the way a question was asked. Source monitoring is the set of mental processes we use to work out the origin of a memory, and it is not perfect.

Key takeaways

A source-monitoring error means remembering information but attributing it to the wrong origin.
The underlying framework was set out by Johnson, Hashtroudi and Lindsay in 1993.
These errors help explain how post-event information slips into eyewitness memory.
The person is not lying; they genuinely cannot tell where a detail came from.
Recording interviews and separating witnesses limits how often misattributions happen.

What is source monitoring?

Marcia Johnson, Shahin Hashtroudi and Stephen Lindsay described source monitoring in an influential 1993 paper in Psychological Bulletin. Their core idea is that memories do not arrive with a label saying where they came from. Instead, the brain judges a memory's source from its qualities: how much perceptual detail it has, how much emotion is attached, how it fits with what else you know.

A memory of something you actually saw usually carries rich sensory detail, such as colour, sound and spatial layout. A memory of something you only imagined or were told tends to carry more detail about your own thinking. Most of the time these cues let us tell real perception from thought or hearsay. A source-monitoring error occurs when the cues mislead us and we assign a memory to the wrong source.

How is a source-monitoring error different from simply forgetting?

Forgetting is losing the information. A source-monitoring error is keeping the information but losing track of its origin. The two can look similar from the outside but are different problems.

Consider a witness who confidently reports that the getaway car was blue. If they never saw the car's colour but later read "blue car" in a question or a news report, the word "blue" is a real memory. What has failed is the judgement about where it came from. They have attributed a detail learned afterwards to their own observation at the scene. This is the mechanism behind many cases of post-event information reshaping an account, and it is closely tied to memory contamination.

How do these errors feed into eyewitness mistakes?

Source confusion is one of the clearest routes by which misleading information enters memory. Loftus's 2005 review of three decades of misinformation research describes how people come to report misleading details as their own memories. When a suggested detail is misattributed to the original event, the witness is not inventing anything on purpose; the source tag has simply been assigned wrongly.

This helps explain a puzzling pattern: witnesses can hold a false detail with real confidence. Because the detail itself is a genuine memory, it can feel just as vivid as an accurate one. That is part of why confidence and accuracy can come apart, discussed in Does Eyewitness Confidence Mean the Memory Is Accurate?.

Several conditions make source errors more likely:

A long delay between the event and recall, so the original memory has faded.
Exposure to a plausible alternative detail from a credible source.
Repeated discussion, which can blur who said what, as in memory conformity.
High similarity between the real experience and the suggested one, which leaves fewer distinguishing cues.

Can you prevent source-monitoring errors?

They cannot be eliminated, because the processes that cause them are part of normal memory. But their frequency can be reduced, and investigators build their methods around that aim.

Interview early. The sooner an account is taken, the less time there is for other sources to intrude.
Ask open questions. Free recall gives the witness fewer suggested details to later misattribute.
Keep witnesses apart. Separating witnesses stops one person's account becoming a hidden source for another's, as covered in Why Should Witnesses Be Interviewed Separately?.
Record everything. A full recording lets reviewers trace exactly when and how a detail first appeared, so a misattributed detail can be spotted.

Researchers also test accounts by asking for specifics that can be checked against physical evidence, rather than treating any single confident statement as settled.

Can imagining something lead to a source error?

Part of the source-monitoring framework deals with telling apart memories of things we actually perceived from memories of things we only thought about or imagined, which Johnson and colleagues call reality monitoring. Because an imagined scene can share some qualities with a perceived one, a vividly imagined detail can later be misremembered as something real.

This matters during questioning. If a witness is repeatedly asked to picture how an event might have happened, or to speculate about a detail they did not clearly see, the imagined version gains the kind of rich detail that memory tends to read as a sign of real perception. Later, the person may attribute that imagined detail to the event itself. The fault is not dishonesty; it is the ordinary difficulty of keeping apart two internally similar memories. It is one more reason investigators favour open recall over prompting a witness to imagine or fill in gaps.

Does a source-monitoring error mean someone is unreliable or lying?

No. A source-monitoring error is an honest mistake about origin, not a deliberate deception. The person believes they are reporting what they saw, and in a sense they are reporting a real memory; the fault is in the source attribution, not in their honesty.

This is why a mistaken detail does not, on its own, show that a witness is dishonest or that their whole account is worthless. It shows that memory is reconstructive and that origin is one of the first things to blur. The sensible response is corroboration: checking the detail against independent evidence rather than relying on the witness's certainty.

Is there an upcoming game built around uncertain accounts?

Murder Book: The Art of Interrogation treats testimony as something to be weighed, not simply believed. It is an upcoming narrative interrogation game from Gungrounds, an independent studio in Zagreb, Croatia, inspired by real police footage and psychological interviews. Players piece cases together from unreliable stories and fragments, examining narrative gaps and contradictions.

Source-monitoring errors are exactly the kind of fault that makes those fragments tricky. A character may offer a detail with full sincerity while being wrong about whether they saw it or absorbed it later. The game frames such a detail as a lead worth testing against the rest of the case, not as proof of guilt or dishonesty. Cases are designed for focused 5–10-minute sessions. Murder Book is in development for PC (Steam) and mobile, with release currently planned for the end of Q2 2027.