Your similarity score was 18% before the deadline and 72% two days after — and the report now shows a match against a classmate's paper. Before you panic about being flagged, know what actually happened: Turnitin runs a final peer-to-peer comparison after the due date, so papers that never matched during the term suddenly surface against each other. Turnitin's official documentation describes this mechanism, the Eric & Jane scenario that shows why the copier's score can be the lower one, and the settings that decide whether the check even runs at your institution. This article explains the mechanism, the confusion it creates, and how to respond when your score changes overnight.
Short answer
Turnitin identifies collusion by comparing submissions to the same assignment against one another — a peer-to-peer check that, for most assignments, runs automatically after the due date. Turnitin's official student guide defines collusion as "identified when a student's work matches with another student's submission on the same assignment or to previously submitted papers," and its help center explains that this comparison "typically happens automatically only after the assignment due date has passed." A score that jumps after the deadline is often this check working as designed, not a new accusation against you.
What collusion detection actually is
The official student guide (Understanding the similarity score for students) opens with the definition:
"Collusion is typically identified when a student's work matches with another student's submission on the same assignment or to previously submitted papers."
Two things to notice. First, the match is against another student's submission — not against a website. Second, it also covers previously submitted papers (including your own earlier work from a different class, which shades into self-plagiarism; more below). The collusion check is not a separate AI-style detector; it is the similarity database comparing papers to papers.
The Eric & Jane scenario
Turnitin's official guide uses a pair of students to show how the mechanism works:
"Consider the following scenario: Eric acquired a copy of his classmate Jane's paper. Eric submits Jane's paper as his own and receives a similarity score of 25%. Jane, who originally wrote the paper, submits her work a few days later and receives a 100% similarity score."
The guide then explains the timing:
"Turnitin can identify that collusion has taken place in this scenario by running a final similarity check against all submitted assignments after the due date."
Read that carefully, because it inverts the usual assumption: Eric (the copier) scored 25%, and Jane (the author) scored 100%. If the check ran before the due date, Eric's paper only matched the internet — no one else had submitted yet. After the due date, the final check matched Jane's paper against everything, including Eric's copy, which is why her score jumps to 100%. The copier's low score is not evidence of innocence, and the author's high score is not evidence of guilt — the numbers reflect submission order, not authorship.
Why the check runs after the due date
Turnitin's help center explains the delay in a dedicated article, "Why are papers submitted to the same Turnitin assignment not matching to each other?":
"Papers submitted to the same Turnitin assignment do not match against each other immediately upon submission."
"To prevent students from matching against their own drafts, peer-to-peer comparison (known as collusion checking) typically happens automatically only after the assignment due date has passed."
"If your Standard (Classic) Assignment is set to allow resubmissions, all Similarity Reports are automatically regenerated within one hour after the assignment due date and time."
The due-date timing serves two purposes: every student gets the same level of scrutiny regardless of when they submitted, and students' own earlier drafts are excluded so that resubmission does not count against them. Before the deadline, reports only check external sources (internet, periodicals, repository) — classmates simply are not part of the comparison yet.
Why your score changed after the deadline
The same help-center page describes the visible consequence:
"Similarity Reports generated before the due date only check submissions against external sources... meaning student papers will not match against one another until the deadline passes."
So a report that was 18% on Monday and 72% on Thursday is most likely the collusion check regenerating the report — the match against a classmate's paper was always there, it just was not visible until the final run. This is the mechanism behind our guide on scores changing after the due date; this article is the mechanism, that one is the phenomenon.
Turnitin's blog on collusion adds the settings that make the check possible:
"The standard and institutional paper repository must be selected to compare student work against previous submissions. If 'no repository' is selected, a collusion check cannot take place."
If your institution did not enable a repository, the check cannot even run — another reason a "classmate match" may have other explanations.
Collusion vs self-plagiarism
Two different patterns both show up as "match with another student's paper," and Turnitin's documentation distinguishes them. The official blog (Effective solutions) uses Celine:
"Celine has submitted the same paper to multiple classes... She's checked her Similarity Report, which shows a match against another student in her institution... This is an example of self-plagiarism."
The instructor-side interpretation guide adds the reverse case:
"A student may have used Turnitin to submit drafts of the same paper, meaning their final draft has resulted in a score of 100%... the instructor can rectify this issue by excluding the student's previous submissions from the Similarity Report."
And the collusion blog notes the boundary:
"Turnitin automatically excludes papers submitted by the same student to the same assignment, preventing previous drafts from skewing similarity scores."
So: same student, same assignment → drafts excluded automatically; same student, different classes → self-plagiarism shows up as a match; different students, same work → collusion scenario. If your match is against a paper you yourself submitted elsewhere, that is not the collusion check — it is self-plagiarism, which requires a different conversation (and a different fix: the instructor can exclude your previous submission).
What a classmate match does and doesn't mean
Turnitin's blog is careful about what a match proves:
"A high similarity score does not always suggest that a piece of writing has been plagiarized, just as a low similarity score does not always indicate that no plagiarism or academic dishonesty, in the form of contract cheating, has occurred."
And its collusion-checking post, quoting Nicholls and Lewis (2017), warns against jumping to conclusions on circumstantial signals:
"Sometimes roommates using the same Wi-Fi router take the class together so this isn't necessarily a sign of collusion. However, if two or more students normally log in with different IP addresses, but suddenly are using the same IP address and starting exams at nearly the same time, it suggests [unauthorized] collaboration."
A shared Wi-Fi or similar phrasing is a hint for the investigation, not a verdict. The report flags or excludes matching text; a human (your instructor) decides what it means, and your response should be a conversation backed by your drafts and revision history — the same evidence playbook as our appeal and version-history guides. What you should not do is treat "matched a classmate" as a conviction you cannot respond to, or try to make your submission evade the check — altering your work to dodge the final comparison is the exact behavior the check exists to catch.
When the check runs in your assignment
The help center ties the timing to assignment configuration:
"If your Standard (Classic) Assignment is set to allow resubmissions, all Similarity Reports are automatically regenerated within one hour after the assignment due date and time... If your assignment is configured to generate reports immediately but treats the first submission as final, Turnitin will not automatically regenerate reports on the due date."
In the newer Standard Assignment, the check can also be forced early — the help center describes a Refresh Reports button that cross-compares everything on demand. The practical point for students: whether your report changes after the deadline is determined by how the assignment was configured, not by anything you did. If the score does move, the collusion check is the most likely reason.
Bottom line
Turnitin's collusion detection is a peer-to-peer comparison of submissions to the same assignment, and for most assignments it runs automatically after the due date. That is why the Eric & Jane scenario produces a 100% score for the original author and a 25% for the copier — the numbers follow submission order, not authorship. A score that jumps after the deadline is usually the final cross-check regenerating your report, and a match against a classmate is a starting point for a conversation, not a verdict. If the matched paper is your own from another class, that is self-plagiarism, which your instructor can exclude. And if the repository was disabled, the check cannot run at all. When a match appears, respond with evidence and a discussion — not with evasion.
Originally published at HumanPen: https://humanpen.net/blog/turnitin-collusion-check-explained?utm_source=devto&utm_medium=article&utm_campaign=syndication-2026-09
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