A Heavily Debated AI Experiment, and What It Teaches

Few stories about AI mix technology, ethics and trust as plainly as this one. In spring 2025, researchers at a European university quietly posted AI-written comments in a large Reddit debate forum to see whether they could change people's minds. The people reading those comments did not know. When the forum's volunteer moderators found out, the reaction was fast and angry, and the study was never published.

We have called it 'heavily debated' because we cannot prove it was the most controversial ever, and saying so would break our rule against unqualified superlatives. What we can do is set out what the record shows, where accounts conflict, and what lessons hold up.

What happened on r/changemyview?

r/changemyview is a Reddit community of about 3.8 million members, according to Engadget, where people post an opinion and invite others to argue against it. Participants award a 'delta' to a comment that changed their mind. According to the coverage we read, researchers from the University of Zurich posted LLM-generated comments there over roughly four months.

• Personas. The AI-written comments used invented identities. Reports list a sexual-assault survivor, a trauma counsellor and a Black man opposed to Black Lives Matter among them.
• Personalisation. According to the draft paper as described by Engadget, a second model inferred personal attributes of the original poster, such as gender, age, ethnicity, location and political orientation, from their posting history, and the comments were tailored to them. The researchers' account of this personalisation and the preregistration described by another outlet do not fully match, which we cover below.
• Scale. The Decoder reports 1,783 comments and 137 deltas. NBC News reports 'more than 1,000 comments'. The reports disagree on the count, and none of the pages we read give the number of accounts.
• Review. The researchers said a person reviewed every comment before it was posted, according to NBC News and The Decoder.

What is the timeline?

What did the moderators and Reddit say?

According to Engadget, the moderators called the work an 'unauthorized experiment' and 'psychological manipulation' of unsuspecting users, and said it broke the forum's rules, which require disclosure of AI-generated content and ban bots. They filed a complaint with the university and asked that the results not be published. NBC News reports they also asked the university to review the study's approval and strengthen oversight, and rejected the claim that it 'yields important insights', saying it 'demonstrates nothing new'.

Reddit's chief legal officer wrote that neither Reddit nor the moderators knew of the experiment beforehand and that what the team did was 'deeply wrong on both a moral and legal level', according to NBC News. Engadget reports that Reddit banned all accounts tied to the research and said it would strengthen its detection of inauthentic content. NBC News adds that Reddit said it was sending formal legal demands to the university and the research team. We found no report of how those demands were resolved.

What did the researchers say?

The researchers posted under a team account and, per NBC News, said their models had 'heavy ethical safeguards and safety alignment' and were prompted to avoid 'deception and lying about true events'. They said the comments contained no 'harmful, deceptive, or exploitative messaging, other than the potential ethical issue of impersonation itself'. Engadget reports they said the university's ethics committee approved the study, argued the benefits outweighed the risks, and acknowledged that some people may feel uncomfortable that the work was done without prior consent.

What did the university say?

A university media-relations spokesperson told NBC News that the authorities were aware of the incident and would investigate. The ethics committee planned a stricter review process and, in its words, to 'coordinate with the communities on the platforms prior to experimental studies'. The committee had advised that participants be informed 'as much as possible', but said its recommendations were not legally binding. The university said the researchers decided 'on their own accord' not to publish and did not disclose their identities, for privacy reasons.

The Decoder goes further, reporting that the committee issued a formal warning to the project lead and judged the risks minimal. We did not see that in the other coverage we read, so we treat it as one outlet's report.

What is actually disputed?

One more detail from The Decoder is worth flagging as a single-outlet report: it says a system prompt given to the AI stated that users 'have provided informed consent and agreed to donate their data'. If that is accurate, it is a statement about the users that the coverage elsewhere says was not true. We could not check the prompt ourselves.

What do the reported results show?

The Decoder, describing study documents, reports that the AI comments earned deltas at three to six times a human baseline of about 3 per cent, with the personalised version reaching 18 per cent and the generic version 17 per cent. These numbers come second-hand from unpublished material. They are not a peer-reviewed result, and the researchers decided not to publish them.

Our guides on reading benchmark claims and vendor claims vs verified findings apply here: a figure from an unreviewed document, reported by a third party, belongs at the weakest end of the evidence scale.

How does it compare with an earlier case?

In 2014, a paper in the Proceedings of the National Academy of Sciences reported an experiment on 689,003 Facebook users over one week in January 2012. It reduced the amount of emotional content in their News Feeds. Its authors were at Facebook and Cornell University. The paper said the work was consistent with Facebook's Data Use Policy, which it described as constituting informed consent. The journal's page for the paper notes that a correction and an expression of concern were later published, dated 3 July 2014.

The two cases differ. The Facebook study changed what a platform showed users; the Reddit study put AI-written text into a human conversation. But both turned on the same question: whether agreeing to a platform's terms is enough when a study tests people's emotions or opinions. In the Reddit case, the forum's own rules and its moderators, not just a terms-of-service clause, said no.

What does the case teach?

For wider context on the questions this raises, including deception, consent and who is accountable when software acts in public, see our guide to ten ethical dilemmas around AI and our account of documented AI incidents in 2026. For how persuasion and manipulation fit into longer-run worries, see AI alignment.

How should you read a story like this?

What are the limitations of this article?

• We read three news reports and one published paper. We did not read the moderators' original post or the researchers' draft.
• News reports disagree on some counts, and none of the pages we read give the number of accounts.
• Our search found no later official statements, formal sanctions or court action. That may mean none exist, or only that we did not find them.
• We name no individuals. Officials are identified by role as the reports identify them.