ELIZA was created by Joseph Weizenbaum at MIT and first described in a January 1966 paper in the Communications of the ACM. Written in MAD-SLIP for the IBM 7094 running on MIT's Compatible Time-Sharing System (CTSS), the program operated by scanning user input for keywords ranked by priority, then applying decomposition rules to break the sentence into parts and reassembly rules to construct a response. Its most well-known script, DOCTOR, simulated a Rogerian psychotherapist—a therapeutic style that conveniently requires little real-world knowledge, since the therapist mostly reflects the patient's statements back as questions. Weizenbaum named the program after Eliza Doolittle from Shaw's Pygmalion, as it could be "taught" to speak increasingly well through improved scripts.

The responses to ELIZA stunned its creator. Users, including Weizenbaum's own secretary, attributed human-like understanding to the program and formed emotional connections with it. His secretary famously asked him to leave the room so she could have a private conversation. Academic psychiatrists, rather than recognizing the program's limitations, seriously proposed deploying it as an automated therapist that could treat multiple patients simultaneously. Weizenbaum later wrote: "I had not realized…that extremely short exposures to a relatively simple computer program could induce powerful delusional thinking in quite normal people." The tendency to read comprehension into computed output became known as the "ELIZA effect"—a phenomenon that remains deeply relevant in the age of large language models.

Disturbed by these reactions, Weizenbaum wrote Computer Power and Human Reason: From Judgment to Calculation (1976), arguing that there are tasks computers ought not to be given regardless of whether they can perform them, and that ELIZA's superficial pattern manipulation was proof that the appearance of understanding is not understanding itself. The original MAD-SLIP source code was long thought lost, but was rediscovered in 2021 in Weizenbaum's papers at MIT by Jeff Shrager and archivist Myles Crowley. The Weizenbaum estate granted permission to release the code under a Creative Commons CC0 public domain license, making the original program freely available for study for the first time in over half a century.

Text adapted from ELIZA on Wikipedia, licensed under CC BY-SA 4.0.