Previous research has shown that if people improve other’s ideas, they subsequently unconsciously plagiarise them at a dramatically higher rate than if they imagine them, or simply hear them again. It has been claimed that this occurs because improvement resembles the process of generation, and that these are confused during retrieval. However, an alternate possibility is tested here: plagiarism may increases because improvement increases personal relevance of the ideas. Two studies were conducted in which there was an initial generation phase, followed by an elaboration phase in which participants imagined the previous ideas, improved them for their own use, or improved them for an older adult’s use. One week later, participants attempted to recall their own ideas, and generated new solutions to the previous problems. In both studies, improvement of doubled the rate of subsequent plagiarism in the recall own task, but this effect was not mediated by whether people improved ideas for their own use, of for use by someone else. Improvement had no effect on plagiarism in the generate-new task. These studies therefore rule out personal relevance, or personal semantics as the source of the improvement effect in unconscious plagiarism.