Across a variety of situations, people strongly condemn plagiarizers who steal credit for ideas, even when the theft in question does not appear to harm anyone. Why would people react negatively to relatively harmless acts of plagiarism? In six experiments, we predict and find that these negative reactions are driven by people's aversion toward agents who attempt to falsely improve their reputations. In Studies 1-3, participants condemn plagiarism cases that they agree are harmless (i.e., stealing credit from an anonymous source). This effect is mediated by the extent to which participants perceive the plagiarizer to have falsely benefitted from plagiarizing. In Studies 4-5, we demonstrate that this effect is not explained solely by participants' negative response to lies or violations of permission. In Study 6, participants condemn a plagiarism case in which the idea's original author actually "benefits", providing the strongest evidence that people condemn plagiarism for reasons beyond perceived harm. We discuss how this work connects to broader questions of intellectual property and impression management.