Why it matters when you are writing
People find this counterintuitive because you cannot steal from yourself. The harm is elsewhere. A reader assumes a paper reports new work; a marker assumes an essay represents effort expended for that course; a journal assumes it is buying something it can publish exclusively. Undisclosed reuse quietly misleads all three, and in the published record it also inflates citation counts and can make one finding look like two independent ones.
Some repetition is unavoidable and entirely legitimate. A methods section describing the same instrument in a follow-up study can only be written so many ways. Editors generally accept this when you say so — a sentence noting that the procedure was reported previously, with a citation, resolves it. Extended verbatim reuse of discussion or introduction sections is treated more strictly, because those are the parts that carry the novel argument.
Two situations come up constantly in doctoral work. Thesis-by-publication chapters that reappear as journal articles are usually fine, but the journal has to be told and the thesis has to be cited. Posting a preprint and then submitting the same manuscript to a journal is standard in most fields and not self-plagiarism at all — though a minority of journals still have policies about it, so check before you post.
