Academics often assume that they are free to reuse their own work - but this is an "act of scholarly misconduct", according to a recent report published by a US plagiarism-detection software company.
The iThenticate "white paper", titled The Ethics of Self-Plagiarism, says that "traditional definitions of plagiarism do not account for self-plagiarism".
The paper quotes Miguel Roig, professor of psychology at St John's College of Liberal Arts and Sciences in New York, who argues that self-plagiarism occurs "when authors reuse their own previously written work or data in a 'new' written product without letting the reader know that this material has appeared elsewhere".
And it is not simply a matter of ethics and trust, the report says, pointing to issues of copyright.
"Authors can quote from portions of other works with proper citations, but large portions of text, even quoted and cited, can infringe on copyright and would not fall under copyright exceptions or 'fair use' guidelines," it says.
Professor Roig has investigated plagiarism in depth and written guidelines for the US Office of Research Integrity.
Speaking to Times Higher Education, he said: "When someone reads a work, he or she is under the assumption that it is original and new. Recycling involves an element of deception."
Although he admits he has no firm data, he believes the problem of plagiarism in general is increasing because it is so easy to copy and paste material electronically.
"Good writing takes effort and cognitive resources. If you lack good command of the language, it's more enticing to use existing text, especially if it is well written. But (in addition), a lot of authors don't believe there's a problem reusing their own work," he noted.
One problem is the lack of guidance on the issue of self-plagiarism. "There needs to be a consensus on the inappropriateness of this practice," Professor Roig said.
He argued that the same standards should apply to reusing material in visual presentations.
"Are they brand new or have they been rehashed several times?" he said. While reusing material was not wrong in itself, he added, "we should be honest about this with our audience".
Most scholarly journals publish guidelines for authors and editors, with rules covering all types of plagiarism, and emphasising the need for transparency about origin.
The UK Committee on Publication Ethics states that best practice for editors is to be "alert for redundant (ie, duplicate) publication" and many publishers use plagiarism-checking software that can pick up self-plagiarism.
But according to Audrey McCulloch, UK executive director of the Association of Learned and Professional Society Publishers, "this isn't as black and white as it sounds. For example, there are only so many ways you can write methods sections."