The “onion problem” in PhD students’ writing: this means adding more and more stuff to your papers. By “stuff”, I mean concepts, theories, methods—-basically, more words on paper, more points to criticize. Dr. Ilkka said it well: the more you write, the more you expose possible weak points to the reviewers. Every sentence becomes under scrutiny, so the more sentences you have, the more room there is to criticize.
But yeah, this is like a psychological default mechanism: when a PhD student sees a comment, their reaction often is, “Let me add something.” Sometimes it’s alright to add, BUT it often means that you have to also take away something.
Another reason why using multiple concepts, theories, etc. is problematic is that all those theories and concepts become dependencies. Those with programming experience know that dependencies should be avoided where we can—–they are potential failure points in the code, because when the reference library changes or is deprecated, the program can stop working. Similarly, when you change a concept in one place of your manuscript, you have to change it in ALL the other places, too. “Parsimony” is a good principle, basically means conceptual simplicity: you can deal with ~3 core concepts and just keep hammering them till the reader knows them inside out.
Why I call this the “onion problem” is that stacking up the paper with more concepts makes it like an onion: the reader peels and peels, but never finds the core. Don’t hide your core under infinite layers: keep it visible at all times. It is funny how less is more, but yeah—-train your mind to go deeper into the concepts/theories you choose, not broad with adding more and more new concepts.