I am planning to write several papers exploring various aspects of the same scientific question. Each of these papers must have an introduction which motivates it and explains the relationship between the problem and what others have studied in the past.
It would be fantastic if I could simply copy-and-paste the same introduction, or, at least, 90% of it. This seems to me to be ethically unproblematic. After all, I need to say the exact same things every time, and I certainly don't mind the self-plagiarism. Am I hurting the reader in any way? I suppose I might be, if the reader desired an introduction which consists of original material, but that is an odd desire, isn't it? Its the research, the stuff that follows the intro, that is original.
Sadly, I have gotten wind that the majority of the research community apparently does not agree with the sentiments expressed in the previous paragraph. This leads me here to ask a series of related questions:
To what extent is self-plagiarism in non-technical bits considered acceptable? I often see authors recycle paragraphs but I have never seen anyone cut-and-paste the entire section outright.
I'd be particularly interested in learning whether norms on this vary across different scientific communities.
How often do scholars find themselves trying to same the same thing in different words to avoid self-plagiarism?
Supposing I insert a sentence to the effect of: "The introductory section 1.2 is taken verbatim from the author's earlier paper ." How likely are journal editors and reviewers to complain about this?
By the way, I am fairly certain they would be very likely to complain about a sentence to the effect of "We refer the reader to  for motivation to study this problem and a discussion of its relation to prior work."
iThenticate has a white paper called The Ethics of Self Plagiarism. Within it are reasons not so self-plagiarise, namely copyright issues and the fact that some definitions of plagiarism include copying one's own work (which is contrary to other definitions).
They also go on to give guidelines on how to avoid self-plagiarism, which are helpful:
• Guideline 10: Authors who submit a manuscript for publication containing data, reviews, conclusions, etc., that have already been disseminated in some significant manner (e.g., published as an article in another journal, presented at a conference, posted on the internet) must clearly indicate to the editors and readers the nature of the previous dissemination.
• Guideline 11: Authors of complex studies should heed the advice previously put forth by Angell & Relman (1989). If the results of a single complex study are best presented as a ‘cohesive’ single whole, they should not be partitioned into individual papers. Furthermore, if there is any doubt as to whether a paper submitted for publication represents fragmented data, authors should enclose other papers (published or unpublished) that might be part of the paper under consideration (Kassirer & Angell, 1995). Similarly, old data that has been merely augmented with additional data points and that is subsequently presented as a new study is an equally serious ethical breach.
• Guideline 12: Because some instances of plagiarism, self-plagiarism, and even some writing practices that might otherwise be acceptable (e.g., extensive paraphrasing or quoting of key elements of a book) can constitute copyright infringement, authors are strongly encouraged to become familiar with basic elements of copyright law.
• Guideline 13: While there are some situations where text recycling is an acceptable practice, it may not be so in other situations. Authors are urged to adhere to the spirit of ethical writing and avoid reusing their own previously published text, unless it is done in a manner consistent with standard scholarly conventions (e.g., by using of quotations and proper paraphrasing) (pg. 19-25).