Conflict of interests and the assignment of referee for a very narrow field Consider a very small scientific field with three major contributors and a total of 15 authors, all of them had coauthored with everyone else. 

In this field how could the editor choose referee for new papers, if he must adhere to the conflict of interests policies?

I know that some editors handle coauthor/students' paper or assign the paper to the previous coauthor of the author. Is that a common practice for a small field?



The small field was naturally extended from some important results back to 50s -70s, some of those researchers are still alive, but inactive. The importance cannot be doubt by the larger field, also because of those important results historically.
Even if the field is small, one can usually find referees in cognate fields with no conflicts: presumably papers by these groups have a bibliography that extends beyond these groups, else the interest is overly narrow and the paper can be rejected as such.

(Aside: the handling editor is not necessarily a “he”.)