In Europe, typically postdocs are hired to work under a professor's research group. This appears to be totally different in the US and some other countries (e.g. Israel), where postdocs are hired by committees. Suppose a hiring committee has 7 members. Are postdoctoral applicants expected to make the case that they are going to work closely and share research interests with at least one of the 7 professors? Let me give a hypothetical scenario from math. Let's say that some candidate for a postdoc just got a paper accepted to the Annals, the topic being Moufang polygons. Nobody on the committee (which consists of a number theorist, a representation theorist, an algebraic geometer, a logician, an analyst, and 2 applied mathematicians) really cares about Moufang polygons. Will the prevailing opinion be, "This is a very strong applicant working in an area that nobody in this department has worked on. Let's pick him/her." Or will they think, "Gee, this person won't interact with the faculty at all since his/her research interests don't overlap." I'm trying to clarify the exact role of a postdoc in the U.S.