I'm an American professor in a branch of theoretical science at a US institution. There is a Japanese postdoc in our group who has nearly finished writing a paper with one of the American postdocs. We're having a bit of a conflict over authorship: the Japanese postdoc wants my name to be on the publication, and I don't think it's appropriate. I chatted about the project with them while it was in progress and made a few suggestions, but fundamentally all the ideas and computations originated with the two postdocs. I think that they should take full credit for the work and I that I should take none, and this seems consistent with US authorship norms to me. However, the postdoc from Japan seems very unhappy that I don't want to sign my name on the paper. He said that if I don't, then he won't feel like he can discuss his future work-in-progress with me because he will think I am disavowing the work or that I have no time for him. I've insisted that I'm happy to discuss his work, I just think that he deserves his share of credit for it and that if my name is attached it might be perceived by others as more my work than his.
I've never encountered this situation before. At times I've been in the opposite situation, when I was a postdoc, feeling that professors were claiming undue credit for my work, so actively wanting a professor who did none of the actual work to claim authorship is difficult for me to understand. But I assume this is a difference in cultural norms; I know professors from certain countries in our field who have such a large number of publications that it's clear they're claiming authorship over this sort of project that they had very little involvement in. (I don't necessarily mean that as a criticism of them; I think that within their local academic culture, this is expected and ethical behavior.)
How should I negotiate this cultural clash?
(Also, to forestall one possible set of comments, I'm in a field where alphabetical author order is conventional, so that isn't an issue here.)