bibtex-autokey-use-crossref

bibtex-autokey-use-crossref is a variable defined in `bibtex.el'.
Its value is

t

Documentation:
If non-nil use fields from crossreferenced entry if necessary.
If this variable is non-nil and some field has no entry, but a
valid crossref entry, the field from the crossreferenced entry is used.
See `bibtex-generate-autokey' for details.

You can customize this variable.