Do note however that many times the first author is not the principal investigator (ie the person that has the funding for the study and is often tenure track faculty) but is often a graduate student or a post-doc. The principal investigator may be in the middle or even at the end of the author list. Thus just excluding first author self-citations is probably inadequate. The chances are high that a grad student or post-doc is going to cite their adviser's previous papers.