The h-index [1] has been claimed to provide a simple way to compare objectively the scientific achievement of researchers and has rapidly become one of the most favoured measures of scientific output [2]. The h-index is an author's number of articles (h) that have received at least h citations [1], and thus depends on the number of a researcher's publications and their impact. Some recent articles have called for cautious use of the h-index [3–7]. In particular, its robustness against self-citations has been disputed [3–5].