Black hole mass and spin coevolution by mergers
  Massive black holes appear to be present in the nuclei of almost all galaxies, but their genesis and evolution are not well understood. As astrophysical black holes are completely characterized by their masses and spins, the observed joint distribution of these quantities contains important clues to their history. We examine the coevolution of mass and spin in binary merger growth scenarios. We find that holes are typically spun down by mergers. Rapid rotation results only if the binary's larger member already spins quickly and the merger with the smaller hole is consistently near prograde; or, if the binary's mass ratio approaches one. If, as some observations have suggested, observed black holes spin rapidly, then this limits the importance of merger scenarios for the growth of black holes. 
Scott A. Hughes and Roger D. Blandford