  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.
