  Gravitational-wave observations give a unique insight into the formation and
evolution of binary black holes. We use gravitational-wave measurements to
address the question of whether GW170729's source, which is (probably) the most
massive binary and the system with the highest effective inspiral spin, could
contain a black hole which is a previous merger remnant. Using the inferred
mass and spin of the system, and the empirically determined population of
binary black holes, we compute the evidence for the binary being
second-generation compared with first-generation. We find moderate evidence (a
Bayes factor of ~6-7) that the mass and spin better match a second-generation
merger, but folding in the expectation that only a small fraction of mergers
are second-generation, we conclude that there is no strong evidence that
GW170729 was the result of a second-generation merger. The results are
sensitive to the assumed mass distribution, and future detections will provide
more robust reconstructions of the binary black hole population.
