  Advanced LIGO detectors at Hanford and Livingston made two confirmed and one
marginal detection of binary black holes during their first observing run. The
first event, GW150914, was from the merger of two black holes much heavier that
those whose masses have been estimated so far, indicating a formation scenario
that might differ from "ordinary" stellar evolution. One possibility is that
these heavy black holes resulted from a previous merger. When the progenitors
of a black hole binary merger result from previous mergers, they should (on
average) merge later, be more massive, and have spin magnitudes clustered
around a dimensionless spin ~0.7. Here we ask the following question: can
gravitational-wave observations determine whether merging black holes were born
from the collapse of massive stars ("first generation"), rather than being the
end product of earlier mergers ("second generation")? We construct simple,
observationally motivated populations of black hole binaries, and we use
Bayesian model selection to show that measurements of the masses, luminosity
distance (or redshift), and "effective spin" of black hole binaries can indeed
distinguish between these different formation scenarios.
