  Observations have shown that the majority of massive stars, progenitors of
black holes (BHs), have on average more than one stellar companion. In triple
systems, wide inner binaries can be driven to a merger by the third body due to
long-term secular interactions, most notably by the eccentric Lidov-Kozai
effect. In this study, we explore the properties of BH mergers in triple
systems and compare their population properties to those of binaries produced
in isolation and assembled in dense star clusters. Using the same stellar
physics and identical assumptions for the initial populations of binaries and
triples, we show that stellar triples yield a significantly flatter mass ratio
distribution from $q=1$ down to $q\sim0.3$ than either binary stars or dense
stellar clusters, similar to the population properties inferred from the most
recent catalog of gravitational-wave events, though we do not claim that all
the observed events can be accounted for with triples. While hierarchical
mergers in clusters can also produce asymmetric mass ratios, the unique spins
of such mergers can be used to distinguished them from those produced from
stellar triples. All three channels occupy distinct regions in total mass-mass
ratio space, which may allow them to be disentangled as more BH mergers are
detected by LIGO, Virgo, and KAGRA.
