  Binary black holes (BBHs) detected by gravitational wave (GW) observations
could be broadly divided into two formation channels: those formed through
field binary evolution and those assembled dynamically in dense stellar
systems. Each of these formation channels, and their sub-channels, populate a
distinct region in the effective spin-mass ($\chi_{\rm eff}-M$) plane.
Depending on the branching ratio of different channels, an ensemble of BBHs
could show a trend in this plane. Here we fit a mass-dependent distribution for
$\chi_{\rm eff}$ to the GWTC-1 BBHs from the first and second observing runs of
Advanced LIGO and Advanced Virgo. We find a negative correlation between mass
and the mean effective spin ($\bar{\chi}_{\mathrm{eff}}$), and positive
correlation with its dispersion ($\sigma_{\chi_\mathrm{eff}}$) at 75\% and 80\%
confidence. This trend is robust against the choice of mass variable, but most
pronounced when the mass variable is taken to be the chirp mass of the binary.
The result is consistent with significant contributions from both dynamically
assembled and field binaries in the GWTC-1 catalog. The upcoming LIGO O3a data
release will critically test this interpretation.
