  The distribution of effective spin $\chi_{\rm eff}$, a parameter that encodes
the degree of spin-orbit alignment in a binary system, has been widely regarded
as a robust discriminator between the isolated and dynamical formation pathways
for merging binary black holes. Until the recent release of the GWTC-2 catalog,
such tests have yielded inconclusive results due to the small number of events
with measurable nonzero spins. In this work, we study the $\chi_{\rm eff}$
distribution of the binary black holes detected in the LIGO-Virgo O1-O3a
observing runs. Our focus is on the degree to which the $\chi_{\rm eff}$
distribution is symmetric about $\chi_{\rm eff} = 0$ and whether the data
provides support for a population of negative-$\chi_{\rm eff}$ systems. We find
that the $\chi_{\rm eff}$ distribution is asymmetric at 95% credibility, with
an excess of aligned-spin binary systems ($\chi_{\rm eff}>0$) over anti-aligned
ones. Moreover, we find that there is no evidence for negative-$\chi_{\rm eff}$
systems in the current population of binary black holes. Thus, based solely on
the $\chi_{\rm eff}$ distribution, dynamical formation is disfavored as being
responsible for the entirety of the observed merging binary black holes, while
isolated formation remains viable. We also study the mass distribution of the
current binary black hole population, confirming that a single truncated power
law distribution in the primary source-frame mass, $m_1^{\rm src}$, fails to
describe the observations. Instead, we find that the preferred models have a
steep feature at $m_1^{\rm src} \sim 40 \,\rm M_\odot$ consistent with a step
and an extended, shallow tail to high masses.
