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With the detection of four candidate binary black hole (BBH) mergers by theAdvanced LIGO detectors thus far, it is becoming possible to constrain theproperties of the BBH merger population in order to better understand theformation of these systems. Black hole (BH) spin orientations are one of thecleanest discriminators of formation history, with BHs in dynamically formedbinaries in dense stellar environments expected to have spins distributedisotropically, in contrast to isolated populations where stellar evolution isexpected to induce BH spins preferentially aligned with the orbital angularmomentum. In this work we propose a simple, model-agnostic approach tocharacterizing the spin properties of LIGO's BBH population. Using measurementsof the effective spin of the binaries, which is LIGO's best constrained spinparameter, we introduce a simple parameter to quantify the fraction of thepopulation that is isotropically distributed, regardless of the spin magnitudedistribution of the population. Once the orientation characteristics of thepopulation have been determined, we show how measurements of effective spin canbe used to directly constrain the underlying BH spin magnitude distribution.Although we find that the majority of the current effective spin measurementsare too small to be informative, with LIGO's four BBH candidates we find aslight preference for an underlying population with aligned spins over one withisotropic spins (with an odds ratio of 1.1). We argue that it will be possibleto distinguish symmetric and anti-symmetric populations at high confidence withtens of additional detections, although mixed populations may takesignificantly more detections to disentangle. We also derive preliminary spinmagnitude distributions for LIGO's black holes, under the assumption of alignedor isotropic populations.

Using spin to understand the formation of LIGO's black holes