We examine the relationship between individual black hole ( BH ) masses in merging binary black hole ( BBH ) systems .
Analyzing the ten BBH detections from LIGO/Virgo ’ s first two observing runs , we find that the masses of the component BHs comprising each binary are unlikely to be randomly drawn from the same underlying distribution .
Instead , the two BHs of a given binary prefer to be of comparable mass .
We show that it is \sim 5 times more likely that the component BHs in a given binary are always equal ( to within 5 % ) than that they are randomly paired .
If we assume that the probability of a merger between two BHs scales with the mass ratio q as q ^ { \beta } , so that \beta = 0 corresponds to random pairings , we find \beta > 0 is favored at credibility 0.987 .
By modeling the mass distribution , we find that the median mass ratio is q _ { 50 \% } = 0.91 ^ { +0.05 } _ { -0.17 } at 90 % credibility . \added While the pairing between BHs depends on their mass ratio , we find no evidence that it depends on the total mass of the system : it is \sim 6 times more likely that the pairing depends purely on the mass ratio than on the total mass .
We predict that 99 % of BBHs detected by LIGO/Virgo will have mass ratios q > 0.5 .
We conclude that merging black holes do not form random pairings ; instead they are selective about their partners , preferring to mate with black holes of a similar mass .
The details of these selective pairings provide insight into the underlying formation channels of merging binaries .