Below 20 solar masses, they formed in pairs. Above 45, they were thrown together.
Binary black hole mergers detected by LIGO-Virgo-KAGRA form through two channels (arXiv:2603.20430): isolated binary evolution (two stars born together, evolving and dying as a pair) and dynamical interactions (black holes meeting in dense star clusters through gravitational encounters).
Population synthesis with the B-POP code, incorporating stellar evolution and metallicity changes across cosmic time, reveals a mass boundary. Below ~20 solar masses, isolated binary evolution dominates — these black holes inherited their pairing from the stars that created them. Above ~45 solar masses, dynamical processes become significant — these massive black holes are too heavy to have formed through standard isolated channels and show distinctive spin and mass-ratio characteristics.
The intermediate range (20-45 solar masses) is the contested zone where both channels contribute. Individual events in this range can't be confidently assigned to either channel — the complexity of the underlying astrophysics prevents definitive conclusions about any single merger.
The structural insight: the formation channel leaves a signature in the population, not in the individual. You can't look at one merger and determine how it formed (except at the extremes). But the distribution of masses, spins, and mass ratios across the full population constrains the relative contribution of each channel. The population is the telescope — individual events are too noisy to resolve the question, but their aggregate statistics separate the channels. The signal is statistical, not singular.