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tokens: 15 Myr. This excess in BH number can be partially explained by an increase in binary interaction rates in these models. Specifically, model 2a has a higher number of binary mergers involving stars with zero-age main sequence (ZAMS) masses between 10 and 25 M ⊙ that would not have otherwise been BH progenitors (at 0.1 Z ⊙ , the minimum ZAMS mass that will form a BH is roughly 20 M ⊙ ). However, these merger products do evolve into BHs with masses roughly in the range of M BH ≈ 5–15 M ⊙ . For models 2e and 2k , the higher number of BHs can be partially attributed to mass transfer in a binary increasing the mass of one of the components past the threshold of BH formation. Naturally, some of these differences are clearly due to stochastic fluctuations in the initial evolution of the model, leading to divergent paths in the dynamical evolution of the system. The main takeaway is that binary evolution processes, coupled with dynamics, can greatly influence BH growth. We leave a detailed study of the extent to which binary evolution processes affect the long-term cluster evolution and dynamics for a future study. It is important to note that many (roughly 55%) of these “excess” BHs formed with masses less than 15 M ⊙ are ejected promptly from their host cluster through natal kicks. Low-mass BHs are expected to form with less mass fallback and thus receive larger natal kicks (e.g., Fryer et al. 2012 ). Thus, this excess of BHs formed is unlikely to have a significant effect on the long-term cluster dynamics. 4. Discussion and Conclusions Our results show that high binary fractions for massive stars lead to an increase in binary-mediated dynamical interactions, which in turn have an important effect on massive BH formation. We have demonstrated that increasing the high-mass binary fraction, consistent with observations, while keeping all other cluster parameters (e.g., cluster masses, virial radii, metallicity) fixed at the values used in Kremer et al. ( 2020b ), may significantly change the early evolution of YMCs. In particular, high-mass binaries facilitate high rates of massive star collisions (occurring through both dynamical encounters and binary-evolution-driven mergers) that can lead to the formation of massive BHs, both within and above the pair-instability mass gap. With the exception of the high-mass binary fraction, the parameters for the models calculated in this Letter are identical to those in Kremer et al. ( 2020b ), which were shown to lead to present-day properties matching well those of Milky Way GCs. If indeed the majority of high-mass stars have binary companions at birth, massive BH formation may therefore be a common occurrence in the early phases of GC evolution. As shown in Figure 2 , the formation of massive BHs is sensitive to small changes in initial virial radius. This is in agreement with previous works on the topic of IMBH formation that have shown that the onset of a collisional runaway occurs abruptly under small