There are few observed high-mass X-ray binaries (HMXBs) that harbor massive
    black holes, and none are likely to result in a binary black hole (BBH) that
    merges within a Hubble time; however, we know that massive merging BBHs exist
    from gravitational-wave observations. We investigate the role that X-ray and
    gravitational-wave observational selection effects play in determining the
    properties of their respective detected binary populations. We confirm that, as
    a result of selection effects, observable HMXBs and observable BBHs form at
    different redshifts and metallicities, with observable HMXBs forming at much
    lower redshifts and higher metallicities than observable BBHs. We also find
    disparities in the mass distributions of these populations, with observable
    merging BBH progenitors pulling to higher component masses relative to the full
    observable HMXB population. Fewer than $3\%$ of observable HMXBs host black
    holes $> 35M_{\odot}$ in our simulated populations. Furthermore, we find the
    probability that a detectable HMXB will merge as a BBH system within a Hubble
    time is $\simeq 0.6\%$. Thus, it is unsurprising that no currently observed
    HMXB is predicted to form a merging BBH with high probability.

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