Effective spin parameter, χ_eff, distribution of field BBHs as a function of redshift, z. Left: modeled intrinsic (underlying) population of field merging BBHs. Center: modeled detectable LIGO–Virgo BBH population assuming simulated O3 detector sensitivity selection effects. Right: modeled Einstein Telescope detectable BBH population assuming a forecast detector sensitivity as in Hild et al. (2011). In all cases, the fraction of non-spinning BBHs decreases as a function of redshift, shifting the χ_eff distribution to larger χ_eff values.