Distribution of BH spin parameters as a function of the BH mass in Box4 (circles), at z = 0. The points are colour-coded according to the number of mergers BH have undergone. The squares and pentagons show the collection of observational measurements of the BH spin parameter by Reynolds (2021; with updates from Bambi et al. 2021) and Mallick et al. (2022), respectively. The hexagon represents the spin estimate reported by Walton et al. (2021) and the diamond represents the measurement obtained by Sisk-Reyn6s et al. (2022).