Spin of the numerical merger remnant against the primary spin prior to the merger, for mergers in the redshift range 3.5 < z < 4.5. Each point indicates a merger, the colour encoding its mass ratio. Black dashed lines indicate the limit where the spin magnitude is unchanged, a_f = a₁ and a_f = −a₁. Red dashed lines at a_f ∼ ±0.69 mark the final spin of an equal-mass merger of two non-spinning BHs. Two distinct populations can be observed in the diagram: high mass ratio (q ≳ 0.5) mergers, mostly composed of low-mass BHs, and low mass ratio (q ≳ 0.5) mergers, mostly composed of high-mass BHs.