  Under the assumption that jets in active galactic nuclei are powered by
accretion and the spin of the central supermassive black hole, we are able to
reproduce the radio luminosity functions of high- and low-excitation galaxies.
High-excitation galaxies are explained as high-accretion rate but very low spin
objects, while low-excitation galaxies have low accretion rates and bimodal
spin distributions, with approximately half of the population having maximal
spins. At higher redshifts (z~1), the prevalence of high accretion rate objects
means the typical spin was lower, while in the present day Universe is
dominated by low accretion rate objects, with bimodal spin distributions.
