One of the major unsolved questions on the understanding of the AGN population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests radio-loud quasars to have higher black hole spin. However, the measurement of black hole spin remains extremely challenging.

We here present results comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet SDSS quasars at 0.3<z<0.8. We find evidence for systematically larger radiative efficiencies by a factor > 1.5 in the radio-loud sample, suggesting that the radio-loud quasar population has on average higher black hole spin that the radio-quiet population. This provides strong observational support for the black hole spin paradigm.