  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 have higher black hole spin. However, the
measurement of black hole spin remains extremely challenging. We here aim at
comparing the mean radiative efficiencies of carefully matched samples of
radio-loud and radio-quiet SDSS quasars at 0.3<z<0.8. We use the [OIII]
luminosity as an indirect average tracer of the ionizing continuum in the
extreme-UV regime where differences in the SED due to black hole spin are most
pronounced. We find that the radio-loud sample shows an enhancement in [OIII]
line strength by a factor of at least 1.5 compared to a radio-quiet sample
matched in redshift, black hole mass and optical continuum luminosity or
accretion rate. We argue that this enhancement is caused by differences in the
SED, suggesting higher average bolometric luminosities at fixed accretion rate
in the radio-loud population. This suggests that the radio-loud quasar
population has on average systematically larger radiative efficiencies and
therefore higher black hole spin than the radio-quiet population, providing
observational support for the black hole spin paradigm.
