  Using a sample of more than 6000 quasars from the Sloan digital sky survey
(SDSS) we compare the black-hole mass distributions of radio-loud and
radio-quiet quasars. Based on the virial black-hole mass estimator the
radio-loud quasars (RLQs) are found to harbour systematically more massive
black holes than radio-quiet quasars (RQQs) with very high significance
(>>99.99%), with mean black-hole masses of <\log(M_{bh}/\Msun)>=8.89\pm0.02 and
<\log(M_{bh}/\Msun)>=8.69\pm0.01 for the RLQs and RQQs respectively. Crucially,
the new RLQ and RQQ samples have indistinguishable distributions on the
redshift-optical luminosity plane, excluding the possibility that either
parameter is responsible for the observed black-hole mass difference. Moreover,
this black-hole mass difference is shown to be in good agreement with the
optical luminosity difference observed between RLQ and RQQ host galaxies at low
redshift (i.e. \Delta M_{host}=0.4-0.5 magnitudes). Within the SDSS samples,
black-hole mass is strongly correlated with both radio luminosity and the
radio-loudness $\mathcal{R}$ parameter (>7 sigma significance), although the
range in radio luminosity at a given black-hole mass is several orders of
magnitude. It is therefore clear that the influence of additional physical
parameters or evolution must also be invoked to explain the quasar
radio-loudness dichotomy.
