  We investigate the effect of a disc-driven jet on the accretion growth of
cosmological supermassive black holes (SMBHs). The presence of a jet enhances
the mass growth rate because for a given luminosity, the mass accretion rate,
is higher (or equivalently, the radiative efficiency e_r is lower for a fixed
mass accretion rate) than that predicted by standard accretion disc theory. As
jets carry away very little of the accreting matter, a larger proportion of the
rest mass can reach the black hole during episodes of jet activity. We show
quantitatively that the conditions required to grow a rapidly spinning black
hole to a mass ~ 10^9 solar masses by redshift z ~ 6, whilst satisfying the
observational constraint e_r > 0.1, are considerably less restrictive for
jet-enhanced disc accretion than for standard disc accretion, which requires
implausibly high super-Eddington accretion rates. Furthermore, jet-enhanced
accretion growth offers a viable explanation for the observed correlation
between black hole mass and radio-loudness of quasars.
