  The dramatic relativistic jets pointing directly at us in BL Lac objects can
be well modelled by bulk motion beaming of synchrotron self-Compton emission
powered by a low Eddington fraction accretion flow. Nearly 500 of these AGN are
seen in the 2nd Fermi Large Area Telescope catalogue of AGN. We combine the jet
models which describe individual spectra with the expected jet parameter
scalings with mass and mass accretion rate to predict the expected number of
Fermi detected sources given the number densities of AGN from cosmological
simulations. We select only sources with Eddington scaled mass accretion rate
$< 0.01$ (i.e. radiatively inefficient flows), and include cooling, orientation
effects, and the effects of absorption from pair production on the
extragalactic IR background.
  These models overpredict the number of Fermi detected BL Lacs by a factor of
1000! This clearly shows that one of the underlying assumptions is incorrect,
almost certainly that jets do not scale simply with mass and accretion rate.
The most plausible additional parameter which can affect the region producing
the Fermi emission is black hole spin. We can reproduce the observed numbers of
BL Lacs if such relativistic jets are only produced by the highest spin
($a_*>0.8$) black holes, in agreement with the longstanding spin-jet paradigm.
This also requires that high spins are intrinsically rare, as predicted by the
cosmological simulations for growing black hole mass via chaotic (randomly
aligned) accretion episodes, where only the most massive black holes have high
spin due to black hole-black hole mergers.
