  We investigate the growth of tidal instabilities in accretion discs in a
binary star potential, using three dimensional numerical simulations. As
expected from analytic work, the disc is prone to an eccentric instability
provided that it is large enough to extend to the 3:1 resonance. The eccentric
disc leads to positive superhumps in the light curve. It has been proposed that
negative superhumps might arise from a tilted disc, but we find no evidence
that the companion gravitational tilt instability can grow fast enough in a
fluid disc to create a measurable inclination. The origin of negative
superhumps in the light curves of cataclysmic variables remains a puzzle.
