  We investigate the influence of the star formation rate on the Tully-Fisher
relation. We find that a simple model which combines the empirically-determined
star-formation rate with the expected properties of galaxy halos provides a
remarkably good fit to the absolute magnitude-rotation speed correlation. We
find that the power-law nature, its slope, normalisation and scatter, are all
readily accounted for if the Universe has a low density parameter, with or
without a cosmological constant and disks are assembled at $z \sim 1 - 1.5$.
Moreover, this agreement is found simultaneously in 4 wavebands. An Einstein-de
Sitter Universe produces disks which are too faint unless the disks are
assembled at $z\sim 0.5$. The scatter in the relation is due to a combination
of the expected range of spin parameters of the halos and the range of
formation redshifts. The source of the scatter opens up possibilities of a
better galaxy distance indicator, if spectroscopic observations of globular
clusters can be used to determine the halo rotation.
