  We provide a physical interpretation and explanation of the
morphology-density relation for galaxies, drawing on stellar masses, star
formation rates, axis ratios and group halo masses from the Sloan Digital Sky
Survey. We first re-cast the classical morphology-density relation in more
quantitative terms, using low star formation rate (quiescence) as a proxy for
early-type morphology and dark matter halo mass from a group catalog as a proxy
for environmental density: for galaxies of a given stellar mass the quiescent
fraction is found to increase with increasing dark matter halo mass. Our novel
result is that - at a given stellar mass - quiescent galaxies are significantly
flatter in dense environments, implying a higher fraction of disk galaxies.
Supposing that the denser environments differ simply by a higher incidence of
quiescent disk galaxies that are structurally similar to star-forming disk
galaxies of similar mass, explains simultaneously and quantitatively these
quiescence -nvironment and shape-environment relations. Our findings add
considerable weight to the slow removal of gas as the main physical driver of
the morphology-density relation, at the expense of other explanations.
