  Numerical simulations of turbulent, magnetized, differentially rotating flows
driven by the magnetorotational instability are often used to calculate the
effective values of alpha viscosity that is invoked in analytical models of
accretion discs. In this paper we use various dynamical models of turbulent
magnetohydrodynamic stresses, as well as numerical simulations of shearing
boxes, to show that angular momentum transport in MRI-driven accretion discs
cannot be described by the standard model for shear viscosity. In particular,
we demonstrate that turbulent magnetorotational stresses are not linearly
proportional to the local shear and vanish identically for angular velocity
profiles that increase outwards.
