  The influence of structural asymmetries (barrier height and exchange
splitting), as well as inelastic scattering (magnons and phonons) on the bias
dependence of the spin transfer torque in a magnetic tunnel junction is studied
theoretically using the free electron model. We show that they modify the
"conventional" bias dependence of the spin transfer torque, together with the
bias dependence of the conductance. In particular, both structural asymmetries
and bulk (inelastic) scattering add {\em antisymmetric} terms to the
perpendicular torque ($\propto V$ and $\propto j_e|V|$), while the interfacial
inelastic scattering conserves the junction symmetry and only produces {\em
symmetric} terms ($\propto |V|^n$, $n\in\mathbb{N}$). The analysis of spin
torque and conductance measurements displays a signature revealing the origin
(asymmetry or inelastic scattering) of the discrepancy.
