The value of the parameter &ngr; from low pressure laboratory measurements disagrees with the value from seismic observations. The parameter &ngr; relates the isobaric change in shear velocity Vs to the change in compressional velocity Vp. Seismic evidence indicates &ngr; exceeeds 2.0 in the lower mantle, whereas data on a variety of minerals at high temperature T and ambient pressure P result in lower values. We reconcile these differences. Our ab initio model calculations on MgO show that &ngr; increases with P and is 2.0--2.5 at lower mantle pressures. There is no need to assume partial melting to explain the seismic data. These calculations also provide insight into the P and T dependence of the dimensionless parameter &Ggr;≡-(1/&agr;G)(∂G/∂T)P, where G is the isotropic shear modulus and &agr; is the volume thermal expansivity. Using measured values of thermoelastic parameters coupled with thermodynamic identities, we seek constraints on ΔS≡-(1/&agr;KS) (∂KS/∂T)P, where KS is the adiabatic bulk modulus, and confirm that P causes ΔS to decrease. We find Poisson's ratio increases with P and T. Altogether these results show that for MgO, &ngr; increases from 1.3 at ambient conditions to over 2 at lower mantle conditions. We expect other mantle minerals to behave similarly. Therefore we find that reconcilication of the mineral physics approach with that of seismic tomography concerning &ngr; does not require special assumptions about the state of the lower mantle.