  (Abridged) We present a joint gravitational lensing and stellar dynamical
analysis of fifteen massive field early-type galaxies, selected from the Sloan
Lens (SLACS) Survey. The following numerical results are found: (i) A
joint-likelihood gives an average logarithmic density slope for the total mass
density of 2.01 (+0.02/-0.03) (68 perecnt C.L). inside the Einstein radius.
(ii) The average position-angle difference between the light distribution and
the total mass distribution is found to be 0+-3 degrees, setting an upper limit
of <= 0.035 on the average external shear. (iii) The average projected
dark-matter mass fraction is inferred to be 0.25+-0.06 inside R_E, using the
stellar mass-to-light ratios derived from the Fundamental Plane as priors. (iv)
Combined with results from the LSD Survey, we find no significant evolution of
the total density slope inside one effective radius: a linear fit gives
d\gamma'/dz = 0.23+-0.16 (1-sigma) for the range z=0.08-1.01. The small scatter
and absence of significant evolution in the inner density slopes suggest a
collisional scenario where gas and dark matter strongly couple during galaxy
formation, leading to a total mass distribution that rapidly converge to
dynamical isothermality.
