  We present an analysis of the evolution of the central mass-density profile
of massive elliptical galaxies from the SLACS and BELLS strong gravitational
lens samples over the redshift interval z ~ 0.1-0.6, based on the combination
of strong-lensing aperture mass and stellar velocity-dispersion constraints. We
find a significant trend towards steeper mass profiles (parameterized by the
power-law density model with rho ~ r^[-gamma]) at later cosmic times, with
magnitude d<gamma>/dz = -0.60 +/- 0.15. We show that the combined lens-galaxy
sample is consistent with a non-evolving distribution of stellar velocity
dispersions. Considering possible additional dependence of <gamma> on
lens-galaxy stellar mass, effective radius, and Sersic index, we find marginal
evidence for shallower mass profiles at higher masses and larger sizes, but
with a significance that is sub-dominant to the redshift dependence. Using the
results of published Monte Carlo simulations of spectroscopic lens surveys, we
verify that our mass-profile evolution result cannot be explained by lensing
selection biases as a function of redshift. Interpreted as a true evolutionary
signal, our result suggests that major dry mergers involving off-axis
trajectories play a significant role in the evolution of the average
mass-density structure of massive early-type galaxies over the past 6 Gyr. We
also consider an alternative non-evolutionary hypothesis based on variations in
the strong-lensing measurement aperture with redshift, which would imply the
detection of an "inflection zone" marking the transition between the
baryon-dominated and dark-matter halo-dominated regions of the lens galaxies.
Further observations of the combined SLACS+BELLS sample can constrain this
picture more precisely, and enable a more detailed investigation of the
multivariate dependences of galaxy mass structure across cosmic time.
