  We introduce a self-similar mass model for early-type galaxies, and constrain
it using the aperture mass-radius relations determined from the geometries of
22 gravitational lenses. The model consists of two components: a concentrated
component which traces the light distribution, and a more extended power-law
component (rho propto r^-n) which represents the dark matter. We find that lens
galaxies have total mass profiles which are nearly isothermal, or slightly
steeper, on the several-kiloparsec radial scale spanned by the lensed images.
In the limit of a single-component, power-law radial profile, the model implies
n=2.07+/-0.13, consistent with isothermal (n=2). Models in which mass traces
light are excluded at >99 percent confidence. An n=1 cusp (such as the
Navarro-Frenk-White profile) requires a projected dark matter mass fraction of
f_cdm = 0.22+/-0.10 inside 2 effective radii. These are the best statistical
constraints yet obtained on the mass profiles of lenses, and provide clear
evidence for a small but non-zero dark matter mass fraction in the inner
regions of early-type galaxies. In addition, we derive the first strong lensing
constraint on the relation between stellar mass-to-light ratio (Upsilon) and
galaxy luminosity (L): Upsilon propto L^[0.14 (+0.16)(-0.12)], which is
consistent with the relation suggested by the fundamental plane. Finally, we
apply our self-similar mass models to current problems regarding the
interpretation of time delays and flux ratio anomalies in gravitational lens
systems.
