  Utilizing deep Hubble Space Telescope imaging from the two largest field
galaxy surveys, the Extended Groth Strip (EGS) and the COSMOS survey, we
examine the structural properties, and derive the merger history for 21,902
galaxies with M_*>10^{10} M_0 at z<1.2. We examine the structural CAS
parameters of these galaxies, deriving merger fractions, at 0.2<z<1.2, based on
the asymmetry and clumpiness values of these systems. We find that the merger
fraction between z=0.2 and z=1.2 increases from roughly f_m=0.04+/-0.01 to
f_m=0.13+/-0.01. We explore several fitting formalisms for parameterising the
merger fraction, and compare our results to other structural studies and pair
methods within the DEEP2, VVDS, and COSMOS fields. We also re-examine our
method for selecting mergers, and the inherent error budget and systematics
associated with identifying mergers using structure. For galaxies selected by
M_*>10^{10} M_0, the merger fraction can be parameterised by f_m = f_0*(1+z)^m
with the power-law slope m=2.3+/-0.4. By using the best available z = 0 prior
the slope increases to m=3.8+/-0.2, showing how critical the measurement of
local merger properties are for deriving the evolution of the merger fraction.
We furthermore show that the merger fraction derived through structure is
roughly a factor of 3-6 higher than pair fractions. Based on the latest
cosmological simulations of mergers we show that this ratio is predicted, and
that both methods are likely tracing the merger fraction and rate properly. We
calculate, utilising merger time scales from simulations, and previously
published merger fractions that the merger rate of galaxies with M_*>10^{10}
M_0 increases linearly between z = 0.7 and z = 3, and that a typical
M_*>10^{10} M_0 galaxy undergoes between 1-2 major mergers at z<1.2.
