  Ultraluminous and luminous infrared galaxies (ULIRGs and LIRGs) are the most
extreme star-forming galaxies in the universe, and dominate the total star
formation rate density at z>1. In the local universe (z<0.3), the majority of
ULIRGs and a significant portion of LIRGs are triggered by interactions between
gas-rich spiral galaxies, yet it is unclear if this is still the case at
high-z. To investigate the relative importance of galaxy interactions in
infrared luminous galaxies, we carry out a comparison of optical morphological
properties between local (U)LIRGs and (U)LIRGs at z=0.5-1.5 based on the same
sample selection, morphology classification scheme, and optical morphology at
similar rest-frame wavelengths. In addition, we quantify the systematics in
comparing local and high-z datasets by constructing a redshifted dataset from
local (U)LIRGs, in which its data quality mimics the high-z dataset. Based on
the Gini-M20 classification scheme, we find that the fraction of interacting
systems decreases by ~8% from local to z<~1, and it is consistent with the
reduction between local and redshifted datasets (6(+14-6)%). Based on visual
classifications, the merger fraction of local ULIRGs is found to be ~20% lower
compared to published results, and the reduction due to redshifiting is
15(+10-8)%. Consequently, the differences of merger fractions between local and
z<~1 (U)LIRGs is only ~17%. These results demonstrate that there is no strong
evolution in the fraction of (U)LIRGs classified as mergers at least out to
z~1. At z>1, the morphology types of ~30% of (U)LIRGs can not be determined due
to their faintness in the F814W-band, and thus the merger fraction measured at
z>1 suffers from large uncertainties.
