  We quantify the degree of lopsidedness for a sample of 149 galaxies observed
in the near-infrared from the OSUBGS sample, and try to explain the physical
origin for the observed disk lopsidedness. We confirm previous studies, but now
for a larger sample, that a large fraction of galaxies show significant
lopsidedness in their stellar disks, measured as the Fourier amplitude of the
m=1 component, normalised to the average or m=0 component, in the surface
density. Late-type galaxies are found to be more lopsided, while the presence
of m=2 spiral arms and bars is correlated. The m=1 amplitude is found to be
uncorrelated with the tidal forces acting on a galaxy via nearby companions.
Numerical simulations are carried out to study the generation of m=1 via
different processes: galaxy tidal encounters, galaxy mergers, and external gas
accretion and subsequent star formation. The simulations show that galaxy
interactions and mergers can trigger strong lopsidedness, but do not explain
several independent statistical properties of observed galaxies. To explain all
the observational results, it is required that a large fraction of lopsidedness
results from cosmological accretion of gas on galactic disks, which can create
strongly lopsided disks when this accretion is asymmetrical enough.
