  Thick disks are a prevalent feature observed in numerous disk galaxies
including our own Milky Way. Their significance has been reported to vary
widely, ranging from a few to 100% of the disk mass, depending on the galaxy
and the measurement method. We use the NewHorizon simulation which has high
spatial and stellar mass resolutions to investigate the issue of thick disk
mass fraction. We also use the NewHorizon2 simulation that was run on the same
initial conditions but additionally traced nine chemical elements. Based on a
sample of 27 massive disk galaxies with M* > 10^10 M_{\odot} in NewHorizon, the
contribution of the thick disk was found to be 34 \pm 15% in r-band luminosity
or 48 \pm 13% in mass to the overall galactic disk, which seems in agreement
with observational data. The vertical profiles of 0, 22, and 5 galaxies are
best fitted by 1, 2, or 3 sech2 components, respectively. The NewHorizon2 data
show that the selection of thick disk stars based on a single [{\alpha}/Fe] cut
is severely contaminated by stars of different kinematic properties while
missing a bulk of kinematically thick disk stars. Vertical luminosity profile
fits recover the key properties of thick disks reasonably well. The majority of
stars are born near the galactic mid-plane with high circularity and get heated
with time via fluctuation in the force field. Depending on the star formation
and merger histories, galaxies may naturally develop thick disks with
significantly different properties.
