  We make use of z = 0 samples of strongly barred and unbarred disc galaxies
from the TNG100 and TNG50 cosmological hydrodynamical simulations to assess the
performance of the simple disc instability criterion proposed by Efstathiou,
Lake & Negroponte (1982) (ELN-criterion). We find that strongly barred galaxies
generally assemble earlier, are more star-dominated in their central regions,
and have more massive and more compact discs than unbarred galaxies. The
ELN-criterion successfully identifies ~75% and ~80% of the strongly barred and
the unbarred galaxies, respectively. Strongly barred galaxies that the
criterion fails to identify tend to have more extended discs, higher spin
values and bars that assembled later than is typical for the bulk of the barred
population. The bars in many of these cases appear to be produced by an
interaction with a close neighbour (i.e. to be externally triggered) rather
than to result from secular growth in the disc. On the other hand, we find that
unbarred galaxies misclassified as barred by the ELN-criterion typically have
stellar discs similar to those of barred galaxies, although more extended in
the vertical direction and less star-dominated in their central regions,
possibly reflecting later formation times. In addition, the bulge component of
these galaxies is significantly more prominent at early times than in the
strongly barred sample. Thus, the ELN-criterion robustly identifies secular bar
instabilities in most simulated disc galaxies, but additional environmental
criteria are needed to account for interaction-induced bar formation.

Astrophysics of Galaxies