  It is shown that both the visibility ${\cal V} = 1/2$ predicted for
two-photon interference experiments with two independent
sources\textcolor{black}{, like the Hanbury Brown-Twiss experiment,} and the
visibility ${\cal V} = 1$ predicted for two-photon interference experiments
with a parametric down-conversion source\textcolor{black}{, like the
Ghosh-Mandel experiment,} can be explained \textcolor{black}{by a discrete
event simulation. This simulation approach reproduces the statistical
distributions of wave theory not by requiring the knowledge of the solution of
the wave equation of the whole system but by generating detection events
one-by-one according to an unknown distribution.} There is thus no need to
invoke quantum theory to explain the so-called nonclassical effects in the
interference of signal and idler photons in parametric down conversion. Hence,
a revision of the commonly accepted criterion of the nonclassical nature of
light\textcolor{black}{, ${\cal V} > 1/2$,} is called for.
