  We present a series of N-body experiments which confirm the reality of the
previrialization effect. We also use weakly nonlinear perturbative approach to
study the phenomenon. These two approaches agree when the rms density contrast,
$\sigma$, is small; more surprisingly, they remain in agreement when $\sigma
\approx 1$. When the slope of the initial power spectrum is $n>-1$, nonlinear
tidal interactions slow down the growth of density fluctuations and the
magnitude of the suppression increases when $n$ (i.e. the relative amount of
small scale power) is increased. For $n<-1$ we see an opposite effect: the
fluctuations grow more rapidly than in linear theory. The transition occurs at
$n=-1$ when the weakly nonlinear correction to $\sigma$ is close to zero and
the growth rate is close to linear. Our results resolve recent controversy
between two N-body studies of previrialization. Peebles (1990) assumed $n=0$
and found strong evidence in support of previrialization, while Evrard \& Crone
(1992), who assumed $n=-1$, reached opposite conclusions. As we show here, the
initial conditions with $n=-1$ are rather special because the nonlinear effects
nearly cancel out for that particular spectrum. In addition to our calculations
for scale-free initial spectra, we show results for a more realistic spectrum
of Peacock \& Dodds (1994). Its slope near the scale usually adopted for
normalization is close to $-1$, so $\sigma$ is close to linear. Our results
retroactively justify linear normalization at 8$h^{-1}$ Mpc, while also
demonstrating the danger and limitations of this practice.
