  Full shell-model diagonalization has been performed to study the structure of
neutron-rich nuclei around $^{20}$C. We investigate in detail the roles played
by the different monopole components of the effective interaction in the
evolution of the N=14 shell in C, N and O isotopes. It is found that the
relevant neutron-neutron monopole terms, $V^{nn}_{d_{5/2}d_{5/2}}$ and
$V^{nn}_{s_{1/2}s_{1/2}}$, contribute significantly to the reduction of the
N=14 shell gap in C and N isotopes in comparison with that in O isotopes. The
origin of this unexpectedly large effect, which is comparable with (sometimes
even larger than) that caused by the proton-neutron interaction, is related to
the enhanced configuration mixing in those nuclei due to many-body
correlations. Such a scheme is also supported by the large B(E2) value in the
nucleus $^{20}$C which has been measured recently.
