Full shell-model diagonalization has been performed to study the structure of neutron-rich nuclei around 20C. We investigate in detail the roles played by the different monopole components of the effective interaction in the evolution of the N=14N=14 shell in C, N and O isotopes. It is found that the relevant neutron–neutron monopole terms, Vd5/2d5/2nn and Vs1/2s1/2nn, contribute significantly to the reduction of the N=14N=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)B(E2) value in the nucleus 20C which has been measured recently.