The very neutron-rich nucleus 20C has been investigated by inelastic scattering on 208Pb and liquid hydrogen targets. Through distorted wave analysis, the reduced electric quadrupole transition probability B(E2;0g.s.+→21+)<18.4 (stat) e2 fm4 and the neutron transition probability Mn2=292±52 (stat) fm4 have been derived. A simple shell model calculation has shown a need for a factor of about 0.4 decrease of the normal polarization charges to elucidate the results. This is interpreted as a decoupling of the valence neutrons from the nuclear core in carbon isotopes heavier than 14C.