  We discuss the mechanism causing the `kink' in the charge radius isotope
shift at the N=126 shell closure. The occupation of the 1$i_{11/2}$ neutron
orbital is the decisive factor for reproducing the experimentally observed
kink. We investigate whether this orbital is occupied or not by different
Skyrme effective interactions as neutrons are added above the shell closure.
Our results demonstrate that several factors can cause an appreciable
occupation of the 1$i_{11/2}$ neutron orbital, including the magnitude of the
spin-orbit field, and the isoscalar effective mass of the Skyrme interaction.
The symmetry energy of the effective interaction has little influence upon its
ability to reproduce the kink.
