  The effect of correlations between the slope and the curvature of the
symmetry energy on ground state nuclear observables is studied within the
extended Thomas-Fermi approximation. We consider different isovector probes of
the symmetry energy, with a special focus on the skin of $^{208}{\rm Pb}$. We
use a recently proposed meta-modelling technique to generate a large number of
equation of state models, where the empirical parameters are independently
varied. The results are compared to a set of calculations using 17 different
Skyrme interactions. We show that the curvature parameter plays a
non-negligible role on the neutron skin, while the effect is reduced in Skyrme
functionals because of the correlation with the slope parameter.
