It has recently been suggested that in addition to the widely accepted order-by-disorder mechanism, the psi_2 state observed in Er2Ti2O7 is influenced by virtual fluctuations of the crystal field and that this would account for an observed gap that is several times larger than can be reproduced in even the most advanced quantitative models. We propose to test this hypothesis by performing measurements of the magnetic ground state in Y-doped Er2-xYxTi2O7 to see if the ground state switches to the psi_3 state above a critical doping level, and compare this to measurements of the excitation gap as a function of doping (to be carried out elsewhere).