In high-temperature superconductors with Tc exceeding the temperature of liquid nitrogen, there is no conventional isotope shift. This suggests an unconventional pairing mechanism, but we notice a systematic upward "polarizability shift" in Tc. The shift in Tc of about 20% occurs in the 1:2:3, 2:2:1:2, and 2:2:2:3 high-temperature superconducting compounds when the more polarizable Ba replaces Sr. A semiquantitative calculation enables us to interpret our result in the framework of the two-hole resonance mechanism. The role of the alkaline-earth ion is to partially screen the electron-electron Coulomb repulsion U, lowering the energy of the d8 state and hence increasing Tc. © 1989 The American Physical Society.