Electric polarizabilities of proton and neutron and the relativistic   center-of-mass coordinate
  We argue that the relativistic correction $\delta{\bf R}_{c.m.}$ to the center-of-mass vector can lead to the approximate equality of the proton and neutron electric polarizabilities in the quark model. The explicit form of $\delta{\bf R}_{c.m.}$ depends only on the non-relativistic potential between quarks. In particular, this correction is the same for the potential generated by Lorentz-vector and -scalar interactions. 
R.N. Lee, A.I. Milstein, M. Schumacher