The origin and the effects of asymmetrical electrical behavior in sputtered PZT (Zr/Ti=65/35) thin film capacitors with Pt electrodes have been studied. The asymmetry and constriction in the P-E hysteresis loops are understood to result from differences in mechanical stress at the top and bottom PZT/Pt interfaces because they experience different thermal cycles during fabrication. A method for correctly positioning asymmetric loops on the polarization axis is suggested. Both d.c. and a.c. electrical stressing (of either polarity) lead to hysteresis relaxation and symmetrization. A post-processing anneal leads to electrically symmetrical devices.