jats:pWeak piezoelectricity, compared with electrostriction, occurs in twinned ferroelastic materials even when the uniform bulk material is centro-symmetric. In a simple computer simulation, polarity is exclusively generated by the flexoelectric effect. Simple twinned structures (parallel twin walls) are electrostrictive and show no piezoelectricity. Complex twinned structures break inversion symmetry by the simultaneous appearance of junctions, kinks, needle domains, etc. Such structures show weak piezoelectricity (d ∼ 10−4 pm/V) under periodic boundary conditions together with significant electrostriction. The macroscopic piezoelectric response is stronger (d ∼ 10−3 pm/V) under free boundary conditions due to the effect of relaxing surfaces.</jats:p>