Strontium titanate is an incipient ferroelectric where the freezing of the soft polar mode is inhibited by quantum fluctuations (Müller K. A. and Burhard H., Phys. Rev. B, 19 (1979) 3593). Exchanging ¹⁶O by its isotope ¹⁸O induces ferroelectricity at T_c = 25 K (Itoh M., Wang R., Inaguma Y., Yamaguchi T., Shan Y. J. and Nakamura T., Phys. Rev. Lett., 82 (1999) 3540). This finding is unexpected and its origin remains unclear since seemingly contradictory data have been obtained. Here we suggest that SrTi¹⁸O₃ (STO18) exhibits self-induced intrinsic inhomogeneity due to optic–acoustic mode coupling, whereby dynamical clusters are formed with a lower symmetry than the embedding matrix. With decreasing temperature these clusters grow in size and volume fraction and remain present in the ferroelectric state leading to an incomplete ferroelectric ground state. The remaining lattice shows soft-mode behaviour. Since the length and time scales of the coexisting components differ considerably, experiments which test these scales can reach contradictory conclusions concerning the phase transition mechanism.