We propose a structural basis for the differences in interaction strengths based on the revised lone-pair model of Walsh et al. In the IV-VI monochalcogenides, the tetrel atoms adopt the 2+ oxidation state with a valence ns 2 lone pair of electrons. The lone pair is not chemically inert and interacts with the anion p orbitals to form bonding and antibonding states. When this interaction is strong, and the antibonding states have substantial cation s character, the antibonding states can mix with the vacant cation p states through on-site hybridisation, resulting in further stabilisation. The interaction is forbidden by symmetry in a centrosymmetric environment, and therefore drives the tetrel atoms toward distorted, low-coordinate geometries in which an "active" lone pair effectively occupies a coordination site and is often projected into a structural void such as the interlayer space in the Pnma and Cmcm structures (c.f. Fig. ). When the tetrel atoms are constrained to a symmetric environment, the lone pair is "strained", which can provide a driving force for anharmonic lattice dynamics. In the extreme, this manifests as imaginary harmonic modes, for example in Cmcm SnSe, as shown in the present work (c.f. Figs ), but also in the Fm 3m and Cmcm phases of SnS and the Fm 3m phase of GeSe. Competing with this, however, is the potential energetic destabilisation associated with the lower coordination number, and if the interaction between the cation s and anion p states is weak the stabilisation from an active lone pair does not outweigh the lower coordination number and a symmetric structure is energetically preferred. In the PbX series, PbO adopts the distorted litharge structure with an active lone pair, whereas the lead chalcogenides PbS, PbSe and PbTe adopt the centrosymmetric rocksalt structure without an active lone pair. Similarly, the strong anion/cation interaction in SnS results in an energetic preference for the Pnma structure and the Fm 3m structure being dynamically unstable, whereas the weaker interaction in SnSe results in the rocksalt phase being both dynamically stable and much closer to the convex hull. In the SM-RTA model used in these calculations, strong threephonon interactions arise from a strong response of the system to atomic displacements at third order. It has been demonstrated that the electron-phonon coupling due to the lone-pair activity in Cmcm SnSe contributes directly to large third-order force constants and strong phonon interactions. Similarly, although the Pb chalcogenides are stable in the rocksalt phase, indicating, in principle, weaker electron-phonon coupling, PbTe has been characterised as an "incipient ferroelectric", which could also be driven by lone pair activity and which has been linked to strong scattering and suppressed heat transport through the longitudinal acoustic (LA) modes. Similarly, while in principle the interaction between Ge and S/Se might be expected to be too weak to drive a structural distortion, both materials readily form in the Pnam structure, and the presence of active lone pairs has been confirmed both experimentally and theoretically. In our view, these observations provide evidence that lone pair activity could support strong three-phonon interactions across all of the IV-VI chalcogenides, even in systems where the electron-phonon coupling is not strong enough to manifest in the second-order force constants as imaginary harmonic modes.