This mechanism explains the strong link between the crystal symmetry and the phonon interaction strengths. In the Fm 3m structure the tetrel atoms are forced by symmetry to adopt a centrosymmetric environment, resulting in a "strained" lone pair and strong three-phonon interactions reflected in large P. In the Pnma structure, on the other hand, the tetrel atoms can adopt a distorted local structure with active lone pairs projecting into the interlayer space, leading to weak three-phonon interactions and low P. In the R3m structure, the tetrel atoms are in a less symmetric environment than in the Fm 3m phase, allowing for some relaxation of the strain, but not as much as in the Pnma phase. Similarly, in the Cmcm structure the tetrel atoms are forced to adopt a more symmetric environment than in the Pnma phase, which leads to larger three-phonon interactions. Both of these systems therefore have P intermediate between the bounds set by the (fully) symmetric Fm 3m and (fully) distorted Pnma phases. We note that while the π phase has a high-symmetry spacegroup the large unit cell allows for the tetrel atoms to adopt a lowcoordinate local geometry, so calculations on this phase will confirm whether (or not) the key descriptor for anharmonicity is the global symmetry of the crystal, or the local symmetry of the tetrel atoms (we strongly suspect the latter). Finally, the strength of the electronic interactions is determined by the energy match between the cation s and anion p states, and this can potentially explain the variation in anharmonicity among different compounds in the same structure type, although to explore this further would require a detailed analysis of the relative energy levels. Finally, our analysis of the averaged lifetimes shows that the three-phonon interaction strengths and anharmonicity, as defined by the P, are not the only important factor, and that the reduced anharmonicity of the lower-symmetry crystal structures is partially offset by the more complex phonon spectra facilitating a larger scattering phase space. Our analysis suggests that this plays a particularly important role in determining the low κ κ κ latt of the Pnma structures.