In summary, we have performed a detailed microscopic analysis of the lattice thermal conductivity of ten group IV-VI chalcogenides within the single-mode relaxation-time approximation. Using the constant relaxation-time model, we find that the phonon group velocities are strongly related to the homogeneity in the chemical bonding, captured by the size and symmetry of the primitive unit cell, with a secondary dependence on the atomic masses of the constituent elements. The phonon lifetimes depend more intricately on the structure type and composition through a balance of the intrinsic anharmonicity, maximised in structures that force the tetrel atoms to adopt symmetric local environments, and the size of the scattering phase space, maximised by low-symmetry structures with complex phonon spectra. These effects are best balanced in the high-temperature Cmcm phase of SnSe, which shows the lowest κ κ κ latt of the ten systems examined, accounting for its ultra-low thermal conductivity and extremely high thermoelectric performance.