Abstract Long‐wavelength crustal thickness variations are observed on telluric planets, particularly on Mars and the Moon whereas Mercury appears to lack such variations. They may stem from two early‐stage positive feedback mechanisms that can emerge in stagnant‐lid planets. Both rely on an increased mantle melting below thinner lids due to lower pressures. The first feedback is between the extraction rate of heat‐producing elements (HPEs) concentrated in mantle melts and the lid HPE content. The second links the lid thickness and its growth rate through the melt‐influenced mantle viscosity. Linear stability analyses and thermal evolution simulations show that the second mechanism is the fastest. We demonstrate that, while the Martian dichotomy can be generated by the HPE‐driven mechanism alone, both mechanisms must interact to produce the observed lunar crustal asymmetry. In contrast, Mercury’s rapid cooling only allows for the fastest mechanism. Crustal thickness variations on Mercury appear limited to sufficiently thin crusts.