This paper highlights the relationship between isostatically supported surface elevation and the lateral buoyancy forces of an orogen. The former is a linear function; the latter is a quadratic function of the thickness of the lithosphere. Thus surface elevation and lateral buoyancy forces may have differing evolutions in the development of an orogen. Indeed, during the evolution of collisional orogens the qualitative relationship between these two parameters may reverse, so that only in the early orogenic evolution, topographically high regions exert a net lateral buoyancy force on topographically low regions. Later in the evolution this may change to a counterintuitive scenario in which there is a net lateral buoyancy force from the topographically lower regions toward the topographically higher regions. To illustrate this fact, we use a simple one-dimensional kinematic model for an orogen which undergoes simultaneous thickening due to convergence and erosion at the surface. It can be shown that for reasonable erosion and thickening rates, surface elevation increases through time until a steady state is reached, while the lateral buoyancy force changes from positive to negative, giving rise to the inward collapse of an orogen. In the field this may be recognized as the occurrence of two successive coaxial convergent deformation events. Present-day examples may be the formation of back arc basins where extension in the low-lying hinterland of an Andean-type mountain belt is caused by a gravitational compressive force exerted by the hinterland onto a belt of significantly higher surface elevation but possibly of lower potential energy. ¿ 2000 American Geophysical Union