Economists behind this theory developed an idea of a "subsistence level" to model the theory. They believed that if real GDP rose above this subsistence level of income that it would cause the population to increase and bring real GDP back down to the subsitence level. It was sort of like a equilibrium level that real GDP would always revert to in this theory. Alternatively, if the real GDP fell below this subsistence level, parts of the population would die off and real income would rise back to the subsistence level.