In this paper has been proposed a solvency-sustainability concept based on "modified" ability to pay criterion, that avoid the looseness of the traditional solvency condition.
Specifically, we have taken into explicit consideration the presence of liquidity constraints and/or of precautionary motives to save, that constraint the saving capacity of the economy, because the saving profile is, to some extent, forced to follow the current income time evolution. As a result, poor countries with more fragile financial markets, and more exposed to the world business cycle (uncertainty) may fail to met debt discipline objectives on a pure ability to pay basis. We have derived a precise theoretical formulation of this intuition, conditioned on specific debt disciplines, that have been employed to evaluate the observed external debt position of several groups of countries. Our conclusion is that, the presence of relevant liquidity constraints can really make the difference between sustainability and non sustainability of a given stock of foreign debt.