‘The rate of growth (y) of any developed country in the long run is equal to the growth rate of the volume of its exports (x) divided by its income elasticity of demand for imports (π)’, he [Anthony Thirlwall] explained.

Our eyes were fixed on the blackboard, attempting to digest the meaning and internalize the implications of this tri-legged animal. That job was not easy. For the animal distilled volumes of legendary work in economic development, encapsulating all of them in a small-sized anti-underdevelopment pill. The teaching of Engel’s law, which implies that the demand for primary goods increases less than proportionally to increases in global income; the Harrod foreign trade multiplier which put forward the idea that the pace of industrial growth could be explained by the principle of the foreign trade multiplier; the Marshall– Lerner condition which implies that a currency devaluation would not be effective unless the devaluation-induced deterioration in the terms of trade is more than offset by the devaluation-induced reduction in the volume of imports and increase in volume of exports; the Hicks super-multiplier which implies that the growth rate of a country is fundamentally governed by the growth rate of its exports; the Prebisch–Singer hypothesis which asserts that a country’s international trade that depends on primary goods may inhibit rather than promote economic growth; the Verdoorn–Kaldorian notion that faster growth of output causes a faster growth of productivity, implying the existence of substantial economies of scale; Kaldor’s paradox which observed that countries that experienced the greatest decline in their price competitiveness in the post-war period experienced paradoxically an increase in their market share and not a decrease; the literature on export-led growth which asserts that export growth creates a virtuous circle through the link between output growth and productivity growth – all of these doctrines were somehow put into play and epitomized within this small-sized capsule. Not only that but the capsule was sealed by the novel and powerful ingredient of the balance-of-payments constraint: ‘in the long run, no country can grow faster than that rate consistent with balance of payments equilibrium on current account unless it can finance ever-growing deficits which, in general, it cannot’.

– Mohammed Nureldin Hussain, The Implications Of Thirlwall’s Law For Africa’s Development Challenges in Growth And Economic Development Essays In Honour Of A.P. Thirlwall, 2006