Here is a basic statement of the law:
“Thirlwall’s law (named after Anthony Thirlwall) states that if long run balance of payments equilibrium on current account is a requirement, and the real exchange rate stays relatively constant, then the long run growth of a country can be approximated by the ratio of the growth of exports to the income elasticity of demand for imports (Thirlwall, 1979).Thirlwall’s paper “The Balance of Payments Constraint as an Explanation of International Growth Rate Differences” (1979) was the foundational work on this law.
If the real exchange rate varies considerably, but the price elasticities of demand for imports and exports are low, the long run growth of the economy will then be determined by the growth of world income times the ratio of the income elasticity of demand for exports and imports which are determined by the structural characteristics of countries. One important example of this is that if developing countries produce mainly primary products and low value manufactured goods with a low income elasticity of demand, while developed countries specialise in high income elasticity manufactured goods the developing countries will grow at a relatively slower rate (Davidson, 1991).”
Thirlwall’s law is a growth model for capitalist economies that focuses on the demand-side and the balance of payments constraint.
For example, if a nation’s growth path takes it down the route where it runs constant balance-of-payments deficits financed by short-term capital inflows, then this in the long run may well lead to unsustainable exchange rate depreciation and, in extreme cases, exchange rate collapse and hyperinflation (McCombie 2012: 19).
It is better, therefore, that in the long run a nation has a tendency to balance current account and long-term capital flows (McCombie 2012: 19).
Specifically, we can note these important factors:
(1) a nation’s export growth depends on world demand and its exchange rate competitiveness. Thirlwall’s law throws aside the unrealistic assumption of global full employment demand or the natural tendency to full employment (Davidson 2011: 246);As Davidson (2011: 248) notes, letting free markets determine balance of payments dynamics is likely to doom many Third world nations to poverty.
(2) internal demand for imports depends on growth of domestic real income and relative prices;
(3) for many countries, a crucial factor determining long-run growth is the growth of exports to ensure a sustainable balance of payments path;
(4) the neoclassical real exchange rate adjustment mechanism is a flawed model of reality. In reality, downwards nominal wage rigidity, cost-based mark-up prices adjusting to changes in the prices of imported factor inputs, and the relatively inelastic demand for many types of export commodities thwart this mechanism.
(5) the upshot is that economic growth needs to take account of the balance-of-payments constraint, and that import substitution and export promotion policies need to take account of this too, and export products should be focussed on goods for which world demand is strong, and growing. An additional point from Post Keynesian trade theory is that manufacturing is crucial to create sectors in which increasing returns to scale and clustering effects (from new industries creating factor-inputs for the manufacturing sector) occur, which will be the key to successful development.
A bibliography on Thirlwall’s law is below:
McCombie, John S. L. 2012. “Balance-of-Payments-Constrained Economic Growth,” in J. E. King (ed.), The Elgar Companion to Post Keynesian Economics (2nd edn.). Edward Elgar, Cheltenham. 19–24.
Davidson, Paul. 2011. Post Keynesian Macroeconomic Theory: Foundation for Successful Economic Policies for the Twenty-First Century (2nd edn). Edward Elgar Publishing, Cheltenham. pp. 246–249, 312.
Lavoie, Marc. 2014. Post-Keynesian Economics: New Foundations. Edward Elgar, Cheltenham. pp. 518–529.
Alonso, José A. and Carlos Garcimartín. 1998–1999. “A New Approach to Balance-of-Payments Constraint: Some Empirical Evidence,” Journal of Post Keynesian Economics 21.2: 259–282.
Ansari, M., Hashemzadeh, N. and Y. Xi. 2000. “The Chronicle of Economic Growth in Southeast Asian Countries: Does Thirlwall’s Law Provide an Adequate Explanation?,” Journal of Post Keynesian Economics 22.4: 573–588.
Atesoglu, H. Sonmez. 1993. “Balance-of-Payments-Constrained Growth: Evidence from the United States,” Journal of Post Keynesian Economics 15.4: 507–514.
Atesoglu, H. Sonmez. 1997. “Balance-of-Payments-Constrained Growth Model and Its Implications for the United States,” Journal of Post Keynesian Economics 19.3: 327–335.
Bekö, Jani. 2003. “The Validity of the Balance-of-Payments-Constrained Growth Model for a Small Economy in Transition: The Case of Slovenia,” Journal of Post Keynesian Economics 26.1: 69–93.
Bértola, Luis, Higachi, Hermes and Gabriel Porcile. 2002. “Balance-of-Payments-Constrained Growth in Brazil: A Test of Thirlwall’s Law, 1890–1973,” Journal of Post Keynesian Economics 25.1: 123–140.
Blecker, Robert A. 1998. “International Competitiveness, Relative Wages, and the Balance-of-Payments Constraint,” Journal of Post Keynesian Economics 20.4: 495–526.
Davidson, Paul. 1990–1991. “A Post Keynesian Positive Contribution to ‘Theory,’” Journal of Post Keynesian Economics13.2: 298–303.
Dutt, Amitava Krishna. 2002. “Thirlwall’s Law and Uneven Development,” Journal of Post Keynesian Economics 24.3: 367–390.
Hieke, Hubert. 1997. “Balance-of-Payments-Constrained Growth: A Reconsideration of the Evidence for the U.S. Economy,” Journal of Post Keynesian Economics 19.3: 313–325.
León-Ledesma, Miguel A. 1999. “An Application of Thirlwall's Law to the Spanish Economy,” Journal of Post Keynesian Economics 21.3: 431–439.
López G., Julio and Alberto Cruz B. 2000. “‘Thirlwall’s Law’ and beyond: The Latin American Experience,” Journal of Post Keynesian Economics 22.3: 477–495.
Loría, Eduardo. 2003. “The Mexican Economy: Balance-of-Payments-Constrained Growth Model: The Importance of the Exchange Rate, 1970–1999,” Journal of Post Keynesian Economics 25.4: 661–691.
McCombie, John S. L. 1993. “Economic Growth, Trade Interlinkages, and the Balance-of-Payments Constraint,” Journal of Post Keynesian Economics 15.4: 471–505.
McCombie, John S. L. 1997. “On the Empirics of Balance-of-Payments-Constrained Growth,” Journal of Post Keynesian Economics 19.3: 345–375.
McCombie, J. S. L. and A. P. Thirlwall. 1994. Economic Growth and the Balance-of-Payments Constraint. Macmillan Press, New York and Basingstoke.
McCombie, J. S. L. and A. P. Thirlwall (eds.). 2004. Essays on Balance of Payments Constrained Growth: Theory and Evidence. Routledge, London.
Moreno-Brid, Juan Carlos. 1998–1999. “On Capital Flows and the Balance-of-Payments-Constrained Growth Model,” Journal of Post Keynesian Economics 21.2: 283–298.
Moreno-Brid, Juan Carlos and Esteban Pérez. 1999. “Balance-of-Payments-Constrained Growth in Central America: 1950–96,” Journal of Post Keynesian Economics 22.1: 131–147.
Pugno, Maurizio. 1998. “The Stability of Thirlwall’s Model of Economic Growth and the Balance-of-Payments Constraint,” Journal of Post Keynesian Economics 20.4: 559–581.
Razmi, Arslan. 2005. “Balance-of-Payments-Constrained Growth Model: The Case of India,” Journal of Post Keynesian Economics 27.4: 655–687.
Sasaki, Hiroaki. 2008–2009. “North-South Ricardian Trade and Growth under the Balance-of-payments Constraint,” Journal of Post Keynesian Economics 31.2: 299–324.
Setterfield, M. 2011. “The Remarkable Durability of Thirlwall’s Law,” PSL Quarterly Review 64: 393–427.
Thirlwall, A. P. 1979. “The Balance of Payments Constraint as an Explanation of International Growth Rate Differences,” Banca Nazionale del Lavoro Quarterly Review 128.791: 45–53.
Thirlwall, A. P. 2011. “Balance of Payments Constrained Growth Models: History and Overview,” PSL Quarterly Review 64.259: 307–351.
Thirlwall, A. P. and M.N. Hussain. 1982. “The Balance of Payments Constraint, Capital Flows and Growth Rate Differences between Developing Countries,” Oxford Economic Papers 34: 498–509.
Vera, Leonardo V. 2006. “The Balance-of-Payments-Constrained Growth Model: A North-South Approach,” Journal of Post Keynesian Economics 29.1: 67–92.