After a critical assessment of both Thirlwalls long-run growth model and Clavijos and Ross (2015) model in which capital accumulation determines output growth, this paper displays an alternative model where capital accumulation and the growth rate of capital productivity determine the long-run growth rate of output which is consistent with a constant position of the balance of payments as a percentage of GDP. Then, the latter is applied to inquire the causes accounting for Mexicos low economic growth rate during 1982-2014. We conclude, on empirical grounds, that the sharp decline in the internal demand for domestic goods and both the rates of net capital accumulation and capital productivity played a major role in the slowdown of the Mexican economy. The income elasticities of the demand for exports and imports also played a role, albeit to a lesser extent.