A key purpose of demand-led growth theory is to extend the ‘principle of effective demand’ to contexts in which productive capacity is best considered variable rather than fixed. The central idea is that, over any time frame, it is demand that determines output, and demand-led variations in income that adjust planned leakages to planned injections. Once it is acknowledged that capacity is variable, it becomes clear that the adjustment of output to demand, and planned leakages to planned injections, can be achieved not only by utilizing existing capacity more fully, but by expanding capacity through investment.

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