The marginal propensity to import (PMI) refers to the change in import spending that occurs as a consequence of a change in disposable income (after-tax income and transfers). For example, a household earns an extra euro of disposable income, and the marginal propensity to import is 0.2, then of that euro, the household will spend 20 euro cents of that euro on imported goods and services.

Mathematically, the function of the marginal propensity to import is expressed as that derived from the function of imports (M) relative to disposable income (Y).

M P M = d M d Y {\displaystyle MPM={\frac {dM}{dY}}}

In other words, the marginal propensity to import is measured as the radius of change in imports relative to the change in income, providing a figure between 0 and 1.

Imports are also seen as an automatic stabilizer that operates by attenuating real GDP fluctuations.

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