AP Syllabus focus: ‘Each factor of production earns the value of its marginal product, which can contribute to income inequality.’
Income inequality can be explained using the logic of factor markets: households earn income by supplying labor, capital, land, or entrepreneurship. Differences in productivity and the value of what is produced lead to unequal earnings.
Core idea: factor payments come from marginal contributions
In competitive factor markets, firms hire inputs up to the point where the marginal benefit of the last unit of an input equals its marginal cost. For labor, the marginal cost is typically the wage rate; the marginal benefit is the additional revenue created by one more worker (or one more hour).
Marginal product links inputs to output
Marginal product (MP): the additional output produced by using one more unit of an input, holding other inputs constant.
MP tends to rise at first with specialization and then fall due to diminishing marginal returns, so the extra output from additional units of an input is usually not constant.
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FAQ
Yes. VMP uses output price ($P$) and fits price-taking firms. Marginal revenue product uses marginal revenue ($MR$) and matters when the firm faces a downward-sloping demand curve, so $MR < P$.
If they work in markets with different output prices, then even similar $MP$ can imply different $VMP$ because $P$ differs across products, locations, or customer segments.
Technology may increase $MP$ a lot for some tasks (complements) and little or not at all for others (substitutes). That uneven change in $MP$ changes $VMP$ unevenly, widening pay gaps.
Scarcity typically reflects that few workers can generate a high $MP$ in a valuable activity. Firms competing for that scarce input bid up pay consistent with its high marginal contribution.
When output can be scaled widely (digital platforms, global audiences), small differences in $MP$ can translate into large differences in total revenue generated, increasing measured $VMP$ and producing very large earnings at the top.
