|Suppose, for example, that Crusoe is gathering coconuts for his personal consumption over a period of time. During his first three hours of labor, he manages to obtain 30 edible coconuts. Thereafter, however, his marginal product declines, because the remaining coconuts are in taller palm trees, which are harder to climb, or in less accessible parts of the island.
If he works just three hours, his marginal product from the third hour is 30 - 18 = 12 coconuts. His marginal productivity in that hour is the marginal utility of those 12 coconuts (given his prior stock of 18).
If he works four hours, however, his marginal product in the fourth hour is only 38 - 30 = 8 coconuts. To make matters worse, each of those 8 coconuts has less marginal utility than the previous ones in his stock. Consequently, his marginal productivity drops precipitously ().