law of diminishing returns　

law of diminishing returns　报酬递减规律，报酬递减法则　

A concept in economics that if one factor of production（number of workers，for example）is increased while other factors（machines and workspace，for example）are held constant，the output per unit of the variable factor will eventually diminish.Although the marginal productivity of the workforce decreases as output increases，diminishing returns do not mean negative returns until（in this example）the number of workers exceeds the available machines or workspace.In everyday experience，this law is expressed as“the gain is not worth the pain”.