Figure11A-1Duality Between Productivity and Cost Due to the inverse relationship between marginal product of labour and marginal cost, and between average product of labour and average variable cost, the marginal cost curve mirrors the marginal product curve while the average variable cost curve mirrors the average product curve. Marginal cost falls when marginal product of labour rises; while marginal cost rises when marginal product of labour falls. When marginal product of labour is maximized (point A), marginal cost reaches its minimum (point A′). Similarly, when average product of labour rises, average variable costs falls; when average product of labour falls, average variable costs rises. When average product of labour is maximized (point B), average variable cost reaches its minimum (point B′).