Which best describes the typical shape of the total product (TP) curve as labor input increases?

• A. A straight line upward
• B. A U-shaped curve
• C. S-shaped, increasing to a maximum and then decreasing
• D. A flat line

Answer: (C)

As you add labor while keeping other inputs fixed, total output rises, but the pace of that rise slows because of diminishing marginal returns. Each extra worker contributes less than the previous one, so the curve slopes downward in its rate of increase. Eventually, total output reaches a peak when the marginal product of labor drops to zero, and if you add even more labor beyond that point, total output can start to fall. This pattern—rising to a peak and then falling—gives an S-shaped shape for the total product curve. The other shapes don’t capture the idea of a peak followed by a decline: a straight line would mean constant marginal product, a U-shape would imply TP falls and then rises, and a flat line would mean no change at all.