Because average total cost is a combination of average variable cost and average fixed cost, the U-shape of the average total cost curve is a result of both underlying averages. At small production quantities, both average fixed cost and average variable cost decline, resulting in a negatively-sloped average total cost curve. However, because of the law of diminishing marginal returns, average variable cost eventually increases, which overwhelms the continuing decline of average fixed cost and results in a positively-sloped average total cost curve.

Average Total Cost Curve

The graph to the right is the average total cost curve for the short-run production of Wacky Willy Stuffed Amigos (those cute and cuddly armadillos and tarantulas). The quantity of Stuffed Amigos production, measured on the horizontal axis, ranges from 0 to 10 and the average total cost incurred in the production of Stuffed Amigos, measured on the vertical axis, ranges from about $3 to over $8.

As noted above, the average total cost curve is U-shaped. For the first 6 Stuffed Amigos, average total cost declines from over $8 to a low of $3. However, for the production beyond 7 Stuffed Amigos, average total cost increases.

While it would be easy to attribute the U-shape of the average total cost curve to increasing, then decreasing marginal returns (and the law of diminishing marginal returns), such is not completely true. While the law of diminishing marginal returns is indirectly responsible for the positively-sloped portion of the average total cost curve, the negatively-sloped portion is attributable to increasing marginal returns, and perhaps more importantly to declining average fixed cost.