According to the law of diminishing marginal returns,when one or more inputs are fixed,a variable input (usually labor)is likely to have a marginal product that eventually diminishes as the level of input increases. The shape of each isoquant can be described by the marginal rate of technical substitution at each point on the isoquant. ❑ According to the law of diminishing marginal returns, when one or more inputs are fixed, a variable input (usually labor) is likely to have a marginal product that eventually diminishes as the level of input increases. ❑ The shape of each isoquant can be described by the marginal rate of technical substitution at each point on the isoquant