Labor’s negative marginal product for L > 5 may arise from congestion in the
chair manufacturer’s factory. Since more laborers are using the same fixed
amount of capital, it is possible that they could get in each other’s way,
decreasing efficiency and the amount of output. Firms also have to control the
quality of their output, and the high congestion of labor may produce products
that are not of a high enough quality to be offered for sale, which can contribute
to a negative marginal product.