Why is increasing marginal cost a short-run phenomenon?
Answer: To understand this, you need to consider why decreasing marginal productivity is a short-run phenomenon as well. We assumed that marginal productivity decreased as the use of labor increased because we ran out of opportunities for labor to use the capital to produce (there were only so many computer work stations for Patricia’s web page designers to work with). In the long run, we could increase the amount of capital, which would increase the marginal productivity of the workers. As marginal productivity increases, marginal cost falls. It is important to remember that we are assuming all the workers are identical – the observed increase in marginal cost is not because the additional workers are in any way inferior to the ones already hired. They just have less capital to work with.