What principle states that adding more of a variable input will eventually lead to a decline in the marginal product?

The principle that states that adding more of a variable input will eventually lead to a decline in the marginal product is known as the Law of Diminishing Returns. This economic concept explains that while increasing the amount of a variable input (like labor) in the production process can lead to an increase in total output, there comes a point where each additional unit of input results in progressively smaller increases in output.

For example, if a farmer adds more workers to a fixed piece of land, initially, production may increase significantly. However, after reaching a certain point, each new worker contributes less to overall production due to limitations in resources like land or equipment. Understanding this principle is crucial for making efficient decisions regarding resource allocation and production levels in various economic contexts.