http://en.wikipedia.org/wiki/Diminishing_returns
In economics, diminishing returns (also called diminishing marginal returns) refers to how the marginal production of a factor of production starts to progressively decrease as the factor is increased, in contrast to the increase that would otherwise be normally expected. According to this relationship, in a production system with fixed and variable inputs (say factory size and labor), each additional unit of the variable input (i.e., man-hours) yields smaller and smaller increases in outputs, also reducing each worker's mean productivity. Conversely, producing one more unit of output will cost increasingly more (owing to the major amount of variable inputs being used, to little effect).