The Law of Diminishing Returns as it applies to Economics:
As employees are added to a fixed resource, the output per employee decreases.
If a company owns one delivery truck, then one driver can drive the truck fifteen (or more) hours per day. Adding an additional driver though, decreases the output per driver to twelve hours per day (24/2=12). Adding a third driver decreases the output per driver to eight hours per day, and so on.