Brooks’s Law is the now-famous claim from Fred Brooks’s book “The Mythical Man-Month”: adding manpower to a late software project makes it later. Brooks drew the lesson from managing the development of IBM’s OS/360 operating system, where extra staff added to a slipping schedule did not bring it back on time.
The reasoning rests on two effects. First, people added to a project must be trained and brought up to speed, which costs the time of the experienced members who do the training, so the team is briefly less productive, not more. Second, the work of coordinating a larger team grows faster than the team itself, because the number of communication paths between people rises with the square of the number of people.
These effects expose a flaw in the “man-month” as a unit of work. Treating effort as interchangeable - assuming that people and months can be swapped freely - works only when a task can be cleanly partitioned and needs no communication between the people doing it. Most software tasks are not like that.
Brooks’s Law is not an absolute, but a caution: there is a limit to how much a late project can be rescued by throwing more people at it, and past a point doing so is counterproductive. The observation has remained one of the most quoted in software project management.