Brooks's Law

Brooks's Law is the observation that "adding manpower to a late software project makes it later," stated by Fred Brooks in his 1975 book The Mythical Man-Month. Brooks drew the lesson from his time as project manager of IBM's OS/360 operating system, a famously over-budget and over-schedule effort. The mechanism Brooks identified has two main parts. First, new engineers added to a running project need time to be brought up to speed on the codebase, conventions, and design decisions; during that ramp-up they consume the time of existing engineers while producing little themselves. Second, the communication overhead between team members grows roughly with the square of team size — n people have n(n-1)/2 pairwise communication channels — so each additional person adds a disproportionate coordination tax. Late projects are typically late because their work isn't perfectly parallelizable; throwing more bodies at sequential dependencies cannot speed them up. Brooks himself called the law an "outrageous oversimplification" and was careful to note exceptions: very early in a project, or when added staff handle cleanly separable work like documentation or testing, growth can still help. The law's enduring force comes from the underlying observation that software engineering effort does not compose linearly the way physical labor sometimes does. This is the "mythical man-month" of the title — the fallacy of treating person-months as a fungible unit of progress. Brooks's Law is commonly discussed alongside Linus's Law and other durable aphorisms about the strange economics of building software at scale.