Brooks’s Law

The Mythical Man-Month by Frederick P. Brooks Jr. (Addison-Wesley, 1975; anniversary edition 1995)

“Adding manpower to a late software project makes it later.” Brooks identified two mechanisms: new members require training from existing members (ramp-up time), and communication channels grow as $n(n-1)/2$, so doubling a team quadruples coordination paths. Most complex software cannot be partitioned like physical tasks, making adding people counterproductive beyond a threshold.