Glossary
Brooks's Law
Brooks law, the mythical man-month
Brooks's law states that adding people to a late project makes it later. The reasons are onboarding and communication effort, which grow faster than the added working capacity.
Fred Brooks derived the statement in 1975 in "The Mythical Man-Month" from developing the OS/360 operating system at IBM. His core argument: people and months are only interchangeable if the work can be divided arbitrarily and those involved do not have to talk to each other. Project work rarely meets either condition.
Three effects combine:
- Onboarding ties up the experienced. Newcomers have to be trained, and by the people who are currently most productive. Net capacity in the first weeks is negative.
- Communication paths grow quadratically. With n people there are
n(n-1)/2possible pairings. Going from 5 to 10 people doubles the team and almost quadruples the number of coordination paths. - Not every task is divisible. Brooks's image for it: nine women cannot produce a baby in one month.
Everyone talks to everyone
Communication paths in a team, every pairing drawn in
5 people
10 paths
15 people
105 paths
n(n-1)/2
The law is not an instruction never to grow a team. It states that adding people costs capacity in the short term and only returns it in the medium term. Staffing up four weeks before a deadline means paying the price without ever seeing the benefit.
The reach of the law is limited. It applies to work that requires coordination, and to the period while onboarding is still running. Where tasks divide cleanly and people work independently of each other, adding staff does scale. Brooks himself described his law as a deliberate sharpening, not a law of nature.
The second reason why more work in parallel rarely finishes sooner lies not in team size but in the number of open tasks. That one is described by Little's law.
