Brooks' law

Brooks' Law stated adding manpower to a late software project makes it later, and the bearing of a child takes nine months, no matter how many women are assigned.

Explanations

One reason for the seeming contradiction is that software projects are complex engineering endeavors, and new workers on the project must first become educated in the work that has preceded them; this education requires diverting the resources already working on the project, temporarily diminishing their productivity while the new workers are not yet contributing meaningfully. Each new worker typically must "ramp up" in this way with not only one, but multiple engineers who must educate the new worker in their area of expertise in the code base, day by day.

Another significant reason is the communication overheads increase as the number of people increase. The number of different communication channels goes up as the square of the number of people; double the number of people and you get four times as many different conversations. Everyone working on the same thing needs to keep in sync, so as you add more and more people, they spend more and more time trying to find out and keep up with what everyone else is doing.