Brooks's Law: prov. "Adding manpower to a late software project makes it later" -

:Brooks's Law: prov. "Adding manpower to a late software project
makes it later" -- a result of the fact that the expected
advantage from splitting work among N programmers is
O(N) (that is, proportional to N), but the complexity
and communications cost associated with coordinating and then
merging their work is O(N^2) (that is, proportional to the
square of N). The quote is from Fred Brooks, a manager of
IBM's OS/360 project and author of "The Mythical Man-Month"
(Addison-Wesley, 1975, ISBN 0-201-00650-2), an excellent early book
on software engineering. The myth in question has been most
tersely expressed as "Programmer time is fungible" and Brooks
established conclusively that it is not. Hackers have never
forgotten his advice; too often, {management} still does. See
also {creationism}, {second-system effect},
{optimism}.
-- The AI Hackers Dictionary