Brooks's Law

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}.