Source : Free On-Line Dictionary of Computing

Brooks's Law
     
         "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}".
     
        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}.
     
        [{Jargon File}]
     
        (1996-09-17)