  Brooks' Law is often referred to in practice and states that adding manpower
to a late software project makes it even later. Brooks' himself gave three
explanation only related to concrete task-related issues, like introducing new
members to the work being done, communication overheads, or difficulty dividing
some programming tasks. Through a description of group developmental psychology
we argue for a fourth explanation to the law by suggesting that the group will
fall back in its group development when new members are added, resulting in
rework setting group norms, group goals, defining roles etc. that will also
change over time. We show that this fourth explanation is important when trying
to understanding Brooks' Law, and that adding the group developmental
perspective might help software development organizations in managing projects.
