  In a variety of open source software projects, we document a superlinear
growth of production ($R \sim c^\beta$) as a function of the number of active
developers $c$, with $\beta \simeq 4/3$ with large dispersions. For a typical
project in this class, doubling of the group size multiplies typically the
output by a factor $2^\beta=2.5$, explaining the title. This superlinear law is
found to hold for group sizes ranging from 5 to a few hundred developers. We
propose two classes of mechanisms, {\it interaction-based} and {\it large
deviation}, along with a cascade model of productive activity, which unifies
them. In this common framework, superlinear productivity requires that the
involved social groups function at or close to criticality, in the sense of a
subtle balance between order and disorder. We report the first empirical test
of the renormalization of the exponent of the distribution of the sizes of
first generation events into the renormalized exponent of the distribution of
clusters resulting from the cascade of triggering over all generation in a
critical branching process in the non-meanfield regime. Finally, we document a
size effect in the strength and variability of the superlinear effect, with
smaller groups exhibiting widely distributed superlinear exponents, some of
them characterizing highly productive teams. In contrast, large groups tend to
have a smaller superlinearity and less variability.
