  We introduce a model of proportional growth to explain the distribution
$P(g)$ of business firm growth rates. The model predicts that $P(g)$ is Laplace
in the central part and depicts an asymptotic power-law behavior in the tails
with an exponent $\zeta=3$. Because of data limitations, previous studies in
this field have been focusing exclusively on the Laplace shape of the body of
the distribution. We test the model at different levels of aggregation in the
economy, from products, to firms, to countries, and we find that the its
predictions are in good agreement with empirical evidence on both growth
distributions and size-variance relationships.
