  Are large biological extinctions such as the Cretaceous/Tertiary KT boundary
due to a meteorite, extreme volcanic activity or self-organized critical
extinction cascades? Are commercial successes due to a progressive reputation
cascade or the result of a well orchestrated advertisement? Determining the
chain of causality for extreme events in complex systems requires disentangling
interwoven exogenous and endogenous contributions with either no clear or too
many signatures. Here, I review several efforts carried out with collaborators,
which suggest a general strategy for understanding the organization of several
complex systems under the dual effect of endogenous and exogenous fluctuations.
The studied examples are: Internet download shocks, book sale shocks, social
shocks, financial volatility shocks, and financial crashes. Simple models are
offered to quantitatively relate the endogenous organization to the exogenous
response of the system. Suggestions for applications of these ideas to many
other systems are offered.
