  Predicting panic is of critical importance in many areas of human and animal
behavior, notably in the context of economics. The recent financial crisis is a
case in point. Panic may be due to a specific external threat, or
self-generated nervousness. Here we show that the recent economic crisis and
earlier large single-day panics were preceded by extended periods of high
levels of market mimicry --- direct evidence of uncertainty and nervousness,
and of the comparatively weak influence of external news. High levels of
mimicry can be a quite general indicator of the potential for self-organized
crises.
