  Two general patterns have been identified for the adoption and subsequent
abandonment of ideas or products within a population. One is symmetric, so that
concepts or products which are adopted rapidly then decline rapidly from their
peak, and those which are slower to move to their peak decline more slowly. The
other is asymmetric, where the decline from the peak is considerably slower
than is the rise to the peak, and vice versa. We posit that these contrasting
patterns arise from two fundamentally different modes of behaviour which are
used by humans in making choices in different contexts. Namely, choice based on
the imitation of the choices of others versus purposeful selection based upon
the inherent attributes of the concept or product. We illustrate the
proposition with the example of internet searches for the phrase 'swine flu' in
a wide range of countries across the world. The methodology offers a general
heuristic for distinguishing between these two general and contrasting modes of
behavioural choice.
