  Social scientists have long sought to understand why certain people, items,
or options become more popular than others. One seemingly intuitive theory is
that inherent value drives popularity. An alternative theory claims that
popularity is driven by the rich-get-richer effect of cumulative
advantage---certain options become more popular, not because they are higher
quality, but because they are already relatively popular. Realistically, it
seems likely that popularity is driven by neither one of these forces alone but
rather both together. Recently, researchers have begun using large-scale online
experiments to study the effect of cumulative advantage in realistic scenarios,
but there have been no large-scale studies of the combination of these two
effects. We are interested in studying a case where decision-makers observe
explicit signals of both the popularity and the quality of various options. We
derive a model for change in popularity as a function of past popularity and
past perceived quality. Our model implies that we should expect an interaction
between these two forces---popularity should amplify the effect of quality, so
that the more popular an option is, the faster we expect it to increase in
popularity with better perceived quality. We use a data set from eToro.com, an
online social investment platform, to support this hypothesis.
