  Tracking endogenous fluctuations in stock prices emerged as a key challenge
for empirical work in behavioural and evolutionary finance. This paper uses new
data from an online discussion forum, Reddit, to quantify social contagion, or
'hype,' in specific stock market movements, using state of the art opinion
dynamics modelling and sentiment analysis. The influence between users on the
WallStreetBets (WSB) subreddit is measured by tracing the probability of a user
starting a fresh discussion on an asset given their previous involvement in a
discussion on the same asset, measured by their comment history. This paper
finds that users who comment on one discussion involving a particular asset are
approximately four times more likely to start a new discussion about this asset
in the future, with the probability increasing with each additional discussion
the user engages in. This is a strong indication that investment strategies are
reproduced through social interaction. This is further validated by findings
that sentiments expressed in the linked submissions are strongly correlated in
a set of spatial regression models. In particular, bearish sentiments seem to
spread more than their bullish counterparts.
