  This paper proposes a new method to predict individual political ideology
from digital footprints on one of the world's largest online discussion forum.
We compiled a unique data set from the online discussion forum reddit that
contains information on the political ideology of around 91,000 users as well
as records of their comment frequency and the comments' text corpus in over
190,000 different subforums of interest. Applying a set of statistical learning
approaches, we show that information about activity in non-political discussion
forums alone, can very accurately predict a user's political ideology.
Depending on the model, we are able to predict the economic dimension of
ideology with an accuracy of up to 90.63% and the social dimension with and
accuracy of up to 82.02%. In comparison, using the textual features from actual
comments does not improve predictive accuracy. Our paper highlights the
importance of revealed digital behaviour to complement stated preferences from
digital communication when analysing human preferences and behaviour using
online data.
