  Societal change is often driven by shifts in public opinion. As citizens
evolve in their norms, beliefs, and values, public policies change too. While
traditional opinion polling and surveys can outline the broad strokes of
whether public opinion on a particular topic is changing, they usually cannot
capture the full multi-dimensional richness and diversity of opinion present in
a large heterogeneous population. However, an increasing fraction of public
discourse about public policy issues is now occurring on online platforms,
which presents an opportunity to measure public opinion change at a
qualitatively different scale of resolution and context.
  In this paper, we present a conceptual model of observed opinion change on
online platforms and apply it to study public discourse on Universal Basic
Income (UBI) on Reddit throughout its history. UBI is a periodic,
no-strings-attached cash payment given to every citizen of a population. We
study UBI as it is a clearly-defined policy proposal that has recently
experienced a surge of interest through trends like automation and events like
the COVID-19 pandemic. We find that overall stance towards UBI on Reddit
significantly declined until mid-2019, when this historical trend suddenly
reversed and Reddit became substantially more supportive. Using our model, we
find the most significant drivers of this overall stance change were shifts
within different user cohorts, within communities that represented similar
affluence levels, and within communities that represented similar partisan
leanings. Our method identifies nuanced social drivers of opinion change in the
large-scale public discourse that now regularly occurs online, and could be
applied to a broad set of other important issues and policies.
