Recently, historians have been trying to understand cultural change by measuring the “distances” that separate texts, songs, or other cultural artifacts. Where distances are large, they infer that change has been rapid. There are many ways to define distance, but one common strategy begins by topic modeling the evidence. Each novel (or song, or political speech) can be represented as a distribution across topics in the model. Then researchers estimate the pace of change by measuring distances between topic distributions.
In 2015, Mauch et al. used this strategy to measure the pace of change in popular music—arguing, for instance, that changes linked to hip-hop were more dramatic than the British invasion. Last year, Barron et al. used a similar strategy to measure the influence of speakers in French Revolutionary debate.
I don’t think topic modeling causes problems in either of the papers I just mentioned. But these methods are so useful that they’re likely to be widely imitated, and I do want to warn interested people about a couple of pitfalls I’ve encountered along the road.
One reason for skepticism will immediately occur to humanists: are human perceptions about difference even roughly proportional to the “distances” between topic distributions? In one case study I examined, the answer turned out to be “yes,” but there are caveats attached. Read the paper if you’re curious.
In this blog post, I’ll explore a simpler and weirder problem. Unless we’re careful about the way we measure “distance,” topic models can warp time. Time may seem to pass more slowly toward the edges of a long topic model, and more rapidly toward its center.