  Discovering and quantifying the drivers of language change is a major
challenge. Hypotheses about causal factors proliferate, but are difficult to
rigorously test. Here we ask a simple question: can 20th Century changes in
English English be explained as a consequence of spatial diffusion, or have
other processes created bias in favour of certain linguistic forms? Using two
of the most comprehensive spatial datasets available, which measure the state
of English at the beginning and end of the 20th century, we calibrate a simple
spatial model so that, initialised with the early state, it evolves into the
later. Our calibrations reveal that while some changes can be explained by
diffusion alone, others are clearly the result of substantial asymmetries
between variants. We discuss the origins of these asymmetries and, as a
by-product, we generate a full spatio-temporal prediction for the spatial
evolution of English features over the 20th Century, and a prediction of the
future.
