  Languages emerge and change over time at the population level though
interactions between individual speakers. It is, however, hard to directly
observe how a single speaker's linguistic innovation precipitates a
population-wide change in the language, and many theoretical proposals exist.
We introduce a very general mathematical model that encompasses a wide variety
of individual-level linguistic behaviours and provides statistical predictions
for the population-level changes that result from them. This model allows us to
compare the likelihood of empirically-attested changes in definite and
indefinite articles in multiple languages under different assumptions on the
way in which individuals learn and use language. We find that accounts of
language change that appeal primarily to errors in childhood language
acquisition are very weakly supported by the historical data, whereas those
that allow speakers to change incrementally across the lifespan are more
plausible, particularly when combined with social network effects.
