  Languages vary considerably in syntactic structure. About 40% of the world's
languages have subject-verb-object order, and about 40% have
subject-object-verb order. Extensive work has sought to explain this word order
variation across languages. However, the existing approaches are not able to
explain coherently the frequency distribution and evolution of word order in
individual languages. We propose that variation in word order reflects
different ways of balancing competing pressures of dependency locality and
information locality, whereby languages favor placing elements together when
they are syntactically related or contextually informative about each other.
Using data from 80 languages in 17 language families and phylogenetic modeling,
we demonstrate that languages evolve to balance these pressures, such that word
order change is accompanied by change in the frequency distribution of the
syntactic structures which speakers communicate to maintain overall efficiency.
Variability in word order thus reflects different ways in which languages
resolve these evolutionary pressures. We identify relevant characteristics that
result from this joint optimization, particularly the frequency with which
subjects and objects are expressed together for the same verb. Our findings
suggest that syntactic structure and usage across languages co-adapt to support
efficient communication under limited cognitive resources.
