Lupyan & Dale (2010)
Language Structure Is Partly Determined by Social Structure
The authors study demographic factors including population size, area spoken, and linguistic contact for over 2000 languages included in WALS. They conclude that all factors are negatively correlated with morphological complexity measured across a variety of features (type, case, verb morphology, agreement, evidentials/possibility, negation/plurality, tense, aspect, mood, possession, articles, demonstratives pronouns). They argue that exoteric languages, those with larger populations, which are spoken over a large area, or have heavy language contact tend to shift complexity from morphology to lexical items.
The authors propose the Linguistic Niche Hypothesis which states that language structures are subjected to different evolutionary pressures in different social environments. Specifically they claim that esoteric languages have mostly L1 learners which can benefit from the excess redundancy provided by complex morphology while exoteric languages have a large degree of L2 learners due to their roll as interface languages. L2 learners have difficultly learning complex morphology and so do not benefit from the added redundancy. Highly exoteric languages, therefore, tend to have reduced morphological complexity creating higher overall fitness for the language by affording L2 learners an easier learning experience.
Lupyan & Dale present compelling models showing the general trend they describe. Their hypothesis relies on the strong link between language structure and learning which needs further exploration. Their notion of language fitness and redundancy (adopted from Nowak et al. 2002) is intriguing but relies on an incredibly simplified model of the relationship between form and meaning. More sophisticated models based on Information Theory should supplant these simplistic models of language redundancy,complexity, and learnability. This would allow an explicit connection to formal learning theory to be developed to further test the Linguistic Niche Hypothesis.