Population size, learning, and innovation determine linguistic complexity

There are a number of claims regarding why linguistic complexity varies, for example: i) different types of societal structure (e.g. Wray & Grace, 2007), ii) population size (e.g. Lupyan & Dale, 2010), and iii) the proportion of child vs. adult learners (e.g. Trudgill, 2011). This simple model of interacting agents, capable of learning and innovation, partially supports all these accounts. However, several subtle points arise. Firstly, differences in the capacity or opportunity to learn determine how much complexity can remain stable. Secondly, small populations are susceptible to large amounts of drift and subsequent loss, unless innovation is frequent. Conversely, large populations remain resilient to change unless there is too much innovation, which leads to a collapse in complexity. Next, if adult learners are prevalent, we can instead expect less sustained complexity in large populations. Finally, creolisation does not imply simplification in smaller populations.