Last week I put up a link to an online experiment. Here’s the results! You can still do the experiment first, if you like, here. Source code and raw results at the bottom.
Languages evolve over time under a pressure to be learned by a new generation. Does learning two languages at once effect this pressure? My experiment says … maybe.
These pressures include ones for learnability (compression) and expressivity (able to express a large variety of meanings, Kirby, Cornish & Smith, 2008). Bilingualism seems like an unlikely ability since learning an extra language leaves the speaker potentially no more expressive at a cost of an increase in the amount of effort required to learn it. There is no pressure for one language structure (e.g. English) to adapt to another language (e.g. Mandarin) so that they can become optimally learnable and expressive as a single medium. That is, there’s no reason to assume that expressivity and learnability pressures apply across languages (which are not being used by the same people).
Nevertheless, children display an aptitude and a willingness to learn and use multiple languages simultaneously, and at a similar rate to monolingual children. Therefore, languages do seem to have adapted to be learnable simultaneously. Does the compatibility of languages point to a strong innate property of language? In contrast, it might point to underlying similarity in the structure of languages, brought about by universal principles of communication.