New research seems to prove that those languages that have a history of adult learning, i.e. languages with largest expansive histories and, normall, large modern populations of speakers, tend to have a much more simpler grammar than those languages restricted to small communities, which are comparatively much more complex (and therefore harder to learn).
Gary Lupyan and Rick Dale, Language Structure Is Partly Determined by Social Structure. PLoS ONE 2010. Open access.
The authors compared more than 2000 languages from all world regions to reach these conclusions. They argue that a simpler grammar makes easier for adults to learn these languages and hence expansive languages tend to lose complexity (a good example could be the transformation of Latin into Vulgar Latin, replacing declensions by propositions, and then into modern Romances, more akin to each other than to classical Latin). Inversely, they suggest (for further testing) that the complex grammar of small languages might be related to how children learn languages:
What appears to be functionless overspecification may provide infants with multiple cues allowing language acquisition to proceed with less reliance on extralinguistic context.