In this paper, we provide quantitative evidence showing that languages spoken by many second language speakers tend to have relatively small nominal case systems or no nominal case at all. In our sample, all languages with more than 50% second language speakers had no nominal case. The negative association between the number of second language speakers and nominal case complexity generalizes to different language areas and families. As there are many studies attesting to the difficulty of acquiring morphological case in second language acquisition, this result supports the idea that languages adapt to the cognitive constraints of their speakers, as well as to the sociolinguistic niches of their speaking communities. We discuss our results with respect to sociolinguistic typology and the Linguistic Niche Hypothesis, as well as with respect to qualitative data from historical linguistics. All in all, multiple lines of evidence converge on the idea that morphosyntactic complexity is reduced by a high degree of language contact involving adult learners.