One of the most puzzling observations for linguists is the difference between learning a language from birth and later in life: while all normally developing children can attain full native language proficiency, there is considerable variability in ultimate attainment among older speakers who attempt to acquire a second language (L2). There is an ongoing controversy in linguistic research on whether this discrepancy is due to a maturationally constrained window of linguistic development making language learning difficult or impossible after puberty, or to general cognitive factors linked to the fact that the later an L2 is established, the stronger the competition it has to overcome from the more deeply entrenched first language (L1). Studies attempting to resolve this controversy have so far focussed exclusively on the development of L2 skills. New insights may be provided by investigating the first language skills of migrants who have become dominant in the L2 (referred to as L1 attriters). Such speakers learned their L1 as monolinguals during childhood, and were therefore not impeded by maturational constraints in the acquisitional process. Having lived in an L2 environment for a long period of time, however, their seldom-used L1 shows signs of the influence of their highly active L2. A systematic comparison of L1 attriters and L2 learners may therefore be able to shed some light on the question of whether there is a qualitative or merely a quantitative difference between L1 acquisition in childhood and L2 acquisition later in life: If being a native speaker is maturationally constrained, even attrited L1 systems should remain native-like. But if the persistent problems of L2 learners are due to issues such as lack of practice and exposure, and competition between their two language systems, bilinguals who use their second language dominantly should become more similar to L2 speakers.