environment. Another recent investigation, focusing on the development of language in late bilinguals (i.e. adults past puberty), has claimed that maintenance of the mother tongue in an L1 environment requires little to no maintenance for individuals, whereas those in the L2 environment have an additive requirement for the maintenance of the L1 and the development of the L2 (Opitz, 2013). There have been cases in which adults have undergone first language attrition. A 2011 study tested adult monolingual English speakers, adult monolingual Russian speakers and adult bilingual English-Russian speakers on naming various liquid containers (cup, glass, mug, etc.) in both English and Russian. The results showed that the bilinguals had attrited Russian vocabulary because they did not label these liquid containers the same way as the monolingual Russian speakers. When grouped according to Age of Acquisition (AoA) of English, the bilinguals showed an effect of AoA (or perhaps the length of exposure to the L2) in that bilinguals with earlier AoA (mean AoA 3.4 years) exhibited much stronger attrition than bilinguals with later AoA (mean AoA 22.8 years). That is, the individuals with earlier AoA were the more different from monolingual Russian speakers in their labeling and categorization of drinking vessels, than the people with later AoA. However, even the late AoA bilinguals exhibited some degree of attrition in that they labeled the drinking vessels differently from native monolingual Russian-speaking adults. Critical period hypothesis Given that exposure to an L2 at a younger age typically leads to stronger attrition of the L1 than L2 exposure at later ages, there may be a relationship between language attrition and the critical period hypothesis. The critical period for language claims that there is an optimal time period for humans to acquire language, and after this time language acquisition is more difficult (though not impossible). Language attrition also seems to have a time period; before around age 12, a first language is most susceptible to attrition if there is reduced exposure to that language. Research shows that the complete attrition of a language would occur before the critical period ends. All available evidence on the age effect for L1 attrition, therefore, indicates that the development of susceptibility displays a curved, not a linear, function. This suggests that in native language learning there is indeed a Critical Period effect, and that full development of native language capacities necessitates exposure to L1 input for the entire