Using self-paced reading, the present study compared native English and adult L1-Korean-L2-English speakers' processing behaviors during online comprehension of underinformative scalar sentences and non-scalar sentences like "Some/All elephants have trunks and ears." Results indicate that native speakers showed online sensitivity (i.e. slower reading) to underinformative scalar sentences relative to non-scalar sentences, but second language (L2) speakers did not. These results are interpreted in support of the Relevance Theory claim that scalar implicature generation is modulated by contextual and individual factors, in particular by the language condition, that is, whether sentences are presented in L1-English versus L2-English.