We propose a previously unexamined factor instrumental in learning vocabulary accounting for the differences between learning a native and a foreign language: the development of critical thinking in adolescence. We hypothesize that the difficulties experienced in foreign vocabulary development result from the learner's readiness to question new information. Following Gilbert's (1991) claim that rigorous critical thought is the last to emerge and children are prone to accept propositions uncritically, we suggest that it is to this absence of doubt that children owe their success in remembering lexical items after a single exposure, a phenomenon referred to as "fast mapping." The rationale is that the mental belief systems are memory's filtering mechanism for what to retain: information labelled as questionable is allowed to decay without being granted access to long-term memory. We present the results of an experiment suggesting that memory of new language forms is enhanced by the learner's conviction in their validity.