It is often asserted that first language acquisition and adult second language learning are “fundamentally different”. In contrast to L2 learning, first language acquisition is believed to rely almost entirely on implicit learning. Children, it is often asserted, are unable to focus on form, and because of this, explicit learning and aptitude (as measured by foreign language aptitude tests such as the MLAT and PLAB) are irrelevant. There are also important differences in motivation: adult learners typically have the explicit goal of learning a new language, while children's motivation is simply to interact with others, and learning occurs as a by-product of engaging in communicative interaction.
While acknowledging that there are important differences between first and second language learning, I argue that they tend to be overstated. I argue that language aptitude plays a significant role in both L1 and L2 language learning, that children are not only able, but also highly motivated to pay attention to form, and that in both populations, acquisition depends on cooperative interaction between implicit and explicit processes. Thus, the same cognitive mechanisms are involved in both types of learning, albeit not necessarily to the same extent.