context of second-language acquisition has focused on what level of access learners may have. there is an ongoing debate among generative linguists surrounding whether L2 users have full or partial access to universal grammar. This can be seen through acceptability judgment tests. For example, one study found that during a comprehension task, while English L1 speakers learning Spanish may accept the imperfect aspect in appropriate conditions, even at higher levels of proficiency, they do not reject the use of the Preterite tense in continuous and habitual contexts. Universal grammar theory can account for some of the observations of SLA research. For example, L2 users often display knowledge about their L2 that they have not been exposed to. L2 users are often aware of ambiguous or ungrammatical L2 units that they have not learned from any external source, nor their pre-existing L1 knowledge. This unsourced knowledge suggests the existence of a universal grammar. Another piece of evidence that generative linguists tend to use is the poverty of the stimulus, which states that children acquiring language lack sufficient data to fully acquire all facets of grammar in their language, causing a mismatch between input and output. The fact that children are only exposed to positive evidence yet have intuition about which word strings are ungrammatical may also be indicative of universal grammar. However, L2 learners have access to negative evidence as they are explicitly taught about ungrammaticality through corrections or grammar teaching. Individual variation There is considerable variation in the rate at which people learn second languages, and in the language level that they ultimately reach. Some learners learn quickly and reach a near-native level of competence, but others learn slowly and get stuck at relatively early stages of acquisition, despite living in the country where the language is spoken for several years. The reason for this disparity was first addressed with the study of language learning aptitude in the 1950s, and later with the good language learner studies in the 1970s. More recently research has focused on several different factors that affect individuals' language learning, in particular strategy use, social and societal influences, personality, motivation, and anxiety. The relationship between age and the ability to learn languages has also been a subject of long-standing debate. Age The issue of age was first addressed with the critical period hypothesis. The strict version of this hypothesis states that there is a cut-off age