Research on variation between individual learners seeks to address the question: Why do some learners do better than others? A flurry of studies in the 1970s, often labelled the "good language learner studies", sought to identify the distinctive factors of successful learners. Although those studies are now widely regarded as simplistic, they did serve to identify a number of factors affecting language acquisition. More detailed research on many of these specific factors continues today.
Tests of language aptitude have proven extremely effective in predicting which learners will be successful in learning. However, considerable controversy remains about whether language aptitude is properly regarded as a unitary concept, an organic property of the brain, or as a complex of factors including motivation and short-term memory. Research has generally shown that language aptitude is quite distinct from general aptitude or intelligence, as measured by various tests, and is itself fairly consistently measurable by different tests.
Language aptitude research is often criticized for being irrelevant to the problems of language learners, who must attempt to learn a language regardless of whether they are gifted for the task or not. This claim is reinforced by research findings that aptitude is largely unchangeable. In addition, traditional language aptitude measures such as the Modern Language Aptitude Test strongly favor decontextualized knowledge of the sort used in taking tests, rather than the sort used in conversation. For this reason little research is carried out on aptitude today. However, operators of selective language programs such as the United States Defense Language Institute continue to use language aptitude testing as part of applicant screening.
Main article: Critical Period Hypothesis
It is commonly believed that children are better suited to learn a second language than are adults. However, general second language research has failed to support the Critical Period Hypothesis in its strong form, which argues that full language acquisition is impossible beyond a certain age.