The document discusses individual differences in language aptitude. It defines aptitude as a learner's capacity for learning a task based on their enduring characteristics. Language aptitude refers to cognitive differences between learners and their ability to learn a language. Intelligence is broader and refers to general mental ability transferable across tasks. Major research on language aptitude was conducted between 1920-1930 and included the Modern Language Aptitude Test (MLAT) in 1959 and Pimsleur Language Aptitude Battery (PLAB) in 1966. Language aptitude tests predict learning success under optimal conditions but do not measure if a learner can acquire a language.