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. Between 1920-1930, researchers developed early language aptitude tests to identify learners who may struggle. These tests analyzed or synthesized specific language learning abilities but lacked theoretical basis. Later tests like the MLAT and PLAB measured abilities like number learning, spelling, sound discrimination that correlated with language achievement. Research on the relationship between aptitude, age, intelligence, teaching methods, and learning outcomes is ongoing but aptitude tests continue to predict language learning success.