The document discusses several factors that can affect success in second language acquisition, including motivation, aptitude, personality, intelligence, and learner preferences. It describes research that attempted to correlate these learner characteristics with L2 proficiency by administering questionnaires and proficiency tests. However, precisely measuring qualities like personality is difficult. Motivation in particular has been extensively studied but the relationship between motivation and success is complex, with the possibility that both influence each other. The document also discusses individual differences in learning styles and beliefs that teachers can take into account.