Drawing on Norton Peirce\\\'s (1995) theory of investment and Darvin and Norton\\\'s (2015) expanded the model of investment, the present study aims to research investment in second language (L2) learning among Iranian English language learners. The participants included 852 male and female English language learners belonging to different age groups and English language proficiency levels. A 42-item questionnaire, developed and validated by the present authors (forthcoming), was administered online and by hand. The results showed that Iranian English as a Foreign Language (EFL) learners moderately invested to learn English language and that there were significant differences in the extent of investment between male and female participants and different language proficiency levels (lowand high-proficiency learners). Moreover, the results of the study demonstrated that there was no significant difference in the extent of investment between different age groups (teenage and adult ones). The study concludes with suggestions for future research on investment in L2 learning and a discussion of how such research can impact language education policy in EFL contexts.

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