Abstract
Between 1898 and 1948, English was the language of instruction for most post-primary grades in Puerto Rican public schools. Since 1949, the language of instruction in all grades has been Spanish. We use this policy change to estimate the effect of English-intensive instruction on the English-language skills of Puerto Ricans. Although naive estimates suggest that English instruction increased English-speaking ability among Puerto Rican natives, estimates that allow for education-specific cohort trends show no effect. This result is surprising in light of the strong presumption by American policymakers at the time that instruction in English was the best way to raise English proficiency. This has implications for medium of instruction policy in former colonies as well as U.S. education policy toward immigrant children