Is Spanish-only schooling responsible for the Puerto Rican language gap?

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 these shifts in language of instruction policy to estimate the effect of English-intensive instruction on the English-language skills of Puerto Ricans. Although naïve 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 is the best way to raise English proficiency. This has implications for school language policies in former colonies as well as for U.S. education policy toward immigrant children.