Why did gender inequality in secondary school access persist after Mexico made attendance compulsory in 1993? This research reveals an interaction between geography, poverty, and sibship structure in contributing to the underrepresentation of girls in Mexico's poorer southern states. Using regional and contextual information about enrollments and development, a multinomial logistic regression model is estimated. The results show that in addition to family and regional poverty, the position of girls within the sibship contributes to their remaining in or dropping out (...) of school following the primary level and their likelihood of employment or domestic work as an alternative to schooling. The results have implications for Mexico's targeted approach to eliminating the effects of poverty on educational opportunity. (shrink)