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While most measures of female empowerment improve or remain unchanged with economic development, the male bias in sex ratios at birth has instead increased in many societies. We identify cultural variation in male-favoring, patriarchal norms following Boserup's hypothesis that the plough increases male comparative advantage in farm labor, combining data on historical plough adoption with detailed birth records from 76 countries to show that the legacy of plough use explains a large portion of the variation in modern sex ratios. We present evidence that plough countries' male-skewed sex ratios are achieved through a mix of in-utero sex-selection, son-based stopping rules, and increased mortality suggestive of neglect or infanticide, and that male-bias intensifies with lower fertility, a pattern not found in non-plough countries. The cultural transmission of plough-driven son preference extends into the sex ratios at birth among US born children of immigrants. Our results highlight the interaction between fertility decisions, historical cultural norms, and economic development in determining modern sex ratios.