End of PNAS-2010-Feng-1002632107.pdf

``Materials and Methods

Data.We use data on emigration, crop yields, and climate for each Mexican state, all averaged over two consecutive 5-y periods of 1995 to 2000 and from 2000 to 2005 (SI Appendix, Table S1). The emigration data are derived from Mexican censuses of 1995, 2000, and 2005. Specifically, we use a residual approach to compute out-migration flows:we start from 5-y population changes and adjust for mortality and internal migration, then calculate percentages of people who migrated within a period among all people aged 15 to 65 y who were in Mexico at the beginning of the period. This measure thus captures relatively long-term population movements rather than more transitory ones. Because of a large number of undocumented Mexican emigrants to the United States (40), all methods aimed at accurately determining emigration rates entail considerable uncertainty. Our method provides the best available estimates of state-level emigration rates, and matches other studies in terms of national totals (40).

To measure crop yields,we focus on corn, because it is the principal food stock of the Mexican diet and the most important crop. Corn accounts for 60% of land under cultivation and agricultural output by value, and provides the main source of livelihood to 40% of people working in agriculture (32). We also examine an alternative measure that combines corn and wheat, as wheat is a major substitute crop for corn and is also widely cultivated in Mexico (41). Yearly information for corn and wheat production and areas of land planted for each state were obtained from the Mexican Ministry of Agriculture, from which we calculate 5-y average crop yields. Our state-level climate data on annual precipitation, annual mean temperature, and summer mean temperature were obtained from the Mexican National Weather Service (SI Appendix, part 2, provides further description).

ACKNOWLEDGMENTS. We thank Douglas Massey, Cynthia Rosenzweig, Sam Schulhofer-Wohl, seminar participants at the Russell Sage Foundation, and two anonymous referees for invaluable comments. The views expressed in the article do not necessarily represent those of the US government.''