Over 90% of U.S. corn and soybeans are planted with genetically modified (GM) seed varieties. We use a flexible, nonlinear functional form to investigate yield differences for corn, soybeans, and wheat between the United States and the European Union (which bans the use of GM technologies). U.S. corn and soybean yields increased relative to EU yields since the introduction of GM technologies. EU wheat yields (for which GM technologies are not commercially available in either region) continue to increase relative to the United States. Thus, the EU ban on GM technologies has likely increased the difference between corn and soybean yields between the two regions.