According to the United Nations Development Program (UNDP), gender inequality is the loss in potential human development that occurs due to differences between the genders in achievements with respect to health, empowerment, and labor market participation. These differences in achievements typically favor men. Gender inequality is especially visible in the Arab world. We compare gender inequality in Arab countries with that in non-Arab countries, especially developed countries of the Organization for Economic Co-operation and Development (OECD). We argue that cross-national differences in gender inequality reflect cross-national differences in patriarchy, in particular differences in how men use their power over women to limit their agency or ability to make decisions for themselves. We set out a causal model to account for cross-national variations in gender inequality. Direct causes include fertility rate, per capita income, polygamy, OECD country, and corruption. Gender inequality in Arab countries is highly variable due to large differences in per capita income and is elevated because of polygamy and corruption. Arab countries can enact policies that would reduce gender inequality, especially improvements to women’s secondary and higher education. We analyze gender inequality in the Arab world and address the following questions: Is gender inequality greater in Arab countries? Among countries in the world generally, what differences in patriarchal practices contribute to differences in gender inequality? Where are Arab countries found with respect to such practices? What policies in Arab countries would reduce gender inequality? Our focus is upon cross-national differences in gender inequality, not upon differences in gender inequality within societies.