The article explores why the gravity equation, a model used to predict trade between countries, is successful. By analyzing two trade theories, the researchers found that differences in factor endowments and increasing returns play a role in explaining trade patterns. The perfect specialization versions of the theories were not supported by the data, but a model incorporating both factors showed some accuracy in predicting trade patterns. Ultimately, the study suggests that factor endowments and increasing returns contribute to different aspects of international trade patterns.