 Important changes in the economic structure included free land distribution to peasants under the concept of "ejido", the nationalization of the oil and railroad companies, the introduction of social rights into the constitution, the birth of large and influential labor unions, and the upgrading of infrastructure.While population doubled from 1940 to 1970, GDP increased sixfold during the same period.Growth while under the ISI model had reached its peak in the late 1960s.