As societies modernize, they move from a relatively homogeneous state to one of greater diversity in several spheres. Among those, changes in educational structure and marital patterns are of great demographic importance, particularly in countries experiencing a high tempo of fertility. Increased education is supposed to result in non-familial aspirations and a greater understanding of the process and ways of controlling fertility. Similarly, marriage postponement tends to shorten the period of exposure to childbearing and results in a lower fertility than is experienced by those marrying earlier, particularly in societies where fertility is confined to marriage and is rarely controlled. However, it is not clear whether there is a threshold at which education or age at marriage becomes important in determining changes in fertility behaviour. Thresholds of fertility decline due to a given level of education have been identified for specific countries at a given point in time, but results vary considerably from country to country. In places where illiteracy is high, the move from illiteracy to literacy seems significant, while in some other societies, the crucial point is the completion of primary or higher education.