Historically, demographic changes in Latin America have been mainly linked to the behaviour of the middle and upper income groups. Fertility and mortality have fallen to relatively low levels among these groups, and future changes are expected to have more to do with the demographic behaviour of the less prosperous groups. This document examines that behaviour, and the contribution of the less prosperous sectors to falls in fertility and infant mortality, distinguishing between the "distribution" effect (changes in the educational profile of the population) and the "rates" effect (demographic changes within the social groups). In relation to infant mortality, fertility, and the most irnmediate factors of fertility among those sectors which are socially and demographically most dis- advantaged, it is observed that the desired number of children is lower than the actual number; however, early marriage remains high and use of modern contraceptive methods is limited. In any case, these groups have already begun the demographic transition, with reductions in fertility and infant mortality, although this is less so than in other sectors of society. As for the contribution of those groups to the evolution of the demographic transition at the national level, it is observed that in various countries, over the past decade, the reductions have come mostly from the contribution of women having the lowest educational levels. In those countries where the transition has progressed the least, the greatest con- tribution has been by women without education, and in those where it has progressed most, it has come from women having only primary edu- cation