87 International cooperation is needed today especially for those peoples who, besides facing so many other difficulties, likewise undergo pressures due to a rapid increase in population. There is an urgent need to explore, with the full and intense cooperation of all, and especially of the wealthier nations, ways whereby the human necessities of food and a suitable education can be furnished and shared with the entire human community. But some peoples could greatly improve upon the conditions of their life if they would change over from antiquated methods of farming to the new technical methods, applying them with needed prudence according to their own circumstances. Their life would likewise be improved by the establishment of a better social order and by a fairer system for the distribution of land ownership. Governments undoubtedly have rights and duties, within the limits of their proper competency, regarding the population problem in their respective countries, for instance, in the line of social and family life legislation, or regarding the migration of country-dwellers to the cities, or with respect to information concerning the condition and needs of the country. Since men today are giving thought to this problem and are so greatly disturbed over it, it is desirable in addition that Catholic specialists, especially in the universities, skillfully pursue and develop studies and projects on all these matters. But there are many today who maintain that the increase in world population, or at least the population increase in some countries, must be radically curbed by every means possible and by any kind of intervention on the part of public authority. In view of this contention, the council urges everyone to guard against solutions, whether publicly or privately supported, or at times even imposed, which are contrary to the moral law. For in keeping with man's inalienable right to marry and generate children, a decision concerning the number of children they will have depends on the right judgment of the parents and it cannot in any way be left to the judgment of public authority. But since the judgment of the parents presupposes a rightly formed conscience, it is of the utmost importance that the way be open for everyone to develop a correct and genuinely human responsibility which respects the divine law and takes into consideration the circumstances of the situation and the time. But sometimes this requires an improvement in educational and social conditions, and, above all, formation in religion or at least a complete moral training. Men should discreetly be informed, furthermore, of scientific advances in exploring methods whereby spouses can be helped in regulating the number of their children and whose safeness has been well proven and whose harmony with the moral order has been ascertained.
Source: Gaudium et Spes (Vatican.va)