Thus, in pursuing their own interests, civil societies, far from causing injury to others, must join plans and forces whenever the efforts of particular States cannot achieve the desired goal. But in doing so great care must be taken. What is beneficial to some States may prove detrimental rather than advantageous to others.
Furthermore, the universal common good requires the encouragement in all nations of every kind of reciprocation between citizens and their intermediate societies. There are many parts of the world where we find groupings of people of more or less different ethnic origin. Nothing must be allowed to prevent reciprocal relations between them. Indeed such a prohibition would flout the very spirit of an age which has done so much to nullify the distances separating peoples. Nor must one overlook the fact that whatever their ethnic background, men possess, besides the special characteristics which distinguish them from other men, other very important elements in common with the rest of mankind. And these can form the basis of their progressive development and self-realization especially in regard to spiritual values. They have, therefore, the right and duty to carry on their lives with others in society. The Proper Balance Between Population, Land and Capital
As everyone is well aware, there are some countries where there is an imbalance between the amount of arable land and the number of inhabitants; others where there is an imbalance between the richness of the resources and the instruments of agriculture available. It is imperative, therefore, that nations enter into collaboration with each other, and facilitate the circulation of goods, capital and manpower. 58Cf. John XIII's encyclical letter Mater et Magistra, AAS 53 (1961) 439.