This notion goes back to the Malthusian theories. According to Malthus (1766-1834), population increases in a geometric progression and the food resources in an arithmetic one. This theory has resurfaced today in a barely modified form: "People are poor because they are too numerous." This assertion is broadcast by the media which tries hard to impose as blinding, evidence that "to be numerous is to be poor."
But we must not say that people are poor because they are too numerous but that they are too numerous because they are poor. To restrain births energetically in order to put an end to poverty is to approach the problem in the reverse.
Population excess is always measured in relationship to a precise, concrete and variable situation. Poverty is always evaluated according to man's capacity to face his environment: A nation is poor because it isn't able to feed it's population. In this sense, poverty is the cause of overpopulation and not the reverse; overpopulation is always relative to a given situation. Now this situation can be changed by man's intervention on the condition that he has both the moral and political desire to do so. There are cases in which people are so materially, intellectually and morally under-equipped that they have no possibility of being properly educated and are, therefore, in this particular situation, too numerous. But that is the point: man can change these situations by organization, teaching and by supplying equipment.
That doesn't mean that the demographic phenomena should not be taken into account, with its relative decline and growth. Public authority must take care of this problem. But here, as elsewhere, one must respect the principle of subsidiarity, the basis of all democracy.1 The intervention of public authorities must be accomplished always with respect to the fundamental rights of man. They may not use arbitrary means at any price.
1 According to the principle of subsidiarity, the public authorities must help individuals and intermediary entities, such as the family, to take the initiative that belongs to them and not act as a substitute for them.