An American friend of mine with whom I discussed this question arrived at a simple conclusion that merits being shared.1
a) What is overpopulation? It's the imbalance between the number of men and the volume of goods that are available. What is poverty? It is the imbalance between the number of men and the volume of available goods. The words "overpopulation" and poverty" have an identical meaning every time they're used to describe the same social situation. They imply, however, very different judgments. In fact, the word "overpopulation" has become a pejorative term to designate "poverty."
b) When one thinks of the situation in "poor countries," he is inclined to help them produce more goods and distribute them better. What is recommended is educational and economic development as well as social justice.
But when one speaks of these same countries in terms of "overpopulation," the solution proposed -- and one had the effrontery to call it "aid"!- consists in sterilizing people and forcing mothers to have abortions, because the men and women are regarded as the cause of the social problems they experience. And that relieves one from the duty of questioning their living conditions.
c) When we speak of "poor people," our hearts are moved: We rise up against the situations of injustice of which the poor are victims: We mobilize and want to express our solidarity.
When we speak of "overpopulation," however, the rich feel that their security is threatened. Elementary concern for justice melts like snow before the sun. Instead of wanting to express our solidarity, we persuade ourselves -- with a big dose of bad faith -- and we persuade the unfortunate people, by trapping them in their inability to judge, that its "for their welfare and that of human society" that they must accept organized contraception, sterilization en masse and abortion.
Briefly, more concerned for their security than for solidarity, the wealthy invoke "overpopulation" to "justify" the coercion practiced on the poor.
1 Cf. Michael Schwartz, "Overpopulation and the War on the Poor," manuscript of talk given at the Third International Conference of the Family of the Americas Foundation in Caracas, Venezuela, Oct, 1985.