"[P]opulation growth is not the consequence of a simple cause and effect relationship between birthrate and death rate. ... In particular, there is a powerful social force which, without compulsion—or even persuasion—leads people voluntarily to restrict the production of children. That force, simply stated, is the quality of life: a high standard of living, a sense of well being and of security in the future. ... Thus, human societies have developed a social means of bringing the birthrate into balance with the death rate. It consists of the improvement of the standard of living. Birth control is, of course, a necessary adjunct to this process; but it can succeed—barring compulsion—only in the presence of a rising standard of living, which of itself generates the motivation for birth control.

http://www.cnr.berkeley.edu/forestry/commoner.html