A March 26, 2009 press release by the Optimum Population Trust explains:
SEX IS MAIN CAUSE OF POPULATION GROWTH
Sex, not religious or cultural beliefs or the quest for economic security, is what increases family size and drives world population growth, according to one of the UK’s leading authorities on family planning.
Conventional economic wisdom, which says that couples in poorer societies actively plan to have large families to compensate for high child mortality, to provide labour, and to care for parents in their old age, is wrong, Professor John Guillebaud will tell a conference on sustainable population today.
Economists overlook the fact that sexual intercourse is more frequent than the minimum needed for intentional conceptions…
(Thanks to investigator Martin Gardiner for bringing this to our attention.)