A quote from Philip Longman in Foreign Policy, March 2006

Single-child families are prone to extinction. A single child replaces one of his or her parents, but not both. Nor do single-child families contribute much to future population. The 17.4 percent of baby boomer women who had only one child account for a mere 7.8 percent of children born in the next generation. By contrast, nearly a quarter of the children of baby boomers descend from the mere 11 percent of baby boomer women who had four or more children. These circumstances are leading to the emergence of a new society whose members will disproportionately be descended from parents who rejected the social tendencies that once made childlessness and small families the norm. [...]

The great difference in fertility rates between secular individualists and religious or cultural conservatives augurs a vast, demographically driven change in modern societies. Consider the demographics of France, for example. Among French women born in the early 1960s, less than a third have three or more children. But this distinct minority of French women (most of them presumably practicing Catholics and Muslims) produced more than 50 percent of all children born to their generation, in large measure because so many of their contemporaries had one child or none at all.