Via Sullivan:

If they want to spread their gospel, then, one might half-seriously conclude that atheists and agnostics ought to focus on having more children, to help overcome their demographic disadvantage. Unfortunately for secularists, this may not work even as a joke. Nobody knows exactly why religion and fertility tend to go together. Conventional wisdom says that female education, urbanisation, falling infant mortality, and the switch from agriculture to industry and services all tend to cause declines in both religiosity and birth rates. In other words, secularisation and smaller families are caused by the same things. Also, many religions enjoin believers to marry early, abjure abortion and sometimes even contraception, all of which leads to larger families. But there may be a quite different factor at work as well. Having a large family might itself sometimes make people more religious, or make them less likely to lose their religion. Perhaps religion and fertility are linked in several ways at the same time.

Or it could be that religion promises those who have nothing a lot more in the afterlife. Many people believe in it for much the same reasons they do in their chances of winning the lottery. Just coincidence that poor families tend toward larger numbers of offspring. Not cause and effect.

Just saying.

Want some rather stronger relation between cause and effect? Try secularism and critical thinking. Again, just saying.