Anthony Gottlieb makes a connection between religious people and fertility in Intelligent Life:
Like other demographers, Eric Kaufmann expects western Europe to become markedly more religious in the course of the 21st century, as a result of the relatively low fertility of unbelievers and immigration from more pious places. Not only do denominations with traditionalist values tend to have higher birth rates than their more liberal co-religionists, but countries that are relatively secularised usually reproduce more slowly than countries that are more religious. According to the World Bank, the nations with the largest proportions of unbelievers had an average annual population growth rate of just 0.7% in the period 1975-97, while the populations of the most religious countries grew three times as fast.
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.
Read the full article here. (HT: Arts & Letters Daily)