FuturePundit is worried that natural selection is currently weeding out people skeptical to the belief in God:

Natural selection in the human race has not stopped. Blogger Audacious Epigone uncovers an interesting pattern in General Social Survey data. Those with firm belief in God and those with firm belief God doesn't exist make more babies. My take: the genes for doubt and skepticism are getting selected against.

Theistic confidence	Desired	Actual
Don't believe	2.26	2.23
No way to find out	2.25	1.95
Some higher power	2.18	1.98
Believe sometimes	2.37	2.34
Believe with doubts	2.34	2.31
Know God exists	2.58	2.64

The more theistic, the greater the number of ideal children for a completed family to contain. It tracks almost identically with the actual number of children given birth to. That's not too surprising, since people are probably biased towards defining their actual family size as the ideal family size.

Granted, those who believe in God surpass the atheists in fertility. But the biggest doubters have the lowest fertility levels. Either the feeling of certainty boosts fertility or some factor causes both certainty and higher fertility.

I think the real answer is much more simple: People who believe in God are more likely to be a part of a religious movement, like Christianity or Catholicism, and these people in turn tend to have more conservative family values. Atheists tend to have more liberal values and thus place more focus on career and self-fulfillment than bringing up lots of children in a family.

Sometimes, due to the nature of surveys, we think we see a cause-effect relationship where it doesn't exist. If there is a relationship, my worry is that this selection process clearly favors less intelligent people. The higher-IQ segment of our population, according to several well-known studies, regards itself as predominantly atheistic. So in essence, we'd like the religious people to cut back on their own bedroom activities, yet promote more conservative values for the liberal-leaning career hunters in our society. Not an easy task.