In the post where I showed that IQ is unrelated to church attendance, readers suggested that a negative correlation would be found between IQ and belief in God. I looked at the question some time ago, but should do a test for statistical significance. The above table shows the mean correct score on the vocabulary test (WORDSUM) by belief in God. As before, agnostics are the smartest group among whites, and are significantly smarter than believers. The same can be said of those who "believe in a higher power" and those who "believe but have doubts." Atheists are not significantly smarter--they probably would be if their sample were bigger. Among blacks, the "believes in a higher power" group has the highest mean but the difference is not statistically significant.
Turning to the fertility question, I divided respondents into a dull group and a bright group based on their Wordsum score, and I limited the sample to people between the ages of 45 and 69 who participated in the survey since 1990. I made these choices to maximize the sample, but to focus on people old enough to have had all their children and who composed more recent cohorts. I also merged the atheist and agnostic categories to maximize cell size.
Among the dull group, believing in some higher power or believing with doubts is associated with having fewer children. "Knowers" and atheists/agnostics do not differ significantly in their fertility.
This changes, however, among the bright sample. Atheists, agnostics, and those who believe in some higher power have significantly fewer children than knowers.
So knowledge of God does not appear to increase fertility among the dull, as Jewish Atheist argued, but belief in God specifically (as opposed to some "higher power") might increase fertility among the intelligent.