From GSS data, I looked at the reported ideal family size* and the actual number of children had, by theistic confidence, among those who had essentially completed their total fertility (age 40-100):
|No way to find out||2.25||1.95|
|Some higher power||2.18||1.98|
|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.
What about the nubile and their young nobles who are a decade or more away from finalizing their families? The ideal number of children by level of theistic confidence for those between the ages of 18-30:
|No way to find out||2.23|
|Some higher power||2.29|
|Believe with doubts||2.49|
|Know God exists||2.65|
At 47, the sample size for atheists is by far the smallest. Anomalously, two respondents from this cohort report an ideal number of children of six. If these two are removed, the average drops to 2.26, in line with the broader trend.
It looks as though early in life (long before family size is finalized) as theistic confidence increases, so does the perceived number of children a person should have. And across all levels of theistic confidence, people tend to realize those ideals**, with the more theistic slightly overshooting them and the less theistic coming up a little short.
It is not that secular people cannot keep up with religious folks. They simply do not want to. In the numbers game, though, the results are what matter. The question regarding Steve Sailer's suggestion that the future may belong to groups who are able to procreate the most is whether or not secularizing social trends are able to overcompensate for greater fertility among the religious.
GSS variables used: AGE(40-100), AGE(18-30), GOD, CHLDIDEL (removed "as many as desired", which is computed as 8), CHILDS
* For the question on the ideal number of children, the "as many as they want" response is filtered out, since it is coded as an 8 (that is, as though giving the response is tantamount to saying that the ideal number of children is eight).
** Incidentally, this increases my confidence in an assertion I made in a previous post that the most determinative factor in the number of offspring a man has is the number of offspring he desires. Except for in relatively rare cases of especially unattractive, socially awkward, or dehibilitated people, other personal attributes are ultimately of minor importance. 'Quality' of offspring, of course, is another matter.