Nevertheless, fertility is inversely correlated with most measures of intelligence and rationality.
The labour involved in raising kids has a higher opportunity cost for more competent people. Also due to regression to the mean, very intelligent people are unlikely to have children smarter than them so this may reduce rewards.
Intelligent, rational people not having kids is basically a tragedy of the commons. Everyone is better of (including the unintelligent and irrational) if this group has more children, but for each individual it is a pretty burdensome undertaking (even when taking into account selfish reasons to have kids) And our whole modern stat...
So I found this post quite interesting:
http://www.gnxp.com/blog/2009/03/gnxp-readers-do-not-breed.php
(I'm quite sure that the demographics of this site closely parallel the demographics on Gene Expression).
Research seems to indicate that people are happiest when they're married, but that each child imposes a net decrease in happiness (parents in fact, enjoy a boost in happiness once their children leave the house). It's possible, of course, that adult children may be pleasurable to interact with, but it seems that in many cases, the parents want to interact with the children more than the children want to interact with the parent (although daughters generally seem more interactive with their parents).
So how do you think being child-free relates to rationality/happiness? Of course, Bryan Caplan (who is pro-natalist) cites research (from Judith Rich Harris) saying that parents really have less influence over their children than they think they have (so it's a good idea for parents to spend less effort in trying to "mold" their children, since their efforts will inevitably result in much frustration). And in fact, if parents did this, it's possible that they may beat the average.
(This doesn't convince me in my specific case, however, and I'm still committed to not having children).