I've seen these sort of claims before and I've never seen a plausible rejoinder to the issue of correlation v. causation. In particular, unhappy people may actually be more likely to have children for some reason. The most obvious way to test this is to repeatedly ask how happy people are and see if it drops when some of them have kids.
In my experience, my probability of having children and my desire to have them are highly correlated with my happiness going up. Primarily I believe this is common cause, as the factors that make me happier also make it far more attractive and practical to have children. Other comments have noted events in the world that made them happy and also made them more inclined to have children.
However, it is also clear that those who come from wealthy or generally happier classes and backgrounds are far less likely to have children; the countries and areas with hi...
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).