The other one is cited in this book (I read it there first) http://www.amazon.com/gp/product/1857883292/ref=as_li_ss_tl?...
Here is a not so scientific national poll about this http://www.dailymail.co.uk/news/article-2135843/Children-liv...
The discussion on moderators on the first study is interesting since it shows that it's not a universal truth - but significantly moderated by age, marital status and sex. For example:
"Finally, age significantly moderated the link between parenthood and life satisfaction. Simple effects analyses revealed that young parents (ages 17-25) were less satisfied with their lives than their childless counterparts (b = -0.39, p < .001); mid-range age parents (ages 26-62) were more satisfied than their childless peers (b = 0.42, p < .001); and older parents (ages 63 and older) did not differ from older non-parents (b = 0.16, p = .29)."
The discussion on recall bias is interesting to - nice to see that mentioned.
US only population too. Would be interested to see how that goes across cultures.
[citation needed]