I can't resist jumping into the do-kids-make-you-happy discussion Noam has bravely entered.
One area that seems to me particularly problematic: the "mental well-being" category that turns up in these studies. Looking at the basic criteria that I suspect most people would use to define this term, of course parents score lower. Why? Um. How do I put this without risking that my children will one day run across this blog post and wind up in therapy forever? Screw it. My kids are already destined for therapy. Children make you crazy. It's not a question of them behaving badly or costing you sleep or messing up the house or wanting to watch Kung Fu Panda 264 nights in a row. It is the feeling of immense vulnerability they bring. Countless books, articles (again, see Michael Lewis--or Caitlin Flanagan), blog posts, and cheesy email chains have been written about the grinding anxiety parents (and, at the risk of stereotyping, especially mothers) have--not just that some harm will befall their kids, but that something bad will happen to them or their husbands or their financial security that will wind up traumatizing their child. Sure, some people wind up less insane than others. (Unlike some of my friends, for instance, I have not taken to stock-piling canned goods in my basement or tracking the progress of bird flu across the globe.) But almost every individual winds up at least a little crazier post-baby than pre-baby. Once you have a kid, you can pretty much erase the word "carefree" from your vocabulary.
On top of this, you have to factor in the concrete stressors placed on Mom's and Dad's work schedules (even for moms who work solely in the home), budgets, personal time (you can pretty much kiss all those Sunday-afternoon S&M marathons adios for at least a few years), etc. And it is the rare couple who doesn't find themselves embroiled in more balance-of-labor arguments post-child. So if by "mental well-being" you're talking about the easiest-to-measure factors like a lack of stress or anxiety, yeah, parents get the short straw.
That said, you know that whole goony-goo-goo cliche about how you don't fully grasp your capacity to love another person until you have a child? Absolutely true. This is no commentary on how ga-ga people are about their spouses. It's just...different. And for all of the anxiety of parenthood, there is also incomprehensible bliss. Noam is dead-on in noting that the definition of happiness must come into play. And while it's inaccurate to suggest that we're talking about the difference between happiness and joy, that's the sort of fuzzy area we're operating in here. As with everything in life, there are trade-offs. You make your choices, and you run with them. And if you're very, very lucky, your offspring never winds up perched atop a clock tower with an automatic weapon.