When you're talking about the overall population as a whole, and expanding your study across socioeconomic lines, maybe the factor they're looking for is the calming factor kids have on the average person's personality. (I know any parent will think I'm crazy, but keep listening.) Being a parent is financially, emotionally and physically stressful. I have a 3 and 6 year old, and that phase where the second was a newborn and the first still always needed something was an absolute nightmare in terms of sleep and emotional well-being. I'm only now starting to climb out of the no-sleep, constant-stress fog.
However, one thing I'm not is impulsive. I don't go tell off my boss when I'm having a bad day or rage-quit my job regardless of how nuts someone drives me. I highly doubt I'm going to get the sudden urge to skydive, or bungee jump or drive 110 MPH on the way home from work. Extend this out further, and people with kids might be less likely overall to smoke, take drugs, drink heavily or any other vice that reduces lifespan. Yes, of course there are plenty of parents who still do this stuff (and they're the most noticeable either on the news or reflected in their kids' behavior.) But I think overall, having responsibility for another human calms people down. People might also subconsciously want to stay healthier so they stick around. Childless people, even married childless couples, might not have that same drive below the surface. I've worked with lots of 20-somethings who just throw up their hands and walk out the door when they're mad about work...less doable when you have kids, even if your spouse has a job. There really is a settling-down that takes place -- it's rare that I see someone who has kids _and_ an angry chip on their shoulder; usually they're the divorced ones pissed off about paying child support, etc. If you're happy, it extends to other aspects of your life. If you're pissed off all the time, you just make everyone miserable.
Do I ever wish I didn't have kids? Sure, it's easy to look at no-kid couples and singles who go through life without a care in the world, have millions of dollars in the bank by the time they're 50 and do whatever they want whenever they want. But I've kind of done that -- we had kids later. I wish we had done more, but if we stay healthy enough we can do that later. And if we do a good job, the kids won't come back to live in our basement. I know it's impossible to defend being a parent, but I'm really glad we did it. It was pretty telling when my father in law, who's Mr. Spock as far as emotions go, told me "Congratulations, having kids was the best thing I ever did." after we found out. He's right, and sometimes it's a pain in the butt, but who else am I going to play Lego Worlds with? :-)