I've been meaning to comment on this piece Andrew linked to the other day, which tries to explain why having children doesn't seem to make us happier even though we often assume it does. The piece reviews some of the survey evidence establishing the result, then spins out a couple plausible theories for why it is. Among them:

One possible explanation for this, according to Daniel Gilbert (2006), is that the belief that ‘children bring happiness’ transmits itself much more successfully from generation to generation than the belief that ‘children bring misery’. The phenomenon, which Gilbert says is a ‘super-replicator’, can be explained further by the fact that people who believe that there is no joy in parenthood – and who thus stop having them – are unlikely to be able to pass on their belief much further beyond their own generation. It is a little bit like Darwin’s theory of the survival of the fittest. Only the belief that has the best chance of transmission – even if it is a faulty one – will be passed on.

I guess I question how we can actually know whether or not having children makes us happier. I just don't see how you can ever get at the relevant counterfactual. That is, what we really want to know is whether Person A is less happy with kids than Person A would have been without them. But there's no way to know that, because there's no way to re-run someone's life to see what it would look like if they weren't a parent. What we measure instead is whether Person A, who happens to have kids, is less happy than Person B, who doesn't have them. (And I suppose we try to match up Person A and Person B so that they look pretty similar demographically.)

Problem is, Person A may be the kind of person who's unhappy with kids, but would be even less happy without them. In fact, I'd venture to say a lot of people fit that description, which is why they made the enormous sacrifice of having kids in the first place. Meanwhile, Person B may be the kind of person who's happy without kids, and would be less happy with them, which is why they decided not to make the sacrifice. I don't see how you avoid this problem empirically, and my guess is that it's a big one.

P.S. I realize that this problem exists to some extent with a lot of social-scientific research, and that we have pretty sophisticated methods for mitigating it. But those methods seem a lot more dubious when you're dealing with something as idiosyncratic as happiness and as life-changing as parenthood--much different than dealing with, say, the returns to education.

To elaborate: The ideal test would be an experiment where otherwise identical people are randomly sorted into two groups, one of which has kids and the other of which doesn't, and then compared. But whereas we can simulate that sort of experiment in the education context (see this explanation), we can't even get close with parenthood. Since no one can really be forced to have a kid the way they can sometimes be forced to attend school, people who seem "identical" to one another are actually pretty different, simply by virtue of having made different decisions about whether or not to have kids. (Or, if they are forced to have a kid, that in itself would have big implications for their happiness and probably dominate the result.) 

When it comes to happiness and parenthood, I think looking at revealed preferences may be our best bet.

Update: Here's one of the studies measuring the effect of children (among other things) on happiness. It's pretty crude--makes no attempt to deal with the problem I highlighted, so far as I can tell. (Here's another. This one looks at "life satisfaction" rather than happiness. Also no attempt to deal with the problem.)

Update II: To clarify, I'm not asking if we can know whether having kids makes us happier than we were before. That seems tough but not impossible to answer. I'm asking if we can know whether having kids makes us happier than we would have been if we decided not to have them, which seems impossible to get at (and the more relevant question for someone contemplating parenthood). For example, it may be that my (as-yet) hypothetical fatherhood years are less "happy" than my immediate pre-fatherhood years, but, all-in-all, more happy than those fatherhood years would have been if I decided not to have kids.

--Noam Scheiber