That’s what their discussing over at Tyler Cowen’s Marginal Revolution. According to a recent economics study, the answer is one.
In comparing identical twins, Kohler found that mothers with one child are
about 20 percent happier than their childless counterparts; and while
fathers’ happiness gains are smaller, men enjoy an almost 75 percent
larger happiness boost from a firstborn son than from a firstborn
daughter.The first child’s sex doesn’t matter to mothers, perhaps because women are better than men at enjoying the company of both girls and boys, Kohler speculates.Interestingly, second and third children don’t add to
parents’ happiness at all. In fact, these additional children seem to
make mothers less happy than mothers with only one child—though still
happier than women with no children.“If you want to maximize your subjective well-being,
you should stop at one child,” concludes Kohler, adding that people
probably have additional children either for the benefit of the
firstborn or because they reason that if the first child made them
happy, the second one will, too.
The longer story is here; here is the paper, and the author’s home page.
I’m guessing lots of you have children. Care to weigh in.
April 21st, 2007 at 8:39 am
I’ve tried to explain this to my neighbor once. She has tons of kids and wondered why I would be content with just two. So (since I’m doing economics) I explained that the marginal cost of that third child would exceed the marginal benefit…don’t think she got it
…But on a micro level this is exactly it…we compare the expected marginal benefit with the expected marginal cost…most of us just don’t understand that we’re doing it 
And let’s say that we know more what to expect in terms of marginal costs and benefits that second time
…
April 21st, 2007 at 12:28 pm
Now you tell me… as I’m 5 months pregnant with #2.
But actually, the argument that having 1-2 children in general makes people happier than not having them makes sense to me. I think it’s because it provides perspective and balance (in weird ways).
April 22nd, 2007 at 3:07 am
Wendy–an initial high cost but scale economies in the long run
But more seriously there is something about having kids that put things in a different perspective as you say. Think it has made me another person, both as an individual as well as professionally. It was a huge change to disregard my utility and instead letting it become a function of somebody else’s utility – that’s a major marginal cost going from 0 to 1 child. And it has changed the way I look upon “place”, taking into account the most scarce resource “time”…the accessibility to things…and instead of valuing thing for me I also take into account how a certain place would shape my children to become the kind of persons I would like them to be.
Not sure if I’d agree that it creates a balance in life, at least not initially. Rather is causes an lack of balance and that it takes quite some time to find a new equilibrium…and I’ve seen just too many examples of couples who never do.
But there is something so cool about seeing a part of you in another person…
April 22nd, 2007 at 9:20 am
Charlotta and Wendy – This is an absolutely fantastic exchange. I think there’s a terrific paper in your insights here (not kidding).
April 23rd, 2007 at 10:59 pm
I’d like to add a different perspective. I’ve got three adult daughters and 4.5 grandchildren. They’re all great people and all quite different. The interactions between them and with them make my life richer, more complex and probably not so much balanced as giving deeper meaning to my existence. I think about things longer term. When I was raising kids, it was about who I wanted them to be. Now it’s who they are. I don’t have to do any more sacrificing of time or place, but I still think about my impact on their lives. I’m definitely happier than if I had none. One vs. two vs. three, I’d say each one adds a variable and every variable gives the formula more dimensions. And the resource of time goes beyond my daily schedule to what I want to have added to the world that’s here in 50 or 100 years.