Big won't be happy to hear about this research.
Tyler Cohen over at Marginal Revolution cites Hans-Peter Kohler's research on family size.
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 [TC: remember the result that fathers with sons are less likely to leave?]. 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.
Here is a story on his work and an argument for having more kids by Bryan Caplan.
As an aside, I regularly read Marginal Revolution despite their conservative bent, and normally that's okay until today I read a link of a link there that had the McCain 2008 petition "Surrender is Not an Option." I may have to rethink my leisure reading. What is that guy thinking?