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.
Europe seems to have this pretty well figured out. And I don't mean to be flip - the European "let's stop at one" approach to childbearing really is well-calculated to maximize a certain kind of parental well-being, narrowly defined. Of course, it's also calculated to seriously diminish the "subjective well-being" of all the second and third children who don't get conceived because their parents decided it wasn't worth the trouble. And while the theory that parents have 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" may be true in many or even most cases, it also reflects a certain degree of deplorable solipsism. The chief reason parents should take on the trouble of conceiving and raising a child is that the child is a good in and of itself - one of the greatest goods there is, in fact, in any moral scheme worth considering - not because they think that it will make them or their already-existing offspring happier.