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.As a first child, I always knew that I added happiness to my parents life but my younger sister didn't. Glad to see that the research now backs me up. :)
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.
via Psychology Today via Marginal Revolution via AndrewSullivan.com