Lo and behold, once again doing what’s best for the planet (rather than, say, advertisers or your in-laws) turns out to be also the best thing for your own happiness.

From the story:

For scientists studying the subject, simply correlating parenthood and happiness can’t answer this question, since happy people might be more likely to have kids to begin with. But a recent study that compared happiness levels in adult identical twins — some of whom are parents and some who aren’t — may be getting to the bottom of the issue.

The study, headed by sociology professor Hans-Peter Kohler of the University of Pennsylvania, found that people with children are, in fact, happier than those without children. But such happiness gains differ for mothers and fathers.

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.