Hans-Peter Kohler is expecting his second child in August. This happy state of affairs is causing more than the usual chatter at dinner parties with friends and neighbors. That’s because Kohler, an associate professor in the Department of Sociology, recently published some surprising research about the effect of babies on personal happiness. A first baby makes both Mom and Dad happier, he found. The second baby, however, has little effect on the father’s happiness and causes the mother’s to decrease.

For his study, Kohler and his colleagues surveyed a large sample of identical twins. The advantage of studying identical twins, says Kohler, is that since they share the same genetic makeup, the researchers could focus on external causes for happiness or misery rather than genetic predispositions.

For their study, Kohler and his fellow researchers bought space in a questionnaire distributed to twins in the Danish Twin Registry’s 2002 omnibus survey.

Twins are a precious resource for researchers, says Kohler, so questionnaire space is always at a premium, with experts from many different fields vying for space to gather information. Kohler had just one page in the survey to find out what he wanted to know—whether having a partner and children makes people happier—so his questions were tightly focused on fertility, partnership history and subjective happiness.

That men and women were both happier in a partnership came as no surprise. “That was pretty much in line with other research,” says Kohler. The strong positive effect of being in a stable union was further heightened by the arrival of the first child. And this is where the results get interesting. Both men and women reported feeling happier once they had a child, but for men having a son made them 75 percent happier than a daughter. “The strong effect of the first child is quite striking,” says Kohler, and the effect of male children on fathers’ sense of wellbeing is a “nontrivial difference, and surprising, since Denmark is pretty gender equalized.”

With the second child, the effect disappears.

And here again the male/female difference—with men’s happiness remaining the same and women’s dropping—is “totally unexpected” says Kohler.

If additional children do nothing for our happiness, why do so many people go on to have a second child? “We can only speculate,” says Kohler. “It would make sense that we would glean information from our social networks. Friends might tell you, ‘We thought we’d be twice as happy but we weren’t.’” In reality, says Kohler, even if it’s true, there’s a stigma attached to saying that your second or third child didn’t make you any happier. “Commodifying children is an awkward thing to do.”

Since much of the developed world is experiencing low fertility, the implications of Kohler’s study are significant. The happiness boost of having a child is big enough that it provides, “underpinnings for the argument that having one child is very important,” says Kohler.

And even if having a second child doesn’t make you happier, many couples will go on to have a second for reasons other than their own wellbeing, such as providing a companion for their first-born. Presumably many will also blithely plan a second because of the happiness the first brought. “We’d be interested to know why people went from the first to the second,” says Kohler.

Meanwhile, he’s looking forward to welcoming his 2-year-old son Boris’ sibling this summer, at which point, he says, “I’ll be able to let you know if my research holds true.”