From the Economist, describing a recent study by a psychologist at the University of California at Riverside:
Parents claimed more positive emotions and more meaning in their lives than non-parents, and a closer look revealed that it was fathers who most enjoyed these benefits. Moreover, further analysis revealed that this enhanced enjoyment came from activities which involved children rather than those (such as watching television alone, or cooking) that did not.
It looks, then, as if evolution has bolted into men a psychological mechanism to keep them in the family. At first sight, it is strange that women do not share this mechanism, but perhaps they do not need to. They know, after all, that the children are theirs, whereas the best a man can do is hope that is true.
The assertion that fathers are happier strikes me as entirely understandable. (Just to run a little personal experiment, I stopped and thought for a moment just now about my happiest moment over the last few days. Easy. Watching my 15-year old at bat in a Babe Ruth game this past Saturday morning. He quickly got into a hole, 0-2. Then he took a deep breath, and with a sense of calm that struck me as almost preternatural, stared the pitcher down. Four pitches later, he was walking to first.) Come to think of it, "happier" isn't quite the right word--nor is it the word that seems to have been used in the study. Children bring all kinds of worries--and, often enough, if temporarily, unhappiness. What they provide unfailingly, though--at least in my experience--is a sense of meaningfulness.
But the second finding? That mothers report no more positive emotions than non-mothers? This runs so entirely counter to all that I myself have observed--my nephew's wife just gave birth to their second child, and there it was, in all the hospital photos on Facebook, that look in his wife's face that she had just done something that made her feel a kind of cosmic contentedness--that I find it utterly baffling.