Our youngest daughter is living at home while her husband finishes his medical residency. One of the many blessings of this old-fashioned arrangement (several generations under one roof) is that we have two babies to pass around. The youngest is just 5 months old. She doesn't have much to say yet, but she has quite a good sense of humor. If you smile at her, she will smile back, and she means it.

I am reminded of her aunt, our third child. When she was 10 months old, she had a great belly laugh. I would tickle her just to get her going, and then call my parents on the phone so they could join in the mirth.

There's something really interesting going on here. When we hear this sort of laughter, we aren't just pleased that our babies have taken a developmental step, as we might be if one rolled over or ate her beans. When we get the 5-month-old to smile, when the 10-month-old makes us laugh, we are doing something together. Laughter is a social practice that even infants incapable of speech can share with us, and they really seem to enjoy doing it.

It's not just that, either. Laughter is infectious. When your 10-month-old is stacking blocks, you might or might not care to participate. When she laughs uncontrollably, you can't help joining in.

Why is that? A neurophysiologist might say it's because social laughter causes the release of endogenous opioids in specific brain regions, and these endorphins are what make us laugh. An internist might say laughing reduces the level of certain stress hormones and stimulates your cardiovascular system, so it's a good way for the body to take care of itself. An anthropologist might say we laugh when others do because it helps us form bonds that link us together in a social network.

But these observations, even if true, leave us wanting some further explanation. Why does hearing a baby laugh trigger the release of endorphins? What is the evolutionary advantage to me in forming a social network with a 10-month-old?

Perhaps, if we dig all the way down, we will find a deeper but simpler truth. Think about the most fundamental human urges -- hunger, thirst, sex, the drive to succeed. They all have three things in common.

First, they arise in us unbidden. Second, we take great pleasure from satisfying them. And third, they are connected to the very stuff of life. If we don't satisfy them, we (and the human race) will waste away. At bottom, we are strings that vibrate in harmony with being.

Maybe laughter is like these things. The impulse is certainly hard to resist. (In fact, there is evidence that you might have problems if you don't find it contagious.) And there is no greater joy in life than uncontrolled laughter. Perhaps the joy that laughter celebrates, like the other things we naturally yearn for, is intrinsic to the nature of things.

After all, God has a sense of humor. He inspired the writers of the Bible to include a few famous jokes, from the mouths (for example) of the prophet Elijah and the man born blind in the ninth chapter of John.

God made us in his image and likeness, and humor subsequently became an important part of every human culture on earth. Our laughing granddaughter is only the most immediate reminder of the joy we are meant for.

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Garvey is president of The Catholic University of America in Washington. Catholic University's website is www.cua.edu.



- Garvey is president of The Catholic University of America in Washington, D.C.