Forty years ago, a study called the Hammond Report analyzed the smoking habits of half a million people. Its conclusion: Smoking is dangerous to one's health. It was a warning that ended up on every pack of cigarettes sold. Ten years later, a researcher took another look at the Hammond Report and found something that had been overlooked -- something just as hazardous as a pack-a-day cigarette habit -- that is, divorce. As Linda Waite and Maggie Gallagher write in their book, THE CASE FOR MARRIAGE, an enterprising Surgeon General might wish to slap a warning label on divorce decrees,...