Traces the author's three-year investigation into what constitutes family, describing how, after receiving an e-mail from a stranger who claimed to be a distant cousin, he embarked on an effort to build the biggest family tree in history. "A.J. Jacobs has received some strange emails over the years, but this note was perhaps the strangest: 'You don't know me, but my wife is your eighth cousin. And we have over 80,000 relatives of yours in our database.'...