Wrong. I structured the problem incorrectly, it seems. The probabilities of the sexes of the two babies are, according to Mlodinow: boy/boy, boy/girl, girl/boy and girl/girl. Given that one of the children is a girl, we can eliminate the first outcome. Therefore, the odds of the second child being a girl are 1/3, or 33%.
Has Mlodinow added an outcome? Or (more likely) am I working with an incomplete understanding of probability? I don't doubt that Mlodinow is right and I am wrong. He does have Stephen Hawking on his side. The problem is that he makes only a passing attempt to persuade me over to his point of view, and thus he utterly fails to correct my (apparent) misunderstanding.
I don't mean to harp on one facet of the book, but there were other problems presented in the book in which the "right" answer was confusing and the given explanation was insufficient. I am admittedly a layman when it comes to advanced probability, but if I cannot understand Mlodinow's reasoning, how can I recommend the book? Surely you shouldn't have to have a bachelor's degree to access The Drunkard's Walk. It's meant to be accessible, or at least "a readable crash course in randomness," according to The New York Times Book Review. That it is not is a great shame, because what the book has to say is so very relevant to all of us, especially at a time when we are met with a deluge of data in our everyday lives.
If the problems I've discussed don't bother you at all, if you completely understand the 33% answer, or if you just think I'm an isolated case (an "outlier," in stat-speak), then by all means pick up the book. Otherwise, I feel compelled to warn you that, after reading the book, it will take a few days for your brain to return to its original, upright position.