I think that people are debating between 3 choices: (a) 100% chance that there is a boy and girl, (b) 2/3 chance that it is a boy and a girl, or (c) 50% chance

I think that people are debating between 3 choices: (a) 100% chance that there is a boy and girl, (b) 2/3 chance that it is a boy and a girl, or (c) 50% chance that it is a boy and a girl. It depends very much on the exact wording of the problem. Like many people in the comments of Jeff's post said that if the person says that "one of my kids is a girl," then common sense makes it sound like ONLY one kid is a girl and the other must be a boy (case a). If the question is worded precisely like question #2 in this wikipedia article: http://en.wikipedia.org/wiki/Boy_or_Girl#Second_question http://en.wikipedia.org/wiki/Boy_or_Girl#Second_question (where you have a population of families and you randomly pick a family which has a girl), then you use the Bayesian approach to conclude that there is a 66% chance that the other kid is a boy (case b). Finally, I think that for case c, if you are just walking around and you're not specifically looking for a family with a girl, and you strike up a conversation and you hear that the person randomly decides to talk about his daughter, then I think the odds are 50% that this person's other kid is a boy. This is because you did not require that you choose a family with a specific gender (like in the wikipedia #2 scenario). First you picked a person to talk to, and then this person just happened to mention he had a daughter. Basically, we did not require that the person has a girl (case b), we only learned that he has a girl (case c).