[kgwgk]

> I have two children…

Oh, you have two children? The probability that they are of the same sex is 1/2.

> and the sex of at least one of them is…

Say no more! If at least one of them is of some sex the odds that they are both of the same sex go down to 1/3.

I said 1/2 before but that was before knowing that at least one of them is either a boy or a girl. That changes everything! (Probability is tricky.)

[phalf]

Nice!

What's really fun about this problem is that you can have very convincing arguments for 1/2 being the correct answer, and very convincing arguments for 1/3 being the correct answer. And for either you can make subtle reformulations that supposedly illustrate how ridiculous this answer is.

And there is no way to know. There is no gold standard for designing an experiment that would show whether 1/2 or 1/3 is correct. You could set up something that generates millions of pairs of (virtual) kids and then count the pairs that fit. But each of these experiments will have built-in the assumption on which the response is ultimately already predicated on.

The only thing really convincing would be if everybody, all "sides", could agree on an experiment with an outcome that they would feel bound to. Then one could settle this once and for all, whether it's 1/2 or 1/3 or 13/27 or 729/1459 or whatnot. But people will never agree on such an experimental setup.

Which tells me that this is not a mathematical problem. This problem is either underspecified or it's contradictory. If it was uniquely specified then we could just use probability theory with its axioms and inference rules to derive at the correct answer. But we obviously can't, since nobody can agree on how to formally note this down.

[kgwgk]

> If it was uniquely specified then we could just use probability theory with its axioms and inference rules to derive at the correct answer.

You’re right.

I wrote in another comment the solution down to this two unspecified elements:

P(you tell me that you have two children including at least one boy | you have two boys)

P(you tell me that you have two children including at least one boy | you have one boy and one girl)

If one assumes that they are equal (why?) the answer is 1/3.

If one assumes that the latter is half as probable the answer is 1/2.

Whatever the assumption that one finds more natural the point is that an assumption is needed.

[HWR_14]

Any arguments for 1/2 are just wrong. This isn't an unknowable or undefined situation. It's counterintuitive, but that's different.

[kgwgk]

Do you agree with the following?

> I tell you I have two children and that (at least) one of them is a boy, and ask you what you think is the probability that I have one boy and one girl.

2/3

> I tell you I have two children and that (at least) one of them is a girl, and ask you what you think is the probability that I have one boy and one girl.

2/3

If you don’t, why not?

If you do, what’s your answer to the following question?

> I tell you I have two children and that I’ve just sent you an email with the sex of (at least) one of them, and ask you what you think is the probability that I have one boy and one girl.

[HWR_14]

I agree 2/3, I agree 2/3. The answer to the last question is 1/2.

[kgwgk]

Thanks for your answer.

What I don’t understand is that when you read the content of that email you will find yourself in either the first situation (I told you that I have two children and that (at least) one of them is a boy) or the second situation (I told you that I have two children and that (at least) one of them is a girl).

In both cases the probability will be 2/3 so why wouldn’t you conclude that the probability is 2/3 without waiting to find out the (irrelevant) details?

[HWR_14]

The odds only sound equal/the details irrelevant because you are only looking at one outcome from the set. In reality, the email will resolve the probabilities to: (BB: 1/3 BG:2/3 GG:0/3} or {BB:0/3 BG:2/3 GG:1/3}. Although the BG values are the same, the rest of the probabilities are not. Therefore, the details are relevant.

I don't have a great explanation as to why that's intuitively true, but it is. I can try again if things are still confusing. But if so it would help to know if you understand the Monty Hall problem.

[kqr]

It is not sufficient to know one of them is of some sex. For the probability to be 1/3, you need to be asked what the probability is that one of them is a specific sex, not just any sex.

[kuboble]

I think the trickiest part is that the other party willingly shared some information and their motives affect probabilities way more than any math.

I find it easier to think about this problem stated like this: let's say you go around asking people " do you have exactly 2 children and at least one of them is a boy?". What are the odds of them having 2 boys if they answered yes.

[HWR_14]

All probability questions suffer from the same bias. The Monty Hall problem doesn't work if the person offering the choice has some agency and motives.