[kgwgk]

> Your last paragraph has correct math. But the math works equally well with "specify a girl if you have one" or "flip a coin and use a random kids gender"

That’s the point.

The math works well with "specify a boy if you have one" and then the answer to A [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.] is 2/3 and the answer to B [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.] is 0.

The math works well with "specify a girl if you have one" and then the answer to A is 0 and the answer to B is 2/3.

The math works well with "flip a coin and use a random kids gender" and then the answer to A is 1/2 and the answer to B is 1/2.

If every parent with two kids says either “at least one is a boy” or “at least one is a girl” there is no way to make the math work so the answer to A is 2/3 and the answer to B is 2/3.

——-

As I explain in another comment for that the two following conditions need to be met:

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

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

There are ways to make the math “work”. For example: if you have two boys or two girls flip a coin and if you get heads talk about the weather, if you get tails say [I have two kids and at least one is a boy/girl], if you have one boy and one girl say [I have two kids and at least one is a …] using a coin flip to decide if you say “girl” or “boy”.

However, they seem quite unnatural and hardly a justification to claim that “any arguments for 1/2 are just wrong.”

[HWR_14]

You misunderstand me.

No matter how you choose the statement "I have at least one (girl/boy)", (prefer one, flip a coins, etc) once you tell me it's always 2/3 boy-girl. Until you tell me it's 1/2. Any algorithm to choose which to say works as long as it's true and you don't convey more information about the children like "my older child is male".

Your counter arguments are wrong, but you don't seem to even acknowledge that I am saying that. I'm willing to try to explain why, but not if you don't want to learn and just want to insist you are correct. Ask yourself how long you would spend trying to explain Monty Hall to someone who kept insisting it was 1/2 to change.

[kgwgk]

> Your counter arguments are wrong, but you don't seem to even acknowledge that I am saying that.

I do acknowledge that you're saying that I'm wrong. That's why we're still exchanging arguments! What I don't know exactly is what do you think that it's wrong with my arguments so I try to find where do we agree - and where we don't.

Do you think that there is something wrong with the answer I wrote down here https://news.ycombinator.com/item?id=37206445 ?

It seems that you don't agree that the answer depends on the (relative) value of P(you tell me that you have at least one boy|you have two boys) and P(you tell me that you have at least one boy|you have one boy and one girl).

> I'm willing to try to explain why, but not if you don't want to learn and just want to insist you are correct.

Well, I could also say that just want to insist that my arguments are wrong but I sincerely hope that you want to learn as much as I do.

> Ask yourself how long you would spend trying to explain Monty Hall to someone who kept insisting it was 1/2 to change.

As long as needed. Souls are saved one at a time. Here we go.

-----

> No matter how you choose the statement "I have at least one (girl/boy)", (prefer one, flip a coins, etc) once you tell me it's always 2/3 boy-girl. Until you tell me it's 1/2. Any algorithm to choose which to say works as long as it's true and you don't convey more information about the children like "my older child is male".

That's wrong and I'm going to try to show you that with an example (the mathematical proof is in the link above). Hopefully I'm not misrepresenting your position - please tell me if I do.

You are in an auditorium with 600 people. Each of them has two kids. (Let's assume there is no strange thing going on like "meeting of parents with twins" and the sex of the kids is equally probable and independent.)

Q: What's the probability that a given person has one boy and one girl?

A: 1/2

Q: How many of them do you estimate that have one boy and one girl?

A: 300

Each of them write into a paper their name and "I have at least one (girl/boy)" (they never lie and if there is a choice the choose however they want: prefer one, flip a coin, etc.).

You have the 600 papers in front of you, but have not read them yet.

Q: What's the probability that a given person has one boy and one girl?

A: Still 1/2

Q: How many of them do you estimate that have one boy and one girl?

A: Still 300

You can win $100 if you guess correctly whether there are more than 350 or less than 350 people with one boy and one girl.

Q: What's your guess?

A: Less than 350, because my estimate is 300.

Q: What will be the probability that a given person has one boy and one girl after you've read the papers?

A: 2/3 because once they tell me it's always 2/3 boy-girl.

Q: How many of them will you estimate that have one boy and one girl after you've read the papers?

A: 400

Q: Do you want you want to change your guess to "more than 350"?

A: No, until I read the papers the probability is 1/2 and my estimate is that 300 people have one boy and one girl.

Q: So you keep your "less than 350" guess even though you know with certainty that in a few minutes you will estimate that the right answer is around 400 and you will wish you had answered "more than 350"?

A: Yes, I'm happy with that. I think I could get the $100 if I answered "more than 350" now but I refuse to do it until I read the papers.

You read the papers.

Q: What's the probability that a given person has one boy and one girl?

A: 2/3

Q: How many of them do you estimate that have one boy and one girl?

A: 400

Q: Do you want to change your guess for the $100 prize?

A: Yes, now I’d like to answer "More than 350". Thanks for letting me change my guess!

Unfortunately you lose, because in a group of 600 pairs of kids we expected around 300 pairs of boy and girl. Writing things on a paper leaves the children unaffected.

[HWR_14]

I think now I understand. "I will make you make exactly one of two statements and each results in a 2/3 chance of BG" doesn't make sense. "You freely made one of two statements and each results in a 2/3 chance of BG" does. I interpreted "it depends on randomness" as "speak up if you have at least one boy" or "speak up if you have at least one girl", each of which would result in 450 saying something in your example and the math works.

So it comes down to if we decide on the question (however we do that) before we look at the kids or after.

[kgwgk]

> I interpreted "it depends on randomness" as "speak up if you have at least one boy" or "speak up if you have at least one girl", each of which would result in 450 saying something in your example and the math works.

The point is that original problem says "I tell you I have two children and that (at least) one of them is a boy". It doesn't say "I tell you I have two children and [when you ask me to confirm whether (at least) one of them is a boy] that (at least) one of them is a boy".

Reasoning from the cases "BB", "BG", "GB", "GG" - and discarding the last one to get p(BB)=1/3 - is implicitly using the cases "BB and I tell you that at least one of them is a boy", "BG and I tell you that at least one of them is a boy", "GB and I tell you that at least one of them is a boy", "GG and I tell you something else like at least one of them is a girl".

That breaks the symmetry between the "I tell you that at least one of them is a boy" and the "I tell you that at least one of them is a girl" problems. Using the cases in the previous paragraph the answer for the former is p(BB)=1/3 but the answer to the latter is p(GG)=1.

If you want to have the same solution when you switch girl <-> boy ["one of them is a boy, what is the probability that I have two boys" <-> "one of them is a girl, what is the probability that I have two girls"] you should treat them equally.

A quite natural way to do so would be to consider the eight equiprobable cases (some of them repeated)

    BB and I tell you that at least one of them is a boy
    BB and I tell you that at least one of them is a boy
    BG and I tell you that at least one of them is a boy
    BG and I tell you that at least one of them is a girl
    GB and I tell you that at least one of them is a boy
    GB and I tell you that at least one of them is a girl
    GG and I tell you that at least one of them is a girl
    GG and I tell you that at least one of them is a girl

but then the answer to both problems is 1/2.

You can make the answer to both problems 1/3 but the eight cases that you would need to consider for that are quite unnatural:

    BB and [... discard this line somehow ...]
    BB and I tell you that at least one of them is a boy
    BG and I tell you that at least one of them is a boy
    BG and I tell you that at least one of them is a girl
    GB and I tell you that at least one of them is a boy
    GB and I tell you that at least one of them is a girl
    GG and I tell you that at least one of them is a girl
    GG and [... discard this line somehow ...]

[HWR_14]

I think at this point we agree on the math and disagree on how the original question should be read. You saw an implicit alternative to the statement as "I have two children, at least one is a girl" and assumed in the question the parent was saying exactly one of that or the original statement "I have two children, at least one is a boy", possibly chosen randomly, from among the true answers. I read it as a simple statement true statement with GG just being an undefined state for the statement generation, maybe resulting in nothing being said. We could argue about parsing English, but it seems less interesting which question was being posed if we agree on the math behind each parsing, which I think we do?

[kgwgk]

> You saw an implicit alternative to the statement as "I have two children, at least one is a girl" and assumed in the question the parent was saying exactly one of that or the original statement

The alternative was made explicit when I asked you what do you think that the probability is for two girls under the alternative.

Given that you think that it's also 1/3 the question is how do you arrive to those 1/3 answers in a coherent way.

> I read it as a simple statement true statement with GG just being an undefined state for the statement generation, maybe resulting in nothing being said.

And with BG and GB being states that generate the statement "I have two children, at least one is a boy".

But then BG and GB cannot generate the statement "I have two children, at least one is a girl".

As a I said there are also ways to justify that the answer to both is 2/3 even though they don't look very nice.

> We could argue about parsing English, but it seems less interesting which question was being posed if we agree on the math behind each parsing, which I think we do?

Maybe we can also agree that this is an undefined situation because the answer depends on how you decide to interpret the question.

More precisely, it depends on your assumptions about the (relative) value of P(you tell me that you have at least one boy|you have two boys) and P(you tell me that you have at least one boy|you have one boy and one girl).