# A couple of couples

Two couples have babies simultaneously in different hospitals. They don't know the expected gender, and each have one child which is F or M. How can they arrange in advance, given a guess each on birth, that at least one couple will correctly guess the gender of the other's child.

Source: This is based on Ed Felten's Coin toss

I am interpreting

that each couple can wait to see what gender their own child is before making a guess about the other couple's child. And that "each have one child which is F or M" to exclude twins and intersex.

Then it seems this works:

The first couple guesses that the second couple's baby has the same gender as their own, and the second couple guesses that the first couple's baby has the opposite gender as their own? Exactly one guess will be correct.

• Damn, guess my guess was wrong. Guess I'll guess again and hopefully my guessing of that guess will be right. – a guy Dec 15 '18 at 20:28
• @aguy your guessing of that guess ought to be a better guess, I guess. – deep thought Dec 15 '18 at 20:32
• You guess I'll guess again, so I'll guess a guess again. – a guy Dec 15 '18 at 20:33
• I like that your answer 'exactly one guess will be correct' is more specific than the question. – Tom Dec 15 '18 at 20:40