Hi everyone, I was recently at a table game with 9 other people, and I was one of two active hands after everyone else had folded.

River card turned, the other player made opening bet.
I called his bet.
He showed his cards, and I folded mine, conceding the pot to him.

He said I had to show my cards because I called his bet. I said it was my right to fold the cards without showing, and that I had conceded the pot to him. He and two other people at the table said it was not my decision to not show cards, but rather the rules that we both paid to see each others cards, and that he was entitled to the information.

Since it was me against 3 (everyone else had nothing to say) I turned up my cards, which were the losing hand.

Do I have to show cards in this scenario?

House rules. Whatever is the valid house rule.... rules.

That said, in a casino.... you would not have to show.

That’s a tricky question. Not only have different games different rules on that, there’s also something called “I want to see that hand”.

IWTSTH means that every player who was dealt into the hand has the right to see any hand that went to showdown. But the main purpose of that rule is to avoid collusion/cheating and not to gain information. So there are 3 possible scenarios here, based on house rules:
1. The other player has the right to invoke IWTSTH without providing a reason
2. The other player has the right to invoke IWTSTH if he provides a reason for doing so
3. The other player has no right to see your hand

I think in the past, 1. was the most common rule but that has switched towards 2. over the last decade or so.

Typically in a home game like this there are people who base rules on what they think is true in casinos, not because they have intentionally created their own rules. Interestingly, in this case they are imposing the rule after you called and mucked. A lot of people would say that since you called him, you don't have to show, but if you showed your hand and HE mucked, he would have to show - because you "paid to see his hand". Of course they are also saying that all the other players get to see, even though they didn't pay anything to see the hand.

In any event, the best ways to handle this are 1) say if that is the house rule, fine, but in most casinos it isn't true (and provide evidence if asked); 2) just go along with it because it really doesn't matter if they see your hand. What they know when you called is that you had a decent hand that wasn't as good as the winner, and when you show they will know that you had a decent hand that wasn't as good as the winner; or 3) if you want to be the ultimate dick, wait til you have the winner and when they make you show say "oh, I didn't realize I had the winner. I guess that is what happens when you made me show my hand, you lost the pot".

I'm not advocating #3, but I know people who have lost pots just because of something like this, and you can bet they never have it happen a second time.

Yes, IWTSTH is a thing. I've never witnessed it actually being called on someone. But I would hope madlex's #2 scenario is becoming the norm where IWTSTH is more common.

(I have seen people call IWTSTH against hand winners that were able to muck because everyone else mucked before showdown)

I remember that was a big point of controversy after Black Friday when a lot of online players started to play live. They were used to being able to see all hands and acted confused when told they couldn’t.

Other than falsely thinking you’re entitled to that information the most common situation is people asking out of spite.

I disagree that IWTSTH is for the prevention of collusion (it is a poor enforcement mechanism regardless). Everything I've read is that it is a vestige from when all hands still in the pot were shown at showdown.

I would always turn it over because who knows you might have the winning hand.