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Declaring riichi while having a yakuman doesn't give more points, and about declaring it in such a situation I found (I don't remember where) two theories:

- you dont: you don't get more points, you just warn the other players you are tenpai.

- you do: declaring riichi might scare some other players into giving up on winning, which could make for a longer game and more change of winning.

On this, I also found two different ideas about declaring riichi in itself:

- it increases your chances of winning, because if someone gives up then the game is longer and you have less chances to discard a winning tile, so you have more time and draws to win.

- it decreases your chances of winning, because if someone goes into defense then it's less likely to discard your winning tile(s).

Does anyone have any data (or ideas/opinions) on this? I can see a logic in both the riichi's views, and if it increases or decreases the change of winning reflects on declaring or not it while having a yakuman.

I don't remember seeing any stats on this specifically and I only have a little to add.

Depending on their situations, declaring Riichi can make opponents switch to some degree of defensive play (as you said). If they run out of their safest tiles or they're still also trying to build their own hand, they might discard tiles that are (generally/sometimes) less risky like dragons and winds - especially if there are a couple of each out already - or ones and nines. This would be beneficial if you're Tenpai for Kokushi (and that will always be a closed hand, of course). Your Kokushi potential could be quite apparent from your discards but - psychology - "why would someone reach on a Yakuman...?"

I found this Reddit thread which you might've seen. One commenter makes the point that a hand that's Tenpai on two pairs for Suu Ankou (always closed again) will only give the Yakuman if you win on a self-drawn tile anyway so, if the Yakuman is your only aim, it doesn't matter what your opponents discard and whether they defend. You would reach to try to deter them from pushing for their own quick(er)/cheap(er) win and to buy yourself more turns - like your point in favour of Yakuman Riichi.

https://www.reddit.com/r/Mahjong/comments/27c8m4/why_would_anyone_declare_riichi_with_a_yakuman/