Why fixed length instead of random/unlimited length tournaments?
In short: because of this.
Your suggestion would very obviously turn into a tournament of nice strategies that will always cooperate once the non-nice strategies are eliminated. The "real world" isn't analogous to any kind of IPD at all, mainly because defectors often can't be punished. If you have a suggestion for a tournament that would to some extent actually model reality, I'm all ears; but random/unlimited length doesn't work.
mainly because defectors often can't be punished.
If each round came with a "situation variable" per bot that determined how 'unfair' the situation was - i.e, high variable means you can get away with defecting - and it wasn't somehow just noise, that might come closer to reality. Outside of the scope of my project, though.
Last year, there was a lot of interest in the IPD tournament with people asking for regular events of this sort and developing new strategies (like Afterparty) within hours after the results were published and also expressing interest in re-running the tournament with new rules that allowed for submitted strategies to evolve or read their opponent's source code. I noticed that many of the submitted strategies performed poorly because of a lack of understanding of the underlying mechanics, so I wrote a comprehensive article on IPD math that sparked some interesting comments.
And then the whole thing was never spoken of again.
So now I'd like to know: How many LWers would commit to competing in another tournament of this kind, and would someone be interested in hosting it?