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Unnamed comments on Prisoner's Dilemma Tournament Results - Less Wrong
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Comments (170)
It looks like malthrin's bot I may have won only because it defected on the last 2 turns. There were enough defect-on-the-last-turn bots for defecting on the last two to be a smart approach, and of the defect-on-the-last-two bots malthrin's was the closest to tit-for-tat.
In the longer evolutionary run, the winning strategy (O) was the one that defected on the last 3 turns. It stuck around until the rest of the population consisted of only defect-on-the-last-two bots (I and C4), and then it took over.
I'd like to see you re-run this competition, adding in a few variants of tit-for-tat which always defect on the last n turns (for n=2,3,4) to see whether I and O are benefiting from their other rules or just from their end-of-game defections.
15 of the 22 strategies in the original tournament were NiceBots, meaning that they sought mutual cooperation until the endgame (formally, they always cooperated for the first 90 rounds against any strategy that cooperated with it for the first 90 rounds). They were mostly tit-for-tat with some combination of vengeance, forgiveness, and endgame defection (Win-stay lose-shift was the one exception).
In the 100-round evolutionary tournament, 15/22 bots survived until the end: the 15 NiceBots all survived and the 7 non-Nice bots all went extinct. In the larger (33-bot, 100-round) tournament, there were 2 exceptions to this pattern. 5 of the 6 new NiceBots survived (C5 went extinct) and 1 of the 5 non-Nice bots survived (C10, which played Nice for the first 84 rounds). [Edited to include C3 among the non-Nice.]
Nice: ABCDEFGHIJKORST C2,4,5,6,9,11 (only C5 went extinct)
Not Nice: LMNPQUZ C1,3,7,8,10 (only C10 survived)
In games between two NiceBots only endgame strategy matters, which means that endgame strategy became increasingly important (and differences in general rules became less important) as the non-Nice bots left the population. Another option for separating general rules from endgame play would be to shorten the game to 90 or 95 rounds (but don't tell the bots) so that we can see what would happen without the endgame.
I think it's unfair to call my strategy (N) Not Nice. There were 15 strategies that would cooperate with a cooperation rock. I just didn't expect anyone to enter permanent revenge mode after so much as a single defection. If there hadn't been any such strategies, or if they had been outnumbered by cooperation rocks, or if there had been enough other testing bots to quickly drive them to extinction I think I would have won the evolutionary tournament.
In my experience, sending out a probe is riskier than it looks, and the only reason to do so is if you have strong reason to believe a pure-C player is in the field.
"NiceBot" is just a label, a shorthand way to refer to those 15 strategies (which notably were the only 15 survivors in the evolutionary tournament). I agree that it's not a very precise name. Another way to identify them is that they're the strategies that never defected first, outside the endgame. Your strategy was designed to defect first, in an attempt to exploit overly-cooperative strategies. Perhaps we could call the 15 the non-exploitative bots (although that's not perfect either, since some bots like P and Z defect first in ways that aren't really attempts to exploit their opponent).
Fixed incomplete descriptions of C2 and C3. C3 started with D,C.
This was the reason that in the version I once competed in, which was evolutionary and more complex, you didn't know which round was last (turned out to be #102). This completely wipes out that issue, and the NiceBots (in the simple case here) end up dividing the world in static fashion.
Or, make each game last longer, so that the last few moves are less important. I agree that which turn you start to defect on appears to have been critical for these rankings.
That would just slow down the differentiation in the evolutionary tournament, since what's important isn't your total score but the ratio of your total score to everyone else's.
Clearly the other features were irelevant when only TfT-but-defect-after-n bots survive. A match between O and C4 or I is always C-C until turn 97, then D-C D-D D-D.
Anyway, what happens when population consists only of TfT-but-defect-after-n bots needs no simulation, a matrix multiplication can do it (also without "random extinctions" due to finite population). Maybe I'll calculate it explicitly if I find some time.