Personally, I believe that only trading techs you invented is just as ridiculous as the other ideas, when it comes to enforcing it. My main idea was simply to introduce the concept of forcing a wait between trades, by requiring an 'emissary' to be sent or each trade. This would slow down tech trades, as until the invention of radio, there would be no practical way of updating the emissary on what he is allowed to trade. Thus only the techs that have already been put forward before either party sees what the other party is putting forward could be traded.
now to something completely different. the reason for trying to delay the tech advance is the same reason we are playing marathon length and not quick. we want to enjoy the full game and that includes playing through the different eras.
the Trade Only Researched Techs rule (hereby abbreviated as TORT) is one attempt at prolonging the enjoyment of the game by avoiding the "I just built my first spearman, next turn he's obsolete"-syndrome which makes the game less fun, imo. being part of a tech race is a good thing, having it result in never experiencing a particular phase of the game is not.
expanding on MMC's thinking could be to limit tech trading to civs you share a border with (or whos borders you can reach from your borders with your boats in one turn), until a certain tech is invented.
it would reduce the overall tech trade early in the game and is sort of realistic as developed sciences didnt jump across continents (until the discovery of the printing press at least). I will name this rule\idea PRETT (ProximityREstrainedTechTrading).
combine that with TORT and I think we just might have what we need, at least for the early game. of course it wont help from the mid/late game and out, but it might push in the right direction.
For the mid/late game situation we would just have to ask for restraint and hope people are paranoid enough to keep tech trading down.
It is either this or a tech trade pr number of turns limit.
now when it comes to enforcing. as I said above, if we cant trust eachother to act honorably then we should either not be playing or not have any rules. but I do believe that some kind of rules are necessary to limit tech-whoring, which is unrealistic and detrimental to the overall development of the game.
perhaps we should just go for NTT. it isnt very realistic, but neither is instantspread, and IMO they both reduce game quality. this is why we could make some rules that will give us a middle road. but all in all, I think I would rather have NTT than techwhoring. though everyone will probably be financial then...
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