Looking through the comments, it seems like most of my thoughts have been captured (economy of scale, collaboration producing non-linear accumulation, etc) But some of the others (risk management, the time axis of logistics) helped me come up with a new one: perishability. When we combine some of the other factors (especially risk) people will at times have a perishable surplus. At these times they would seek to convert this surplus to something non-perishable or some other thing that they need at the time. If we had a society as described above plus uniform starting conditions and everyone used the same dice-rolls (i.e. everyone had the same good/​bad corn year), I believe this reason for trade would cease to exist
Looking through the comments, it seems like most of my thoughts have been captured (economy of scale, collaboration producing non-linear accumulation, etc) But some of the others (risk management, the time axis of logistics) helped me come up with a new one: perishability. When we combine some of the other factors (especially risk) people will at times have a perishable surplus. At these times they would seek to convert this surplus to something non-perishable or some other thing that they need at the time. If we had a society as described above plus uniform starting conditions and everyone used the same dice-rolls (i.e. everyone had the same good/​bad corn year), I believe this reason for trade would cease to exist
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