I've been reading a lot about Medieval history recently. This experiment synthesizes what I've learned so far using feedback loops like this simple example:
graph TD AW --> |Selective breeding| HS HS[Horse strength] --> |Can carry more| AW[Armor weight]
As armor weight increased, horses needed to carry more weight and were selectively bred for this purpose. As horses got stronger, they could carry more, and armor got heavier.
This is a simple stock and flow diagram. Armor Weight and Horse Strength are stocks, and the arrows between them are roughly flows.
Early middle ages (800 - 1200)
Feedback loops (early)
graph TD A[Invading Armies] --> |Need to be repelled| K[Knights] K --> |Require wealth| F[Lord-Vassal relations] K[Knights] --> |Carry armor| AW[Armor weight] AW --> |Selective breeding| HS[Horse strength] K --> |Require| H[Horses] HS --> |Can carry more| AW HS[Horse strength] --> |More power| DH[Draft Horses] H[Horses] --> |More available| DH DH --> |Faster than oxen| FY[Farm Yields] AW[Armor weight] --> |Unidentified horse and rider| H[Heraldry] TFS[Popularity of Three Field System] --> |Better land use| FY[Farm Yields] TFS --> |Increased soil fertility| FY TFS --> |Popular winter crop| O[Oat Yields] O --> |Horses ♥ Oats| DH[Draft Horses] DH --> FY DH[Draft horses] --> |Used for transport| PM[Peasant mobility] PM --> |Cut out merchants| CT[Cheap transport] PM --> |Commute on horseback| TS[Town size] TS --> |City scale benefits| PP[Peasant prosperity] CT --> |Less spent on transport| FP[Farm profits] CT --> NF[Farm distance from rivers] NF --> |More food production| CC[Carrying capacity] FP --> PP CC --> PP FY[Farm Yields] --> PP
Late middle ages (1200 - 1500)
Feedback loops (late)
graph TD AW[Armor weight] --> |Bolts could pierce| C[Crossbows] C --> |Too slow| L[Longbows] L --> PP[Peasant Prosperity] C --> |Attempt to prevent puncture| AW BD[Black Death] --> |Supply of workers decimated| HL[Human Labor Cost] HL --> |Economics| ML[Mechanical Labor] ML --> PP[Peasant Prosperity] HL --> PP PP[Peasant Prosperity] --> |Uprisings| KW M[Mercenaries] --> |Need for control| SA SA[Standing Armies] --> RP[Royal Power] SA --> |Reduced need for knights| KW CAN[Cannons] --> |Castles obsolete| KW[Noble Weakness] L[Longbows] --> |Armor piercing| KW C[Crossbows] --> |Armor piercing| KW MUSK[Muskets] --> |Armor piercing| KW HL[Human Labor Cost] --> |Higher wages| KW CAN --> |Effective siege weapon| RP KW --> FD[Feudal decline]