Shared resources, individual incentives, collective ruin, and the property-rights fix.
Warming up the engine…
Overuse of a rival but non-excludable resource: each user captures the full benefit of extra use while the depletion cost spreads across everyone.
Private use where MB = MC(private); overuse because MC(social) > MC(private)
One more boat on the fishery earns its owner a full catch but shrinks everyone's future stock a little. Multiply by every boat owner reasoning identically and the fishery collapses, rational individuals, ruinous total.
Distinguish from public goods: commons ARE rival (fish run out); the fixes are property rights, quotas, or Pigouvian pricing.
Now prove you have it
Move the curve to where you think it lands, and get told exactly which part you got right.
Real-world scenarios on this model
Equilibrium at B 80, V 24.
Equilibrium: B* = 80.0, V* = 24.0
Current equations
A lake nobody owns. Each new boat looks at the average catch and enters if it beats the cost of fishing. But every entrant drags down everyone else's catch, and nobody counts that harm. Entry keeps going until the average catch barely covers costs, far beyond what is best for the group.