The Tragedy of the Commons

Medieval Version: The "Commons" was land held communally by a village; each villager was expected to graze a "reasonable" number of sheep or cattle on the land at no expense to him.
  You don't overgraze You overgraze
I don't overgraze We both have enough to live on You get rich; I still have enough to live on
I overgraze I get rich; you still have enough to live on The land is ruined by overgrazing and we both starve


Twentieth Century Version
: the "Population Explosion"

  You have 2.2 children You have more than 2.2 children
I have 2.2 children Stable or slowly declining population Stable or slowly growing population
I have more than 2.2 children Stable or slowly growing population Population explosion; we all starve


Twenty - First Century Version
: the "Birth Dearth"

  You have children You don't have children
I have children Both our families pay for both our retirements My kids have to pay for your retirement as well as mine
I don't have children Your kids have to pay for my retirement as well as yours Nobody pays for our retirement; we work till we drop, or starve.