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. |